Simon G
Simon G

The window insulation always plays an important role in building design and thermal comfort, it is one of the main parts needed to be optimized in building envelop.
The window insulation always plays an important role in building design and thermal comfort, it is one of the main parts needed to be optimized in building envelop.
Extended Research Task
Temperature Control in Homes ---- Windows
Extended Research Task
Temperature Control in Homes ---- Windows

Temperature Control in Homes --- Windows 1. Abstract:

The window insulation always plays an important role in building design and thermal comfort, it is one of the main parts needed to be optimized in building envelop. Window insulation is a basic element which can decide the insulation capacity. The large heat loss from the window is the main part of wasting energy, and simultaneously, there is also difficult to explore the new energy source and to improve the current heat generation device efficiency. Therefore, building a proper insulated window system is a good approach to keep an acceptable indoor climate as well as to reduce energy use and negative climate effects. The aim of this extended research task is to introduce readers to different ways of windows insulation that can efficiently reduce heat loss in an average household during Tasmanian winter.

2. Introduction

Tasmania has cold temperature climate with four distinct seasons, with the most distinctive season during the winter month between June and July. During the winter season Tasmania become the wettest and coolest state in Australia with most high lying areas receiving considerable snowfall. Winter maximums are 12 °C (54 °F) on average along coastal areas and 3 °C (37 °F) on the Central Plateau, thanks to a series of cold fronts from the Southern Ocean.1 As mentioned above Tasmania have a quite cold environment to live in during winter time, therefore to maintain a comfortable indoor climate, one not only need to have heating elements, but also have means of reducing heat lost from the indoor climate to the outdoor climate.

But Indoor climate involves lots of elements, these elements will influence the inside occupants feeling about the temperature, humidity and air quality .etc. Generally speaking, there are too much details related to the sensitivity of occupants which can be described as a standard concept, the thermal comfort. Thermal comfort defined as “the condition of human mind in which satisfaction is expressed with the thermal environment”2. Because it is a scale that based on the occupant’s subjective feeling, this task will base the ‘thermal comfort zone’ as 20°C and the outside temperature as 0°C to explore different windows insulation that can optimize minimum heat loss in the following essay.

3. Theory on Heat Loss Field

To realize the heat loss from numerical, the main equation from heat transfer field can explain the detail about how a certain heating energy is losing:

The origin equation Q=KD (Ti-Te), U=K/D
Q: heat transfer rate, W
K: thermal conductivity, W/mK
D: the thickness of material (glass)
U: the U-value of window, W/m2K
Ti: The temperature inside the room
Te: The temperature outside the room (usually the indoor temperature is about 20°C.)
As states above, U-value is the key of the heat transfer. A low U-value can reduce the heat loss indeed.

Another aspect, if there is a way to reduce the air leakage from the window surrounding, then the heat loss can be reduce as well.

4.1. The ordinary condition (as an example) to show the difference

How much energy leakage of the window could be when compare with the other part of building construction. In well insulated buildings the U-factor of walls, roof and floors can be between 0.1 and 0.2 W/m2K. The best windows have U-factors of about 0.7–1.0 W/m2K 3. Even in some old house the U-value can reach more than 1.2 W/m2K.4

One example can show the difference; these three windows are common window in its research:

Window 1, U-value: 2.7 W/m2K 5
Window 2, U-value: 1.8 W/m2K 5
Window 3, U-value: 1.4 W/m2K 5

During winter time, the outside temperature is treated as 0 °C.
According to the equation conversion: q1=2.7 (20-0) =54 W/m2 q2=1.8(20-0) =36 W/m2K q3=1.4(20-0) =28 W/m2K

Even for the best window, taking the number 0.7, the q=0.7(20-0) =14 W/m2K.

In comparison with the roof or floor, the window part is could reach almost 27 times as much than that.

4. Result of Research Finding (Body)

There are different optimization approaches to reduce the heat loss from windows. Both the glazing and frame can be made some change in order to reduce the convection or conduction heat transfer. This section will gradually introduce some technology or reasonable method and outcome problem and explain in detail.

5.2. Different Technology of Glazing

Current main technology in windows glazing involves multi-layer glazing window, vacuum glazing system, gas filling window, and low-emittance coating cavity.

5.3.1. Multi-layer Glazing Window

In Australia, the window of modern building and not too old houses are constructed as two glazing window, but in some area a fair amount of house still uses the single glazing window. The different between this two is the inside gap contains glazing or not, it could be filling with air and other gas, or just vacuum.

A case study is done by Korea scientists about the U-value of different glazing layer6, the results shows an obvious changes. The result can be seen in the following table (figure 1), that with increased of glazing layer, the U-value is decreasing. It has nothing to do with what kind of frame or other means of reducing heat loss from windows.

Figure 1: U-value of different glazing layer 6

5.3.2. Vacuum Glazing

For double or triple glazing window, the air in the gap between each glass sheets can be extracting out to form a vacuum space. Therefore the heat transfer will minimize because of the gaseous heat transfer is negligible, and the internal vacuum space can be stable for a long period in terms of current technology.7

There are apparently potential benefits of vacuum glazing. Firstly, by combination of a vacuum space and low emittance coating, a very high level of thermal insulation could be achieved. Secondly, since the insulating properties of an evacuated space are effectively independent of the width of the space, a narrow vacuum space could be constructed which get good thermal insulation and saves the whole windows volume at the same time.8

5.3.3.1. Basic Fabrication for Vacuum Glazing Window

A sample of the basic fabrication can be seen in figure 2.9 Vacuum glazing is made by thickness of 3 mm or 4 mm soda-lime glass with low emittance surface of inner face; the pillars are made from a high strength, heat resistant, nickel-based alloy named Inconel 718. They are typically 0.25 mm in diameter and 0.15 mm in height. Based on their research of relation between the diameter and length of pillars, typically, vacuum glazing is designed with a glass-to-glass pillar conductance of between 0.40 W/m2K (for 4 mm thick glass, with p=25 mm and a=0.125 mm), and 1.2 w/m2k (for 3 mm thick glass with p=20 mm and a=0.25 mm) 9.

Figure 2: Schematic of vacuum glazing, as produced at the University of Sydney. 9

5.3.3.2. Heat Transfer Process and U-value for Vacuum Glazing Window
The external heat transfer of a glazing system can be significantly complex. It contains the combination of conduction of indoor and outdoor, conduction heat transfer of support pillars, forced and natural convection, and radiation. To simplify, the main part heat transfer can be calculated by the equation: Q=UA (Ti-Te).

According to the standards for determining heat flow through insulating glazing10, the heat transfer coefficient is treated as constant with the value 8.3 W/m2K and 30 W/m2K for the hot side and cold side respectively. The temperature on each side is 21.1°C and -17.8°C, for standard winter condition. The result will show a highly agreement with expects.

There are not only one factor need to be consider, for the rest, which is also important, such as gaseous conduction, radiated heat flow, pillar conduction, and edge conduction still should be taken into calculation.

As the experiment carry out by University of Sydney, the U-value of the vacuum glazing system can reach 0.80W/m2K for the ASTM winter conditions which is sated above that inside 21.1°C and outside -17.8°C10, this shows a promising option for a typical Tasmanian winter.

5.3.3. Vacuum Triple-Glazing

As it state above, the triple glazing window and vacuum glazing window are two advanced window system. Nevertheless, the technology is combining both of them together. In terms of the Switzerland scientists project 11, the U-value can be reduce to a less than 0.20 W/m2K value which is a highly improved project than that done by University of Sydney. It is different from the double-glazing window. This kind of triple-glazing window introduces the stainless support pillars in the gap with the position on the edge of each glass sheets. It still meets the same challenge of ordinary double glazing, but the triple system got more serious burden than the double system.

5.3.4.3. Basic Fabrication for Vacuum Triple Glazing Windows

An example of the basic design of the triple glazing window is shown in Figure 3. In all cases, the thermal conductivity of soda-lime glass was set to 1 W/mK and the emittance of uncoated glass surfaces was set to be 0.9. The support pillar emittance was set to 0.1 and the support pillar height is 0.2 mm. The selected emittances for the coated glass inside the cavity were 0.03, 0.06 or 0.09; two or four of the surfaces in the cavities were assumed to be coated. The thermal conductivity of the pillar material was set to 16 either a conservative value for stainless steel of 20 W/m2K, or to a fictitious material value of 1 W/m2K. The considered pillar radii ranged between 0.1 and 0.2 mm, the pillar separation between 10 and 60 mm. Glass sheet thicknesses was set to 4 and 6 mm. 11

Figure 3: Triple glazing window sample. 11

5.3.4.4. Heat Transfer Process

Simultaneously, the main processes are almost the same with the double and triple glazing sample. The group shows the result by computer simulation, they forming a 20 K temperature different between indoor and outdoor. In this case, 80% of the total flow through the glazing is due to heat conduction in the support pillars. Finally, the result of thermal transmittance is 0.2 W/m2K. 11

5.3.4. Gas Filled Glazing

This kind of optimization is filling gas with good thermal property between each 17 glazing and gets a reduction of heat loss. The most common material is absorbing gas, the rest are silica aerogel, chromogenic materials and phase change materials (PCM) 12. Usually, the gas filled window can be fill with lots kind of gas, these gas sometime are noble gas or some simple gas such as the carbon dioxide (see Figure 4). For safety and valid, the filled gas must be non-toxic, low conductivity and get high viscosity.

Figure 4: the gas filled window sample 12

The window in Figure 3 is filled with the noble gas argon. This kind of window be designed to reduce heat loss while admit the solar gain, it has low emissivity glass which can reduce heat loss in winter and refuse outdoor heat come inside in summer. In terms of this project, the U-value is achieved around 0.3 W/m2K.

Other researchers have filled in the glazing cavity with SF6 (Sulphur Hexafluoride), CO2 (Carbon Dioxide), NO2 (Nitrogen Oxide) and NH3 (Ammonia) or some other gases. The range of the width between each glazing was set from 6 to 20 mm, and kept at a steady state. The result in Figure 5, Figure 6 and Figure 7 shows the relation between the gap width and the U-value of the window.13

Figure 5: The relation between gap width and the U-value of 2 as media 13

Figure 6: The relation between gap width and the U-value of SF6 as media 13

Figure 7: The relation between gap width and the U-value of NH3 as media 13

From the result, it obviously that the U-value decrease with the increase of the gap width, and with low emissivity coating of 0.065, the U-value is more acceptable than the high one. It is better to keep the low emissivity coating of 0.065 and control the width around 16-20 mm.

5.3. Optimization of Windows Frame

Window frames are available in a variety of materials. Frames can be composed of a single material or made of a combination of different materials such as wood clad vinyl or aluminium-clad wood. Each framing material has its advantages and disadvantages. The new strategy of construct the frame is built a hollow frame with insulates filled in. It improves the insulation function of the frame and save the material.

5.4.5. The Common Frame Material

Many types of materials are used to construct the window frame. The most popular materials are timber, polymer and metal14.

Here is a list of common frame material with their determined U-value: 14
Balsa: 0.055 W/m2K
Cypress: 0.097 W/m2K;
Fir: 0.11 W/m2K;
Maple or Oak: 0.166 W/m2K;
Yellow Pine: 0.147 W/m2K;
White Pine: 0.112 W/m2K;
PVC (polyvinylchloride): 0.09 W/m2K;
Aluminium alloy (Al-Mg-Si):177 W/m2K;

To date, the polymer plays an important part in energy saving design of window frame. As shown above, it got a relative low U-value when compare with some timber. It also gets both light and high strength property that better than some low heat transfer rate timber, because some of them get worse strength.

Still the aluminium alloy frame can be found everywhere in some old house in country like China. Both of the window and door frame of some new house even constructed by the aluminium alloy. From the value indicates, this kind of window do bad in energy saving. To some degree, they just got the property of beauty and safe.

5.4.6. Frame fabrication and simulation result

The research of filling different insulates in different kind of frame is done by some Norway scientists. They do the simulation method to get the possible result. There are totally five kind of window that named as A (foam insulation aluminium frame), B (thermally broken aluminium frame), C (foam insulated wood frame), D (solid wood frame), E (PVC frame).

For the basic design, all the frames were simulated with triple glazing. 95 precent argon and 5 percent air filling in the cavity, and set two low-e coatings with an emissivity of 0.037. The resulting glazing U-factor was 0.710 W/m2K. For all of the frames, the spacer effective conductivity was varied between 0.02 W/mK and 10 W/mK.15

(For details on the separate result of frame A, B, C, D, and E see: Appendix 1)

The overall result of this research is shown in the graph below (figure 7); it shows the variation in total U-value of the windows as a function of the space conductivity. Figure 7 also shows the total windows U-value as a function of material/thermal break conductivity and emissivity. 15
Figure 8: Total window U-factor from A to E 15

According to the overall result it would seems the solid wood frame and the PVC frame have a clear advantage in turns of reducing heat loss, compared to the other 3 frame materials and it also shows that a complex insulate frame is better than a single material insulates frame. Therefore when optimizing the performance of windows frame, solid wood should be the best choice for domestic use.

5.4. Possible Experiment & Problem

5.5.7. New phenomenon introduction

Water condensation on windows is a common problem for public. It always can be seen between the window panes or on the inside. In both cases, the condensation indicates that the window is of poor quality or has a high U-value. For poor quality window, humid air leaks into a space that is supposed to be closed or properly ventilated. In the latter case water condenses on the inner window surface because the window is considerably colder than the room. That means there is certain heat loss out from the room through the window. 16

High efficient window always equipped with some kinds of energy saving material. Low-emissivity and solar control coatings in windows can considerably reduce the energy needed for heating and cooling buildings. The heat conductance of such windows, the U-value, can be as low as 1 W/m2K. For the glazed area of a window, values down to 0.5 W/m2K can be realized. In some climates, especially the climates in northern parts of Tasmania, where the radiated cooling current towards a clear night sky can be high, this has led to a new phenomenon: external water condensation on windows16. It is experienced when the temperature of the external glass pane of a window goes below the outside dew point due to radiative cooling. It is appears during clear nights on well-insulated windows for which the thermal losses do not balance the radioactive cooling of the external glass surface16.

As Werner said, at most case, the condensation on the window indicates a bad thermal insulation condition. But for some energy efficient window, because of it contain low emittance coating or other new technology material, it is a sign of excellent thermal insulation property as more water drops appears. 16

5.5.8. Possible Experiment

* High and Low emissivity test

To explore if there is a simple way to monitor surface temperature and to visually discern external condensation on windows samples exposed to a clear night sky. Also test whether there is a simple way to compare different coated glass samples to see which one is more prone to condensation.

The experiment should be undertaking during night with outdoo`r air temperature about 13 °C. Two windows samples, an double glazing sample and a vacuum glazing sample, of size 5 cm×10 cm of equal thickness were heated to about 50 °C and placed horizontally on a piece of polystyrene. A thermocouple below each sample measured the temperature. The heating to 50 °C was done to monitor the difference in cooling rate more clearly. The experiment started at midnight.

5. Discussion

It can be observed that the with the glazing layer increase the glazing U-value will decrease. Just because of multi-glazing layer contain gaps between each glazing, therefore no matter what kind of insulation material filling inside, it has a certain insulation function. The single glazing window do not have gaps, much energy will be lost. It obviously save twice as much heat loss with the triple glazing system of U-value of 2.6 W/m2K when compare with the single glazing got a value of 5.3 W/m2K. If a quadruple-glazing window system can be built, it will get three gaps. At that condition, the insulation function must be considerable great no matter the insulation gas filling in or not.

There are not too many words to talk about the low E coating, because it is just a small particle in this field, but it still indicates this kind of coating is necessary to contain in the glazing optimization. There is a certain difference when the glazing system add a low e coating material, in terms of Figure1, compare with the double glazing with U-value of 3.30 W/m2K, a low e coating coated one got a 2.90 W/m2K which reduce almost 14% heat loss.

The gas filling glazing is an advanced improvement when compare with the common glazing-air-glazing window, it take the advantage of the space and the compressive of gas. Insulation gas is easy to moving, filling, and did not obstruct the vision. In terms of research done by other scientist12; it’s a satisfactory result for their gas filling glazing research. Nevertheless, what makes the different is Arasteh’s group adopts the noble gas while Reilly’s group chose some ordinary gas. The noble gas may be really expensive, but it has stable chemical property and bad heat transfer property, their result got an excellent value of 0.3 W/m2K, but in terms of price the SF6 (Sulphur Hexafluoride) filling is still quite effective with u-values less than 1.5 W/m2K which means it still a really good value.

Every coin gets two sides. The energy efficient window also has trouble of condensation, but this kind of condensation is new, because it is the outside external surface condensation. The researcher Werner16 aims to carry out work on glazing covered material to solve the problem rather than do the structure work of the whole window system. It is really a good strategy, because it will not cause bad influence on the window, also avoid the extra new problem occurs. It can be observed that the titanium dioxide-coated glass performs well in each testing experiment. It may condensates but the surface is still clear and obstructs less vision.

6. Conclusion
Back to the topic, to get a better indoor temperature with less heating supplied during a typical Tasmanian winter, it is essential to build a space with less heat loss which means the common large heat loss part, such as window part must be optimized in order to meet the demand.

According to all researchers‟ work, the insulation strategy should be focus on the window glazing and frame at the same time, each of them cannot be missing or ignore. On the glazing part, multi-glazing layer is much better than single glazing, it can be found that triple glazing window always got a better insulation performance in every research. Therefore to date, the focus should be change towards the triple-glazing because the double one will be replaced step by step. What is more, the gap between two glazings also cannot be ignored, no matter filling in some insulation gas or extract the air to get a vacuum space. For gas filling strategy, it is better to use the noble gas such as argon, but if the cost of whole window system cannot be accept, the gas can be change to some cheap compounds also with a stable chemical property, such as SF6. For vacuum space strategy, the support pillars should be placed on the edge, therefore, it will not obstruct the vision.

Window frame must be optimized at the same time. Frame with single material will not be considered, and it is the same with the metal frame, because both of them will cause either bad insulation or inside surface condensation. Hollow inside with insulants filled frame is the best choice, with wood or PVC covered, an expanding foam filled frame will performance very well.

At last, low emissivity coating is another important element which worked to reduce heat loss in winter and refuse outdoor heat come inside in summer. And the coating material must be paid attention, because low U-value efficient window meet the external surface condensation problem, the titanium dioxide-coated glass will do a favour to solve this problem.

Reference List 1. Statistic taken from the Australian Government Bureau of Meteorology Tasmania in winter 2012: Dry, except in the northwest. http://www.bom.gov.au/climate/current/season/tas/archive/201208.summary.shtml © Copyright Commonwealth of Australia, Bureau of Meteorology (ABN 92 637 533 532)
Viewed on: 10/03/2014 2. ANSI/ASHRAE Standard 55 Thermal Environment Conditions for Human Occupancy (2004)
Publisher: ANSI/ASHRAE
Viewed on: 10/03/2014 3. Key elements of and material performance targets for highly insulating window frames，Energy and Buildings，vol.43, Issue 10, pp.2583–2594 http://www.brikbase.org/sites/default/files/best3_hart.pdf Author: Gustavsen, A., Grynning, S., Arasteh, D., Jelle, B.P., Goudey, H
Publisher: Elsevier B.V. All rights reserve (2011)
Viewed on: 13/03/14 4. Implementation of energy-efficient windows in Swedish single-family houses, Applied energy, vol.89, Issue 1, pp.329–338 http://www.diva-portal.org/smash/get/diva2:489863/FULLTEXT01.pdf Author: Naira, G., Mahapatrab, K., Gustavssonb, L.
Publisher: Eco technology and Environmental Science Department of Engineering and Sustainable Development, Mid Sweden University, Östersund, Sweden (2011)
Viewed on: 12/03/14 5. Table of Default U values – Pilkington http://www.pilkington.com/~/media/Pilkington/Site%20Content/UK/Reference/TableofDefaultUValues.ashx Viewed on: 12/03/14 6. Article: Evaluation of inside surface condensation in double glazing window system with insulation spacer: A case study of residential complex http://www.researchgate.net/publication/239360410_Evaluation_of_inside_surface_condensation_in_double_glazing_window_system_with_insulation_spacer_A_case_study_of_residential_complex Author: Seung-Yeong Song, Jae-Hun Jo, Myoung-Souk Yeo, Young-Don Kim, Kyoo-Dong Song
Copyright: Building and Environment - BLDG ENVIRON 01/2007; 42(2):940-950. DOI:10.1016/j.buildenv.2005.10.015 Viewed on: 16/03/14 7. Vacuum Glazing: When inert gas is replaced by vacuum http://www.bine.info/fileadmin/content/Publikationen/Englische_Infos/projekt_0108_engl_internetx.pdf Publisher: BINE Informationsdienst
Viewed on: 15/03/14 8. Current status of the science and technology of 36 vacuum glazing http://solarenergyengineering.asmedigitalcollection.asme.org/mobile/article.aspx?articleid=1457703 Author: Collins, R.E., Simko, T.M Viewed on: 15/03/14 9. Schematic of vacuum glazing http://www.intraprojects.com/Principle%20Vacuum.htm Viewed on: 16/03/14 10. Standard procedures for determining the steady state thermal transmittance of fenestration systems, ASTM Standard E 1423-91. Annual Book of ASTM Standards 04.07.
Publisher: American Society of Testing and Materials
Viewed on: 14/03/14 11. Triple vacuum glazing: Heat transfer and basic mechanical design constraints, Solar energy, Vol.80, Issue 12, pp. 1632–1642 (2006)
Author: Manz, H., Brunner, S. , Wullschleger, L.
Viewed on: 04/03/2014 12. Overview on insulated glazing windows http://www.armadacanberra.com/double-glazed-overview.html Copyright © 2014 Armada. All Rights Reserved.
Viewed on: 15/03/2014 13. The effects of infrared absorbing gasses on window heating transfer: A comparison of theory and experiment. (1990)
Solar energy material 20,
Author: Reilly, M.S., Arasteh, D., Rubin, M.
Viewed on: 04/03/2014 14. Injection modeling simulation analysis of natural fiber composite window frame, Journal of Materials Processing Technology (2008)
Vol.197, Issues 1–3, 1 pp.22–30 http://www.sciencedirect.com/science/article/pii/S0924013607005833 Author: Rahman, W.A.W.A., Sin, L.T., Rahmat, A.R.
Viewed on: 24/03/2014 15. Key elements of and material performance targets for highly insulating window frames, Energy and Buildings (2011) vol.43, Issue 10 http://eetd.lbl.gov/sites/all/files/publications/lbnl-5099e.pdf Author: Gustavsen, A., Grynning, S., Arasteh, D., Jelle, B.P., Goudey, H.
Viewed on: 25/03/2014 16. Condensation tests on glass samples for energy efficient windows, Solar Energy Materials and Solar Cells. (2007) Vol.91, Issue 7, http://www.researchgate.net/publication/222774553_Condensation_tests_on_glass_samples_for_energy_efficient_windows Author: Werner, A. & Roos, A.
Viewed on: 02/04/2014
Appendix 1: Frame simulation results 15
Appendix Figure 1, foam insulation aluminium frame 15

The cross-section of frame A (Figure 1): The purple elements show the placement of the polyurethane (PUR) foam. The light green elements show the unventilated cavities of the frame. The dark blue elements show the frame's aluminum skeleton. The thin layer of aluminum cladding covers solid polyurethane elements, minimizing direct connections between inside and outside. It is calculated the thermal transmittance values for various configurations of spacers which effective conductivity ranging from 0.02 to 10W/mK. Thermal insulation conductivities is detected from 0.005 to 0.089 W/mK for the frame. 15
Appendix Figure 2, thermally broken aluminium frame 15

The cross-section of frame B (Figure 8): The black elements show polyamide thermal 22 breaks (with 25% glass fibre of thermal conductivity 0.173 W/mK) and the light blue area represents the aerogel (with thermal conductivity 0.057 W/mK). The light green elements are the unventilated air cavities within the frame. The blue and dark red elements depict the outer and inner aluminium skeleton, respectively. The outer aluminium skeleton has an emissivity of 0.9, and the inner one has an emissivity of 0.6. The performed calculations for various combinations of spacer and thermal break conductivities. The spacer effective conductivity was varied from 0.02 to 10 W/mK, and the thermal break conductivity was set from 0.005 to 0.1733 W/mK. 15
Appendix Figure 3, foam insulated wood frame 15

The cross-section of frame C (Figure 3): The pink areas show the placement of the PUR insulation material, it is insulated with a continuous 17-mm-thick layer of PUR foam, and the brown areas represent the wood (Nordic pine with thermal conductivity 0.12 W/mK) areas of the frame. The light green elements are the unventilated air cavities within the frame. The performed calculations for various combinations of spacer and thermal break conductivities indicate the spacer effective conductivity was varied between 0.02 and 10 W/mK, and the thermal break conductivity from 0.005 to 0.029 W/mk.15
Appendix Figure 4, solid wood frame 15

The cross-section of Frame D (Figure 4): Solid core wood is shown in brown. Air cavities are shown in light green. The spacer effective conductivity was varied between 0.02 and 10 W/mK, and the wood conductivity from 0.005 to 0.12 W/mk. 15
Appendix Figure 5, PVC frame 15

The cross-section of Frame E (Figure 5): The brown areas show the PVC skeleton of the frame, and the light green areas show air cavities. The blue areas show the supporting steel and the continuous parts of the hardware used for opening the frame. The calculations for various effective spacer conductivities from 0.02 to 10 W/mK, and PVC surface emissivity between 0.02 and 0.9. Hollow inside frame could be a control group. 15

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[ 1 ]. U-value is the measure of heat loss in a building element such as a wall, floor or roof. It can also be referred to as an ‘overall heat transfer co-efficient’ and measures how well parts of a building transfer heat.