中 華 大 學 碩 士 論 文
Increase in Energy Efficiency of Residential Buildings in Taiwan, R.O.C.
系 所 別：土木工程學系碩士班 學號姓名：M09604039 柯立夫 指導教授：劉 俊 杰 博 士
中 華 民 國 102 年 8 月
建築物是主要的能源消耗者。氣候變遷的主因是溫室氣體排出，而這些溫室氣體也 源自於建築物所造成的大量能源消耗。建造的建築物維持約十年，甚至於五年或只有一 年 。現今的建構和設計方式大大的影響了當下的營業成本，但是也影響了環境條件和 未來多年的營業成本。中華民國的總能源消耗量在這二十年來急遽成長。減少能源使用 和建築排出的溫室氣體，最經濟有效的方式是改善建築的熱性能建材。空調和電暖器是 所有家電中最主要的耗能產品。空調的負荷佔一般建物所消耗的約 50%能源。因為亞熱 帶氣候不得不使用空調營造體感溫度的舒適，所以可以依如何使用這些家電來改善。
本篇論文探討利用居家建築的絕緣性能的改善以達如何降低使用空調和電暖器。維 持建築裡可接受溫度(使用暖氣或冷氣) 用掉了大部分的全球耗能 。絕緣物減少了不需 要的熱損或熱增益，且可降低暖氣或冷氣系統的能源需求。在熱的時候，最大的熱能來 源為太陽輻射。太陽輻射可以透過窗戶直接進入建築裡，且可加熱建築遮蔽外構為更高 溫，藉由外構增加熱能轉換。所以改善後的外構可讓熱能留在屋外也讓舒適溫度留在屋 內，進而有效降低空調使用。絕緣物不但可減少不需要的熱損或熱增益，且可降低暖氣 或冷氣系統的能源需求，同時達到體感的舒適溫度和減少溫室氣體的排放。本論文分析 了一些個案研究與居家經驗，以證明建築絕緣物是增進建物能源效率的有效方式。
The building sector is a major energy consumer. The main cause of climate change is greenhouse gas emissions. Source of these is also great energy consumption by buildings.
Buildings are built to last decades nor for a year or five. Constructing and designing way today have a great impact on operating costs now, but this will also affect the environmental conditions and operating costs for many years in the future. R.O.C.'s total energy consumption has grown greatly over the past two decades. The most cost-effective way to reduce energy use and greenhouse gas emissions caused by buildings is by improvements in buildings thermal properties. Air conditioners and space heaters are the most significant energy consumers among all the household appliances. The air conditioning load accounts for around 50% of the energy consumed by a typical building. Because of the subtropical climate, the use of air conditioners for creating thermal comfort is necessary. Improvement can be in the way of how these appliances are used.
This thesis focuses on how to lower usage of air conditioners and heaters by improving the insulation capabilities of residential buildings. Maintaining acceptable temperatures in buildings (by heating and cooling) uses a large proportion of global energy consumption. In hot conditions, the greatest source of heat energy is solar radiation. This can enter buildings directly through windows or it can heat the building shell to a higher temperature, increasing the heat transfer through the building envelope. As improved building envelope can lower the use of air conditioners by keeping heat out of the building as well as keeping comfortable temperature inside building. Insulation reduces unwanted heat loss or gain and can decrease the energy demands of heating and cooling systems as well as creating thermal comfort and reducing greenhouse gas emissions. Case studies and home experiments are analyzed in this thesis and used as a proof that building insulation is very effective method to increase the building energy efficiency.
Keywords:Energy Efficiency, Building Insulation, Energy saving, Thermal Comfort.
First and foremost, I thank to my wife and my daughter Julia for patience and understanding of my study. I am grateful for their help and support.
Thank to my supervisor, Prof. Chun-Chieh Liu for guiding me and helping with my research and study. Thank you for your support and suggestions.
I am very fortunate and grateful to our department head Prof. Jason Wu for his huge help in editing my many mistakes and help with my study. You are truly an outstanding person, an able educator and, I thank you from the bottom of my heart.
Not least of all, I want to thank all the teachers for their patience and my classmates for their support. Thank to my classmate Sophie for her huge help with editing my thesis and patience through the process. Thank you.
© Copyright 2013 by Jozef Krivak All Rights Reserved
TABLE OF CONTENTS
摘要 ... i
ABSTRACT ... ii
Acknowledgements ... iii
LIST OF TABLES ... vii
LIST OF FIGURES ... viii
Chapter 1 Introduction ... 1
1.1 Energy efficiency of residential buildings in Taiwan, R.O.C. ... 1
1.2 Research objectives ... 3
Chapter 2 Literature Review... 6
2.1 CO2 Emissions increase ... 6
2.2 Energy consumption and energy policy ... 7
2.3 Thermal Transmittance ... 10
Chapter3 Research on Energy Efficiency of different parts of Building Envelope ... 12
3.1 Study on walls and insulation capabilities of different materials and methods of construction ... 12
3.1.1 Thermal Resistance: R-value ... 13
3.1.2 Thermal mass as an important factor for energy efficiency of walls ... 14
3.1.3 Thermal mass “Home Experiment”. ... 17
3.1.4 Autoclaved Aerated Concrete (AAC)... 20
3.1.5 “Baoshan” Case Study ... 24
3.1.6 External wall insulation and it’s improvement in thermal properties of walls ... 28
3.2 Insulated glazing and glass coatings in windows ... 32
3.2.1 Factors that affect window’s thermal performance ... 32
3.2.2 Heat gain and loss in windows ... 34
3.2.3 Sunlight transmittance ... 35
3.2.4 Insulated glazing ... 35
3.2.5 Gas fills and lower heat transfer between glass panes... 37
3.2.6 Tints (color) and coatings in windows ... 37
3.2.7 U-value for windows and it’s calculation ... 40
3.3 “Green roof” as the effective roof insulation... 43
3.3.1 Roof insulation ... 43
3.3.2 Green roofs for reduction of energy use: A case study. ... 48
3.3.3 Green roof “Home Experiment”. ... 53
Chapter 4 Results of the study and comparisons ... 58
4.1 Walls ... 58
4.2 Windows ... 59
4.3 Roofs ... 61
Chapter 5 Conclusion: Improvement in Energy Efficiency in Residential Buildings with Insulating the Building Envelope. ... 62
REFERENCES ... 64
LIST OF TABLES
Table 2.1. Acceleration of Atmospheric CO2 from decade to decade. 07 Table 2.2. Typical thermal transmittance values for common building structures. 11 Table 3.1. R-values of commonly used building materials. 13 Table 3.2. Data collected during the thermal mass increase experiment. 18
Table 3.3. Cost effectiveness of AAC and brick walls. 26
Table 3.4. AAC wall temperature measurement data. 27
Table 3.5. R-values for different thickness of EPS panels. 30 Table 3.6. R-values for different thickness of mineral wool panels. 30
Table 3.7. Examples of whole window U –factors. 42
Table 3.8. Data measured for green roof experiment. 54
Table 3.9. Price calculation for the 18m2 green roof . 55
Table 4.2. Examples of whole window U –factors. 60
LIST OF FIGURES
Figure 1.1 Historical Electricity Prices by Year in Taiwan, R.O.C. 2
Figure 1.2 Heat transfer through building envelope. 4
Figure 2.1. Atmospheric CO2 at Mauna Loa Observatory, Hawai. 6 Figure 2.2. Residential electricity consumption in Taiwan, R.O.C. 8 Figure 2.3. Average household appliances electricity consumption. 9 Figure 3.1 Application of decorative stone on the concrete wall. 17
Figure 3.2. Final aesthetic result of the wall. 19
Figure 3.3. Cellular structure of AAC with air voids. 20
Figure 3.4. Construction process. Building with AAC blocks. 24 Figure 3.5. Example of different sizes of EPS wall insulation panels. 29 Figure 3.6. Example of placement of the EPS panels on a brick wall in a corner detail. 31
Figure 3.7. Factors affecting window performance. 33
Figure 3.8. Different glazing types of windows. 36
Figure 3.9. Difference in reflected solar radiation in double-pane insulating
windows with and without low-e coating. 38
Figure 3.10. Roof of the Chicago City Hall. 48
Figure 3.11. Basic layers of a green roof. 45
Figure 3.12. Layers of a green roof include thermal insulation to increase
energy efficiency. 46
Figure 3.13. Building #251 and #253, Medical Center, Houston Texas. 48
Figure 3.14. The Gulf Freeway Office Building, Houston Texas. 50
Figure 3.15. Layering the green roof. 53
Figure 3.16. Layers of green roof used for experiment. 54
Figure 3.17. Completed green roof. 55
Figure 3.18. Completed green roof. 56
Figure 3.19. Completed green roof. 57
Chapter 1 Introduction
Buildings are some of the biggest energy consumers in the world, accounting for one- quarter to one-third of all energy use and a similar amount of greenhouse gas emissions (Hong, W. et al., 2007). In tropical climates the construction of well-insulated buildings is important for reducing the energy use for cooling or heating, especially in small residential buildings. As Taiwan climate is very hot in summer and potentially cold in winter this brings thermal discomfort due to non-insulated constructions. This thermal discomfort is avoided by widely use of conventional air conditioners in summers and conventional heaters in winters. All of these appliances use significant amount of electric energy.
1.1 Energy efficiency of residential buildings in Taiwan, R.O.C.
Conventional A/C for middle-sized room uses about 2kW of electricity in an hour and conventional electric heaters use from 0.7 to 1.8 kW/h of electricity. This is not negligible amount of electric energy that we are spending for creating our thermal comfort in residential buildings as well as in larger commercial buildings.
Because Taiwan has a subtropical climate nearly every building in Taiwan is equipped with an air conditioner or a central air conditioning system. On a daily basis the air conditioning load accounts for around 40 % of the energy consumed in a typical building, while lighting accounts for another 35 %. The cooling load is the main cause for summer peak power demand that is almost 1.4 times that of winter, sometimes causing power shortages (Hong, W. et al., 2007). With high electricity demand the price for electricity is also increased.
Historical increase in electricity prices is shown in Figure 1.1.
Figure 1.1: Historical Electricity Prices by Year in Taiwan, R.O.C. Unit : N.T.$/KWh (Taiwan Power Company, 2012)
The environmental view is as strong as the economic one. The Intergovernmental Panel on Climate Change in its 2007 report noted that: It is often more cost-effective to invest in end-use energy-efficiency improvement than in increasing energy supply to satisfy demand for energy services. Efficiency improvement has a positive effect on energy security, local and regional air pollution abatement and employment (Intergovernmental Panel on Climate Change, 2007).
The building sector is a major energy consumer. The main cause of climate change is greenhouse gas emissions. Source of these is also a great energy consumption by buildings.
Buildings are built to last decades nor for a year or five. Constructing and designing way today have a great impact on operating costs now, but this will also affect the environmental conditions and operating costs for many years in the future. It is not just about turning off the lights or turning down the A/Cs.
2.2000 2.3000 2.4000 2.5000 2.6000 2.7000 2.8000 2.9000 3.0000
World’s environmental problems are not going to be solved only by building green buildings and making old buildings greener. But it is one of the most important areas on which we have to focus to adapt to climate change. As factories became vastly more productive over the past decades, we can expect much more out of energy use five years from now.
1.2 Research objectives
This thesis focuses on how to lower usage of air conditioners and heaters by improving the insulation capabilities of residential buildings. Maintaining acceptable temperatures in buildings (by heating and cooling) uses a large proportion of global energy consumption.
When well insulated, a building is energy-efficient, thus saving the owner money provides more uniform temperatures throughout the space. There is less temperature gradient both vertically (between ankle height and head height) and horizontally from exterior walls, ceilings and windows to the interior walls, thus producing a more comfortable occupant environment when outside temperatures are extremely cold or hot has minimal recurring expense. Unlike heating and cooling equipment, insulation is permanent and does not require maintenance, upkeep, or adjustment lowers the rating of the carbon footprint produced by the house. (“Thermal Insulation.” Wikipedia, 2013)
Insulation reduces unwanted heat loss or gain and can decrease the energy demands of heating and cooling systems. In hot conditions, the greatest source of heat energy is solar radiation. This can enter buildings directly through windows or it can heat the building shell to a higher temperature, increasing the heat transfer through the building envelope. (“Building Insulation.” Wikipedia, 2013)
A building gains heat from actual outdoor temperature and humidity levels. Mostly it gains heat from its exposure to sunlight, from solar radiation. The hot sun heats the walls and the roof, the sunlight passes through the windows and warms the floors it lands on. The sum of all of this heat accumulation is known as the heat gain of the building (“Heat Gain.”
Warmair.com. Web. 2013).
As shown in Figure 1.2 in hot summers the established heat gains of the building through different parts of building envelope are:
Windows and doors: up to 25%.
Roof: up to 25%.
Outside walls: up to 35%.
Ground floors: up to 15 %
Figure 1.2: Heat transfer through building envelope. (Pfiddle, Sustainability in Older Buildings, WordPress.com, 2013).
Focus of this thesis is to improve building envelope and then improve insulating capabilities of building to reduce heat gain and heat loss. As improved building envelope can lower the use of air conditioners by keeping heat out of the building as well as keeping comfortable temperature inside building. The main objectives of study are windows, walls and roofs.
Improvements like this can help people to live in more comfortable apartments and work in more comfortable offices. Around the world are being developed many green building projects. Designers and constructors are using improved technological solutions to achieve greater energy efficiency but the most difficult challenge is changing people's mind to accept this existing solutions.
Chapter 2 Literature Review
Environmental impact is also an important concern. From 2004 to 2030 the world’s CO2 emissions are projected to increase around 50%. More than 57 % of that increase will come from Asia, with China alone accounting for 30 %, putting great pressure on environ- mental sustainability in Asia and the world(Hong, W. et al., 2007). The concentrations of CO2
in the atmosphere are increasing at an accelerating rate from decade to decade. The latest atmospheric CO2 data is consistent with a continuation of this long-standing trend. Figure 2.1 shows Atmospheric CO2 levels in past 50 years.
Figure2.1 Atmospheric CO2 at Mauna Loa Observatory, Hawai (Tans, P., NOAA/ESRL, Web.
For the past ten years, the average annual rate of increase is 2.07 parts per million (ppm). This rate of increase is more than double the increase in the 1960s. Atmospheric CO2 acceleration data for past 50 years are shown in Table 2.1.
Table 2.1. Acceleration of Atmospheric CO2 from decade to decade. (Scripps Institution of Oceanography's CO2 Program. 2013)
Decade Total Increase Annual Rate of Increase
1963 – 1972 9.00 ppm 0.90 ppm per year
1973 – 1982 13.68 ppm 1.37 ppm per year
1983 – 1992 15.24 ppm 1.52 ppm per year
1993 – 2002 16.73 ppm 1.67 ppm per year
2003 – 2012 20.74 ppm 2.07 ppm per year
2.2 Energy consumption and energy policy
The world’s total energy consumption from 1971 to 2004 increased 87 %, with an annual average growth rate of 1.9 %. For around 43 % of the total energy consumption increase is responsible Asia, where final energy consumption increased 275 %, with an average annual rate of 4.1 percent, more than twice as fast as the global average. The world’s final energy consumption is expected to increase 1.5 times from 2004 to 2030(Hong, W. et al, 2007). The R.O.C.'s residential electricity consumption for past 20 years is shown in Figure 2.2.
Figure 2.2. Residential electricity consumption in Taiwan, R.O.C. ( Taiwan Power Company, 2012)
The R.O.C.'s total energy consumption has grown greatly over the past two decades, going from 53.25 million kiloliters of oil equivalent in 1991 to 111.92 million kiloliters in 2011, which is an average annual growth of 3.78% (“Energy Statistics Hand Book 2011”, Bureau of Energy, Ministry of Economic Affairs, R.O.C., 2011).
About 30 % of the world’s total energy is consumed by buildings. The main causes of climate change are greenhouse gas emissions. Buildings are responsible for about 30 % of the world’s greenhouse gas emissions as well.
The McKinsey Global Institute (2007) has studied the issue on a worldwide basis and estimates that from the five most cost-effective measures taken to reduce greenhouse-gas emissions four involve building efficiency. The measures are building insulation, lighting systems, air conditioning and water heating. The only non-building issue that is on to the top five list is improved efficiency for commercial vehicles. Better efficiency means that consumers are able to use less energy to create the same level of comfort. The most cost- effective way to reduce energy use and greenhouse gas emissions is by improvements in buildings. The McKinsey stated that, globally, energy demand will accelerate from 1.6% a
0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 50,000
year in the past decade to 2.2% a year in the next 15 years unless energy efficiency measures will be adapted.
Over the past 20 years, Taiwan has substantially revised its national energy policy to find a balance between securing energy supply for its economic growth and strengthening its environmental policies. Energy efficiency and conservation is a major component of Taiwan’s energy policy, with the building sector one of the six main target sectors. This sector alone is expected to contribute 8.7 % of the total 28 % energy-saving goal for 2020(Hong, W. et al., 2007).
The main household energy uses in Taiwan are for cooling. Households are increasing their energy consumption through greater use of electrical goods, more home floor space per capita, and higher levels of cooling and heating comfort.
Figure 2.3. Average household appliances electricity consumption. Unit: Watts (W)
200 180 400
42" LCD TV refrigerator 468L desktop computer rice cooker water boiler
middle sized air conditioner conventional heater light bulb
As shown in Figure 2.3, air conditioners and space heaters are the most significant energy consumers among all the household appliances. The air conditioning load accounts for around 50% of the energy consumed by a typical building. Because of the subtropical climate, the use of air conditioners for creating thermal comfort is necessary. Improvement can be in the way of how these appliances are used.
2.3 Thermal Transmittance
Thermal transmittance, also known as U-value, is the rate of transfer of heat (in watts) through one square metre of a structure divided by the difference in temperature across the structure. It is expressed in watts per square metre per kelvin, or W/m²/K. Well-insulated parts of a building have a low thermal transmittance whereas poorly insulated parts of a building have a high thermal transmittance.
Φ = A × U × (T1 - T2) (2.1)
where Φ is the heat transfer in watts, U is the thermal transmittance, T1 is the temperature on one side of the structure, T2 is the temperature on the other side of the structure and A is the area in square metres.
most walls and roofs can be calculated using ISO 6946
if there is metal bridging the insulation in which case it can be calculated using ISO 10211
for most ground floors it can be calculated using ISO 13370
for most windows the thermal transmittance can be calculated using ISO 10077 or ISO 15099.
ISO 9869 describes how to measure the thermal transmittance of a structure experimentally.
Table 2.2 shows examples of thermal transmittance values for common building structures. In practice, the thermal transmittance is strongly affected by the quality of workmanship. If insulation is fitted poorly, the thermal transmittance can be considerably higher than if insulation is fitted well (“Thermal Transmittance.” Wikipedia, 2013).
Table2.2. Typical thermal transmittance values for common building structures. (“Thermal Transmittance.” Wikipedia: The Free Encyclopedia. Web. 2013).
Types of Building Structures Thermal Transmittance (W/m²K)
single glazed windows 4.5
double glazed windows 3.3
double glazed windows with advanced coatings 2.2
triple glazed windows 1.8
well-insulated roofs 0.15
poorly insulated roofs 1.0
well-insulated walls 0.25
poorly insulated walls 1.5
Chapter 3 Research on Energy Efficiency of different parts of Building Envelope
In Chapter 3, the study explains research methods and important definitions that are need to understand the problematic of energy efficiency in different types of materials and construction details used in building envelope. Based on these definitions, statistical data, case studies and on-site experiments this thesis will prove that using different materials, construction details and improvement in building envelope’s efficiency leads to higher thermal comfort, savings in electricity consumption as well as environmental benefits.
3.1 Study on walls and insulation capabilities of different materials and methods of construction
One- third of heat gain or loss in an un-insulated building goes through the walls.
Walls in most homes represent more exterior surface area than the floors or ceilings, they also present more opportunity to lose and gain heat. The rapid depletion of the world's energy resources, high profile environmental issues such as climate change, are beginning to take precedence and legislation will demand that the thermal performance of any building is strictly governed.
Majority of residential buildings in Taiwan are build of concrete or bricks. These building materials are very good heat conductors and then have low thermal resistance. As shown in Table 3.1, the major building materials used in Taiwan for residential buildings have low thermal resistance and then low insulating capability.
Table 3.1. R-values of commonly used building materials. (MIL Handbook 1003/19, Military Handbook -- Design Procedures for Passive Solar Building, 1987).
Low insulating capability of building materials results to high heat gains or lose and then very low energy efficiency. This disadvantage can be improved by using different building materials with high thermal resistance or by insulating existing walls and then improving the thermal capabilities. External wall insulation is an effective way to stop wasting energy needed for cooling or heating the building.
3.1.1 Thermal Resistance: R-value
Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow (heat per time unit or thermal resistance).
“R” means resistance to heat flow. The higher the R-value, the greater the insulating value. R-value is the standard way of describing how effective insulation is to heat gain. The higher the R-value, the better the insulation will resist heat gain. Engineers determine the R- value for a insulation by adding up all the total assembly. Each material has a specific resistance to heat gain and when calculated together we get the total R-value for the isolative quality of the exterior wall. Then the inverse of the total is taken and that gives us the U-value.
Materials and Description Thickness R-value (h·ft²·°F/Btu)
Concrete block 10 cm 0.71
Concrete block 20 cm 1.11
Concrete block 30 cm 1.28
Brick 10 cm 0.8
Brick 20 cm 1.6
R-Value is the measurement of thermal resistance. The bigger the number, the better the insulation effectiveness. R-value is nothing more than the reciprocal of U-value (“R-Value &
U-Value”. Universal Design Consortium LLC. Web. 2013)
Increasing the thickness of an insulating layer increases the thermal resistance. For example, doubling the thickness of fiberglass batting will double its R-value, perhaps from 2.0 m²K/W for 110 mm of thickness, up to 4.0 m²K/W for 220 mm of thickness. (“R-value (insulation).” Wikipedia. Web. 2013). R-values are additive. For instance if you have a material with an R-value of 12 and another material with an R-value of 3, then both materials combined have an R-value of 15.
3.1.2 Thermal mass as an important factor for energy efficiency of walls
Very important factor in building envelope is the thermal mass. Thermal mass is the ability of a material to absorb heat energy. A lot of heat energy is required to change the temperature of high density materials like concrete, bricks and tiles. They are therefore said to have high thermal mass. Lightweight materials such as timber have low thermal mass.
Appropriate use of thermal mass throughout your home can make a big difference to comfort and heating and cooling bills. Correct use of thermal mass moderates internal temperatures by averaging day/night extremes. This increases comfort and reduces energy costs.
Thermal mass acts as a thermal battery. During summer it absorbs heat, keeping the house comfortable. In winter the same thermal mass can store the heat from the sun or heaters to release it at night, helping the home stay warm. Thermal mass is not a substitute for insulation. Thermal mass stores and re-radiates heat. Insulation stops heat flowing into or out of the building. A high thermal mass material is not generally a good thermal insulator (Institute for Sustainable Futures, 2010).
Several massive building envelope technologies (masonry and concrete systems) are gaining acceptance by builders today. It is believed that building envelopes made of concrete,
earth, insulating concrete forms (ICFs), and solid wood (log) may be helpful in lowering building heating and cooling loads. For centuries, the vast majority of European and Middle Eastern residential buildings have been built using massive wall technologies. They have made life without air conditioners relatively comfortable even in countries with hot climates such as Spain, Italy, or Greece. Heating and cooling energy demands in buildings containing massive walls with high R-values could be lower than those in similar buildings constructed using lightweight wall technologies.
A comparative study of the energy performance of massive wall technologies have demonstrated that heating and cooling energy demands in buildings containing massive walls of high R-value could be lower than those in similar buildings constructed using lightweight wall technologies.
Comparative analysis of the space heating and cooling energies from two identical residences, one with massive walls and the other containing lightweight wood-frame exterior walls was adopted and used the DOE-2.1E computer code to simulate three single-family residences in ten representative U.S. climates. Over ten thousand whole building energy simulations were performed during this study. The heating and cooling energies generated from these building simulations served to estimate the R-value equivalents for massive walls.
For each analyzed material configuration, four different sets of thicknesses were considered and organized according to their R-value;
- R - 3.03 m2K/W (17.2 hft2F/Btu),in total: 10.6 cm (4-in) of foam, 15.2-cm. (6-in) of concrete.
- R - 2.29 m2K/W (13.0 hft2F/Btu), in total: 7.6 cm (3-in) of foam, 10.2-cm. (4-in) of concrete.
- R - 1.58 m2K/W (9.0 hft2F/Btu), in total: 5.2 cm (2-in) of foam, 10.2-cm. (4-in) of concrete.
- R - 0.88 m2K/W (5.0 hft2F/Btu), in total: 2.5 cm (1-in) of foam, 10.2-cm. (4-in) of concrete.
These energy savings were defined as a difference between energies required to heat and cool the house containing massive walls vs. the same house constructed with wood frame technology. Energy savings for massive walls house were estimated between 3 and 7 MBtu/year for R-1.8 to 4.4 m2K/W walls.
It is possible for buildings with high R-value walls to save up to 8% of annual energy consumption. During the design process, an architect may save 5 to 18% of future whole building energy use simply by replacing traditional light-weight walls with massive systems or ICF walls. The thermal mass benefit is a function of wall material configuration, climate, building size, configuration, and orientation.
The most effective wall assembly was the wall with thermal mass (concrete) applied in good contact with the interior of the building. Walls where the insulation material was concentrated on the interior side, performed much worse. Whole building possible energy savings in houses built with ICF walls were estimated and can be between 6 and 8%.
Field experiments have demonstrated that heating and cooling energy demands in buildings containing massive walls of relatively high R-values can be lower than those in similar buildings constructed using equivalent R-value with lightweight wall technologies (Kostny, J. et al., Thermal Mass - Energy Savings Potential in Residential Buildings, 2001).
3.1.3 Thermal mass “Home Experiment”.
In summer of 2011 thermal mass of a living room wall in a house located in Taoyuan County was increased by a decorative stone. The wall is 20 cm thick made of concrete. On only half of the wall was used for thermal mass increase by adding 3 cm thick decorative stone (Figure 3.1).
Figure 3.1 Application of decorative stone on the concrete wall. (Krivak J., 2011) This experiment was tested by measuring temperatures on both sides of the wall. For measurement of the temperature was used professional infrared thermometer Optex Thermo- Hunter PT-3S. The data were collected on August 28th, 2013. Room was tempered using A/C conditioner on 25°C.
Weather conditions: Cloudy with outside temperature of 35.3 °C. Data collected are shown in Table 3.2.
Table 3.2. Data collected during the thermal mass increase experiment. (Krivak J., 2013)
20 cm concrete wall
20 cm concrete wall + 3 cm decorative stone
Outside temperature on the wall surface
Inside temperature on the wall surface
Inside room temperature
The results have proved that the part of the wall with increased thermal mass by just 3cm thick decorative stone is cooler by 1.1°C and then radiates less heat into the room. Added decorative stone helped to increase thermal properties of the outside wall. The final aesthetic result of added decorative stone to the concrete wall is shown in Figure 3.2.
Figure 3.2. Final aesthetic result of the wall. (Krivak J., 2013)
3.1.4 Autoclaved Aerated Concrete (AAC)
The suggested solution for newly planned constructions is to use AAC as wall filling.
AAC is a versatile lightweight construction material and usually used as blocks. Compared with normal “dense” concrete, aircrete has a low density and excellent insulation properties.
The low density is achieved by the formation of air voids to produce a cellular structure (see Figure 3.3). These voids are typically 1mm - 5mm across and give the material its characteristic appearance. Blocks typically have strengths ranging from 3-9 Nmm-2 (when tested in accordance with BS EN 771-1:2000). Densities range from about 300 to 750 kg m-3; for comparison, medium density concrete blocks have a typical density range of 1350-1500 kg m-3 and dense concrete blocks a range of 2300-2500 kg m-3 (“Autoclaved aerated concrete (AAC, Aircrete)” Understanding Cement, Web. 2013).
Figure 3.3. Cellular structure of AAC with air voids. (Web. 2013)
AAC is not a “new” innovation. Autoclaved Aerated Concrete has been around for over 80 years. Invented in 1923, AAC has been used extensively in Europe and Asia. It comprises over 40% of all construction in the United Kingdom and 60% in Germany. More AAC is produced worldwide than any other building material with the exception of regular concrete (“Autoclaved Aerated Concrete also known as AAC.” Aac- autoclavedaeratedconcrete.com, Web 2013).
The main advantages and preferences of AAC are:
When used to build external walls, AAC along meet all strict requirements made by various countries without use of any other auxiliary thermal protecting materials. It can be illustrated that AAC with thickness of 4-5cm performs the same function of thermal protection as one layer of common bricks. Therefore AAC is not only the structure material but also the thermal materials. The R-value for 20 cm thick AAC block is 8.12 (h·ft²·°F/Btu) compared to 20 cm thick concrete block 1.11 (h·ft²·°F/Btu) (Institute for Sustainable Futures, “Your Home – Technical manual.” 2010).
The dry specific gravity of AAC is between 300kg/m3 to 650kg/m3. This is about 1/3 of common bricks and 1/4 of common concrete. Therefore it reduces the total weight of the construction and is very easy to process on-site. AAC is easily cut to any required shape.
AAC's light weight saves labor.
The test result shows that AAC has excellent acoustic performance. Walls could reduce noise range from 30-52dB depending on thickness of walls and different surface disposed (“Advantages of Ytong Block”. Xella International Gmbh., Web. 2013).
The raw material adopted and AAC itself are absolutely non-combustible. Experiment results show that 10-cm thick walls made of AAC could stand against fire for at least 4 hours.
Hence AAC is the perfect building material for fireproof walls (“Autoclaved Aerated Concrete also known as AAC.” Aac-autoclavedaeratedconcrete.com, Web. 2013).
AAC can be sawn, drilled, nailed and machined using normal wood working tools.
Also, simple construction details allow the designer, detailer and contractor to quickly and confidently complete project without any anxiety over difficulty or voluminous details.
Standardized sizes makes AAC excellent tool for architects to adapt exact dimensions and shapes to the prefabricate blocks.
The manufacturing of AAC materials is a pollution free process that makes the best use of a minimum amount of energy and natural resources, resulting in a premier green building material. All the materials are completely natural. The block consists of sand, lime and cement - natural abundantly available raw materials that are obtained from responsibly managed extraction sites.
High energy efficiency is one of the determining characteristics of autoclaved aerated concrete. AAC's cellular structure gives it a thermal efficiency 10 times higher than that of aggregate concrete and two to three times better than clay brick. Consequently, buildings made of AAC are warm in winter and cool in summer. With buildings accounting for 40% of the EU's energy requirements, greater use of AAC, both in construction and renovation, offers an immediate solution to cutting the energy consumption of residential and non-residential buildings.
AAC's excellent inherent thermal insulation properties reduce the need for space heating and cooling, also cutting carbon dioxide emissions and combating climate change and also make the use of additional insulation materials unnecessary. Insulation requirements can be met by using AAC alone. By contrast, aggregate concrete, clay brick and calcium silicate
masonry units need to be used in combination with insulation products, thereby adding to their cost and environmental impact.
AAC is energy-efficient over its whole life cycle. Its production requires less energy than other construction materials and its light weight saves energy in transportation (European Autoclaved Aerated Concrete Association, Energy Efficiency, 2011).
The use of autoclaved aerated concrete has a range of environmental benefits:
Insulation: most important, the insulation properties of AAC will reduce the cooling and heating costs of buildings constructed with autoclaved aerated concrete, with constant savings over the lifetime of the building.
Materials: lime is one of the principal mix components and requires less energy to produce than Portland cement, which is fired at higher temperatures. Sand requires only milling before use, not heating. Lime may require less energy to manufacture compared with Portland cement but more CO2 is produced per ton (cement approx. 800-900 kg CO2/ton
compared to lime at 1000 kg CO2 per ton).
Carbonation: less obviously, the cellular structure of aircrete gives it a very high surface area. Over time, much of the material is likely to carbonate, largely offsetting the carbon dioxide produced in the manufacture of the lime and cement due to the calcining of limestone (“Autoclaved aerated concrete (AAC, Aircrete)” Understanding Cement, Web.
3.1.5 “Baoshan” Case Study
In 2009 I was part of a construction project of an 8 apartment villa in Baoshan area, Hsinchu. The plan was to use traditional clay bricks as a walls filling. After my suggestion and cost calculations the owner accepted that we will use AAC blocks as the wall filling. Because there wasn’t any company that makes AAC in Taiwan I had to import all the AAC blocks from overseas. The cost of the material along with transportation and all necessary fees was at the time the exact price as the traditional clay bricks. Therefore the designer and the owner decided to use the AAC blocks as the building material for all walls of the villa. Because of relatively new construction material in Taiwan I had to personally train the contractor and help the contractor with building process. Figure 3.4 shows the construction process on site.
Figure 3.4. Construction process. Building with AAC blocks. (Krivak J., 2009)
One of the main attributes of the AAC is the construction speed. Because of the size and lightweight the construction process is more than three times faster than clay bricks. This was not well received with the contractor due to their contract with owner that the contractor is paid by the time of work and not the amount of work that contractor had done. For example, if traditional clay bricks are used the contractor would need about three weeks to complete all the walls but with AAC blocks the work could be done only in one week. That means that contractor will lose earnings for 2 weeks.
This situation can be very easily solved with proper contracts between owner and the contractor. It is not standard that contractor is paid by the time spend for work but for the work that been done by quantity. If paid by time needed for work there is a speculation that the contractor would intensively slow their work speed due to higher earnings.
The benefit was the construction speed and much higher quality. The most difficult challenge was changing people's mind to accept this solution.
Price calculation: The cost of AAC was calculated as all expenses needed to buy and transport AAC on working site.
For this project was bought total of 87 m3 of AAC blocks of size (600*250*125mm) for inner walls and (600*250*250mm) for exterior walls.
Price for AAC blocks at Shanghai Ytong factory was at the time 64 US$ per 1m3 .
After all expenses, included fright, customs clearance, taxes and on site transport and import provision, the price for AAC blocks was established as 4140NT$ for 1m3 include adhesive needed for building with AAC.
Very important factor in comparison between AAC and bricks is price for material, labor and time effectiveness. Table 3.3 shows the calculations for both AAC and bricks.
Because of the difference in thickness of the walls (25cm AAC, 20cm brick) the price will be calculated per m2 of the construction.
Table 3.3. Cost effectiveness for AAC and brick walls. (Krivak J., 2013)
Description Units AAC blocks
Clay brick (200*100*50mm) Cost of the material m2
Time factor 1 person 14m2
Labor price 1 day 3600,- NT$
Labor price per m2
1 person 260,-NT$ 1030,-NT$
From the Table 3.3 it is clear that using AAC blocks as a wall building material is much more cost effective than using traditional clay bricks. As shown the cost of AAC material is slightly lower. The building effectiveness is 4 times higher than in clay bricks because of the size and lightness of the AAC blocks. As a result the labor price per m2 for walling is 4 times lower than bricks.
The measurements of the temperatures on surface were done by professional infrared thermometer Optex Thermo-Hunter PT-3S. Data were collected on August 7th,2013. Data were measured on the ouside and inside surface as well as inside and outside temperature.
Results are shown in Table 3.4. Weather conditions: Sunny with outside temperature of 36, 2
Table 3.4. AAC wall temperature measurement data. (Krivak J., 2013)
Wall built with AAC blocks 25 cm thick.
Outside temperature 36.2°C -
Outside temperature on surface
Inside temperature on surface
Inside room temperature 27.1°C -9.1°C
The results showed that using AAC as a building material helped to increase thermal properties of the building and then lowered the initial cost of the construction as well as maintenance cost.
3.1.6 External wall insulation and it’s improvement in thermal properties of walls
For already existing as well as for newly panned wall construction insulation is the best way to improve their thermal properties. Only external walls need to be insulated. For internal walls there is no need. Walls should be insulated from exterior side of the building envelope and insulation has to be done in proper way with care of thermal bridging. A thermal bridge occurs when there is a gap between materials and structural surfaces. The main thermal bridges in a building are found at the junctions of facings and floors, facings and cross walls;
facings and roofs, facings and low floors. They also occur each time there is an opening (doors, windows, loggias…). These are structural thermal bridges. These thermal bridges vary in importance according to the type of wall or roof (insulated or not). In a building that is not properly insulated, thermal bridges represent low comparative losses (usually below 20%) as total losses via the walls and roof are very high (about >1W/m2K)(“What is a Thermal Bridge?”. Isover.com, Web. 2013)
There are two most common insulating materials: mineral wool and expanded polystyrene (EPS). External insulation involves fixing these insulating materials to the outer surface of the wall. The insulation is then covered with a special render to provide weather resistance. A steel or fiberglass mesh is embedded in this render to provide strength and impact resistance.
The reason EPS is such a good insulating material is the millions of air pockets which are formed when the blocks are molded. These pockets impede the flow of heat, making EPS an excellent insulator keeping things cool in the summer and warm in the winter. High quality EPS wall insulation is available in a variety of sizes (see Figure 3.5).
Figure 3.5. Example of different sizes of EPS wall insulation panels. (Web. 2013) EPS or mineral wool thermal insulation systems are a highly suitable and cost-effective type of thermal insulation for new construction and renovation projects. This type of insulation protects the existing exterior wall against thermal loads and it also provides protection against the weather. Insulation materials made of the EPS and mineral wool are resistant to aging and decay, and also extremely firm and dimensionally stable.
The R-values for EPS various by thickness of the panels and are shown in Table 3.5.
Table 3.5. R-values for different thickness of EPS panels. (STN 73 0540 , Slovak Technical Standards, 2002)
Thickness of the EPS panel (mm) R-value (m2k/W)
The R-values for mineral wool panels are shown in Table 3.6.
Table 3.6. R-values for different thickness of mineral wool panels. (STN 73 0540 , Slovak Technical Standards, 2002)
Thickness of the mineral wool panel (mm) R-value (m2k/W)
In compare with mineral wool, the EPS panels perform slightly better as an insulation material.
31 Application and processing:
The insulating panels made of EPS are fastened to the masonry or concrete by means of adhesives, dowels, or rail mounts. Figure 3.6 shows an example of EPS panels fastened to the masonry by dowels in a corner detail. The panels should be installed from the bottom to the top so as to be tightly abutting and arranged in a masonry bond (avoid intersecting joints). In order to avoid cracks and to mechanically protect the facade, a reinforcement of the surface is needed on outside areas. It consists of a reinforcement compound and a corresponding reinforcement fabric. After sufficient drying, the outer coating is applied in the form of a suitable and system-compatible finishing plaster or final coat.
Figure 3.6. Example of placement of the EPS panels on a brick wall in a corner detail.
3.2 Insulated glazing and glass coatings in windows
Windows provide our homes with light, warmth, and ventilation, but they can also negatively impact a home's energy efficiency. The earth receives an enormous amount of solar energy from the sun. This energy is referred to as Solar Heat Gain. Solar Heat Gain is the measurement of the increase in temperature in a space, object or structure that results from this solar radiation. In the summer months when air-conditioning loads are greatest, we do not want that powerful energy entering the home to become heat. In a hot climate we need windows that effectively reduce solar heat gain. We can reduce energy costs by installing energy-efficient windows in our home or improve energy efficiency to existing windows.
3.2.1 Factors that affect window’s thermal performance
A window's energy efficiency is dependent upon all of its components. Window frames conduct heat, contributing to a window's overall energy efficiency, particularly its U- factor. Glazing or glass technologies have become very sophisticated, and designers often specify different types of glazing or glass for different windows, based on orientation, climate, building
Window systems are comprised of glass panes, structural frames, spacers, and sealants.
In recent years, the variety of glass types, coatings, and frames available for use in window systems has increased dramatically. Figure 3.7 shows the factors that affect the window’s performance.
Figure 3.7. Factors affecting window performance. (Ander, G., Windows and Glazing, 2010) Specification of window and glazing systems is essential to the energy efficiency and comfort of all buildings. In residential structures optimum window design and glazing specification can reduce energy consumption from 10% - 50% below accepted practice in most climates. In internal-load dominated commercial, industrial, and institutional buildings, properly specified fenestration systems have the potential to reduce lighting and HVAC costs 10%-40% (Ander, G., Windows and Glazing, 2010).
3.2.2 Heat gain and loss in windows
Windows, doors, skylights can gain and lose heat through:
Direct conduction through the glass or glazing, frame, and/or door.
The radiation of heat into a house typically from the sun.
Air leakage through and around them.
These properties can be measured and rated according to the following energy performance characteristics:
U-factor is the rate at which a window, door, or skylight conducts non-solar heat flow.
It's usually expressed in units of Btu/hr-ft2-oF. For windows, skylights, and glass doors, a U- factor may refer to just the glass or glazing alone. NFRC U-factor ratings, however, represent the entire window performance, including frame and spacer material. The lower the U-factor, the more energy-efficient the window, door, or skylight.
Solar heat gain coefficient (SHGC) is the fraction of solar radiation admitted through a window, door, or skylight -- transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gain from the sun. Building’s climate, orientation, and external shading will determine the optimal SHGC for a particular window, door, or skylight. For more information about SHGC and windows, see passive solar window design.
Air leakage is the rate of air movement around a window, door, or skylight in the presence of a specific pressure difference across it. It's expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating is tighter than one with a high air leakage rating.
3.2.3 Sunlight transmittance
The ability of glazing in a window, door, or skylight to transmit sunlight into a home can be measured and rated according to the following energy performance characteristics:
Visible transmittance (VT) is a fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through the glazing of a window, door, or skylight. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window, door, or skylight should be determined by your home's daylighting requirements and/or whether you need to reduce interior glare in a space.
Light-to-solar gain (LSG) is the ratio between the SHGC and VT. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat. This energy performance rating isn't always provided (“Energy Performance Ratings for Windows, Doors, and Skylights”. U.S Department of Energy. Web.
3.2.4 Insulated glazing
Insulated glazing, more commonly known as double glazing or double-pane, and increasingly triple glazing/pane are double or triple glass window panes separated by an air or other gas filled space to reduce heat transfer across a part of the building envelope. Figure 3.8 shows the different glazing types of windows. Most windows are manufactured with the same thickness of glass used on both panes but special applications such as acoustic attenuation or security may require wide ranges of thicknesses to be incorporated in the same unit. The glass panes are separated by a "spacer". A spacer is the piece that separates the two panes of glass in an insulating glass system, and seals the gas space between them. To reduce heat transfer through the spacer and increase overall thermal performance, manufacturers may make the spacer out of a less-conductive material such as structural foam.
Figure 3.8. Different glazing types of windows. ( Web. 2013)
The maximum insulating efficiency of standard insulated glazing windows (IGU) is determined by the thickness of the space containing the gas between panes. Too little space between the panes of glass results in heat loss by diffusion between the panes, while if too large a gap is used, convection currents are not damped out by the gas viscosity and transfer heat between the panes. Typically, most sealed units achieve maximum insulating values using a gas space of 16–19 mm. When combined with the thickness of the glass panes being used, this can result in an overall thickness of the IGU of 22–25 mm for 3 mm glass to 28–31 mm for 6.35 mm plate glass.
3.2.5 Gas fills and lower heat transfer between glass panes
An established way to improve insulation performance is to replace air in the space between glass panes with a lower thermal conductivity gas. Gas convective heat transfer is a function of viscosity and specific heat. Gases such as argon, krypton and xenon are often used since they do not carry heat in rotational modes, resulting in a lower heat capacity than poly- atomic gases. Argon has a thermal conductivity 67% that of air, krypton has about half the conductivity of argon. Krypton and Xenon are very expensive. These gases are used because they are non-toxic, clear, odorless, chemically inert, and commercially available because of their widespread application in industry. Some manufacturers also offer sulfur hexafluoride as an insulating gas, especially to insulate sound. It has only 2/3 the conductivity of argon, but it is stable, inexpensive and dense. However, sulfur hexafluoride is an extremely potent greenhouse gas that contributes to global warming.
In general, the more effective a fill gas is at its optimum thickness, the thinner the optimum thickness is. For example, the optimum thickness for krypton is lower than for argon, and lower for argon than for air. Since it is difficult to determine whether the gas in an IGU has become mixed with air at time of manufacture (or becomes mixed with air once installed), many designers prefer to use thicker gaps than would be optimum for the fill gas if it were pure (“Insulated Glazing.” Wikipedia: The Free Encyclopedia. Web. 2013).
3.2.6 Tints (color) and coatings in windows
The properties of a given glass can be altered by tinting or by applying various coatings or films to the glass. Glass tints are generally the result of colorants added to the glass during production. Some tints are also produced by adhering colored films to the glass following production.
Tints are usually selected for aesthetic purposes. Some tints also help reduce solar gains. Coatings, usually in the form of metal oxides, can also be applied to glass during production. Some of these coatings, called "low-emissivity" or "low-e," help reduce radiant
heat transfer between panes of glass by blocking some or all of the IR wavelengths. These coatings can dramatically lower the window U-factor.
Care should be taken in specifying tints and coatings, as their application can dramatically impact window heat loss and heat gain. Mis-specification can result in the exact opposite of the desired performance.
In the 90s a new kind of hot climate windows coating designed for hot southern climates was introduced. The generic name for the new technology is low solar gain low-e coating, but it can also be called hot climate low-e coatings. Low-emissivity (Low-E) windows feature glass that has a thin coating on the glass within its airspace that reflects thermal radiation or inhibits its emission reducing heat transfer through the glass. The difference in reflected thermal radiation in double-pane insulating windows when one has low- e coating is shown in Figure 3.9.
Figure 3.9. Difference in reflected solar radiation in double-pane insulating windows with and without low-e coating. (“Low-e”, Allweather Windows & Doors, 2013)
This coating or film allows light in, but prevents some solar rays from being transmitted through the glass. These new coatings work by extending the spectral selectivity characteristic of a conventional high solar gain low-e coating, bringing it from outside the
solar spectrum to the edge of the visible portion of that spectrum. The result is a new kind of spectral selectivity through which the infrared portion of the solar spectrum accounting for more than half the energy from the sun is reflected by the new coating back outside the building. This invisible reflected solar IR radiation returns to the outside and is thereby prevented from entering the cool interior of the building. The result is a major reduction of the solar heat gain through the window without significant loss of visible light transmitted through the glass. The new coating reflects almost half of the incident solar radiation back outside while still allowing copious quantities of daylight to enter. Thus the window does not appear tinted. It's as bright as a pane of clear uncoated glass.
This is possible because less than half the energy in solar radiation incident on a window is visible light. The rest is invisible infrared radiation and a smaller quantity of (also invisible) ultraviolet radiation. The special coating can also be called a strongly spectrally selective one, meaning that it selects out only the visible radiation to be allowed inside. In consequence the solar heat gain through an un-insulated window can be dropped to half what it would be for single pane clear glass, without significant reduction of the admitted daylight (“How Windows Work – About Solar Gain”. Florida Solar Energy Center. Web. 2007)
3.2.7 U-value for windows and it’s calculation
In windows there are always three different U-values:
Uw (w = window) – overall value of the window Ug (g = glazing) - U-value of the glazing
Uf (f = frame) – U-value of the frame
The heat transfer coefficient Uw relates to the entire window. This value also incorporates the U-values for the glazing and the frame Uf. The overall value Uw is also influenced by the linear heat transfer coefficient (g = glazing) and the size of the window.
U-value of window glazing: Ug
The Ug-value is a function of the type of gas filling of the intermediate space between the glass sheets, the distance between the sheets and the number of sheets.
Typical U-values for thermally insulated windows are:
Double insulated glazing 24 mm with argon filling: 1.1 W/m2K Triple insulated glazing 36 mm with argon filling: 0.7 W/m2K Triple insulated glazing 44 mm with argon filling: 0.6 W/m2K Triple insulated glazing 36 mm with krypton filling: 0.5 W/m2K
U-value of window frame: Uf
The Uf-value for the frame-sash combination is defined by means of measurement or calculation. The area for the calculation of the Uw-value is the cross-section of the profile.
41 Linear heat transfer coefficient Ψg
The value Ψg for the edge seal of the glazing is first and foremost a function of the used material for the insulated glazing spacer. The standard material with the worst thermal properties is Aluminium. Spacers with improved thermal insulation are referred to as “warm- edge” spacers. These spacers are made of stainless steel or plastic. A larger edge cover of the insulated glazing in the sash profile further enhances the Y-value of the edge seal.
Examples of Ψ-values:
Aluminium spacer: approx. 0.08 W/mK
„Warm edge" spacer: approx. 0,04 W/mK
U-value of window: Uw
The heat transfer coefficient for windows and window doors Uw is usually calculated in the standard window size 1.23 m x 1.48 m. The U-value worsens as the size decreases, larger windows feature better values. This is because U-values achieved in glazing are better than in the frame material and therefore a larger glass area is able to produce a better thermal insulation value.
Uw-value calculation: The following formula is used to determine the heat transfer coefficient:
Ug = heat transfer coefficient of the glazing Uf = heat transfer coefficient of the frame
Ψg = linear heat transfer coefficient of the insulated glazing edge seal Ag = glass area
42 Af = frame area
Aw = Ag + Af
lg = length of inside edge of frame profile (or visible periphery of the glass sheet) ( “U-value for windows”. Inoutic. Web. 2013).
As shown in Table 3.7, the best performance have the windows that are double or triple glazed with low-e coating and gas filled space between panes. The difference also wary in used frames.
Table 3.7. Examples of whole window U –factors. (ASHARE Handbook: Fundamentals, American Society of Heating, Refrigerating and Air-conditioning Engineers, 1993)
U-factor ( Btu/hr-ft2 F) Aluminum frame
without thermal break
Aluminum frame with thermal break
Wood or Vinyl frame with insulated spacer Single glass, air
1.30 1.07 ---
Double glass, air space
0.81 0.62 0.48
Double glass, low-e, air space
0.70 0.52 0.39
Double glass, low-e, space with argon
0.64 0.46 0.34
Triple glass, low-e, space with argon
0.53 0.36 0.23