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Walls and slabs

Facts about radiant energy, windows and comfort.

The biggest radiator known to man is the sun and the biggest absorber of radiant energy is the one we walk on everyday - its called earth - and earth cools itself every night with radiant energy. So does radiant heating and cooling work? Well if it didn't - we'd all be dead.

Radiant heating and cooling is essential for our survival yet in the world of HVAC it is the most powerful yet least understood of all mechanism for conditioning bodies and buildings.

This is demonstrated by the fact that most HVAC systems in America are designed to respond to the air temperature sensors, which in fact do not reflect radiation problems,  particularly those caused by windows.

For our purposes in building science we know this radiant energy by two forms; that being shortwave and long-wave electromagnetic energy  (see Fig. 5 ).

Shortwave energy is the direct high intensity energy from the sun. You see shortwave energy in colors and feel it as heat. Shortwave falls in the categories of ultra-violet (UV), visible light (VL) and infrared (IR). How important is shortwave energy? Well the difference on earth between cold and dark and hot and light depends on the flow of this electromagnetic energy.

Long wave energy is the indirect (diffuse) or filtered low intensity energy that bounces off the environment. It falls primarily under the category of far infrared. Body and room temperatures (Fig. a & b ) are in the long wave spectrum. Though these images appear hot, they are in fact cool - less than 90F in the case of the child and less than 75F in the case of the building.

facts about your skin, click to learn more
Figure a. Thermographic image of a small child on a floor.
Image Credit: Dr. A. Tucker / Science Photo Library

click to learn about thermography

Figure b. Thermographic image of a radiant heated room. Image credit: (C) Bob Rohr

Both short and long wave energy is converted to heat when its strikes an opaque surface of lower temperature. Opaque means it's not transparent or translucent.

Window glazing is virtually opaque to long wave radiation but not shortwave. It’s the shortwave energy passing through the window that provides visible light and also heat’s up in the interior space.

When you read on the internet about problems with passive heating or how solar gains and radiant floor heating can cause discomfort, what you are reading about is the lack of control over the two forms of radiant energy. This lack of control comes from poor planning (ignoring building orientation to south, length to width ratio's and excessive window to wall ratio's) and inadequate external solar control such as shading and blinds; the last passive line of defense is the building enclosure which windows are an integral part.

By their very nature windows almost always create asymmetric thermal environments. Symmetrical means the same, asymmetrical means different. Asymmetric thermal environments means the two different surface temperatures on opposites sides of the human body. Think about you in front of a side hot the other not...that's radiant asymmetry.

Specifically, a window influences thermal comfort in these ways:

long-wave radiation from the warm or cold interior glass surface to the building interior and occupants

transmitted short wave solar radiationto the building interior and occupants

  induced air motion (convective drafts) caused by a difference between the glass surface temperature and the adjacent air temperature.

Comfort effects from windows are fundamentally different based on seasons.

The winter effect is mainly driven by inside window surface temperature, which are correlated with window U-factor (Fig. 3 and 4) and outside temperature.

The summer effect is driven by a combination of the inside surface temperature and transmitted short and long wave solar radiation, which are heavily influenced by the optical properties of the window; that being single, double and triple pane with various fills (ex. argon) and coatings (ex. low e).

Comfort aspects to consider:

the effect of window surface temperature on long-wave radiation heat exchange between the body and the window, and

  the effect of solar radiation transmitted by the window and absorbed by the body and interior surfaces.

Practical application

the closer a person is to a window, or the larger the window, the greater the impact on comfort.

Suggested reading

Zhang, M., Kuehn, T., Carmody, J., Energy Simulation of HVAC Systems with High Performance Windows, University of Minnesota at Twin Cities, 2005

See: Walls for cold climates

Download: Sill to Sash

The Sill to Sash video explains the latest window developments, the use of standards and certification, and shows how you can use this information to buy the right products.

Sill to Sash is sponsored by the Federal Government Climate Change Action Fund, the Siding and Window Dealers Association of Canada, and the Fenestration Canada.

more to come back soon!


Windows for cold climates:
For additional support visit our visitor services page.

Windows (fenestration) are one of the weakest links in the building enclosure due their inefficiencies as an insulator which also includes the thermal bridging through structural elements that create the opening for the window unit.

This topic is part of our Professional Development curriculum. Several on-line webinars and multiday programs are offered through the year - many are at no cost or available with government subsidies.  Be sure to also check out our new Donate to Educate program.


U Value: measure of the rate of conductive heat transfer through the glazing due to a temperature change between inside and out-side surfaces also known as the overall heat transfer coefficient. The U values shown in Figure 3. are for center of glass and for a complete window (glazing unit + frame). The smaller the U value the better the window is at stopping heat flow. The R value of a window is the reciprocal of the U value (i.e. R=1/U).  In cold climates (heating dominated) low U value windows should be used on all northward facing windows.
Solar Heat Gain Coefficient (SHGC): the solar heat gain coefficient of the total window system represents the solar heat gain through the window system relative to the incident solar radiation. A value of 1.0 indicates that 100% of the solar gain enters the building. A value of 0.0 indicates no solar gain is entering the space. Low solar heat gain (LSG) coefficients can significantly reduce cooling requirements. However in cold climates (heating dominated) high solar heat gain (HSG) is preferred on southward facing windows.
Shading Coefficient (SC): is a dimensionless number representing the ratio of solar heat gain through the window system relative to that through 1/8" (3mm) thick clear glass at normal incidence. The lower the number, the less solar gain is admitted. ex. SC = SHGC x 1.15. Low shading coefficients can significantly reduce cooling requirements (see above regarding heating dominated climates).
Visible Transmittance (VT or Tvis): percent of the visible spectrum striking the glazing that passes through the glazing. This value changes with angle of incidence.
Visible and Solar Reflectance: percent visible light or solar energy that is reflected from the glazing.
UV Transmittance: percent transmittance of ultraviolet-wavelength solar energy (0.30 to 0.38 microns). High UV penetration will fade fabrics and can damage sensitive artwork.


Window shading coefficient and visible light transmission are governed mainly by the type of glass and coating used in the glazing unit.
Low values are indicative of the performance characteristics associated with highly reflective glass and high numbers are indicative of clear glass performance. 

U values should be evaluated for the complete window (glazing unit frame) and take into account the impact of edge conditions and frame construction on the window's performance.

Coatings are hard coat or soft coat, and available with high solar heat gain (HSG), moderate solar heat gain (MSG), and low solar heat gain (LSG).

The differences between coatings affects what portion of the IR spectrum is blocked.

High performance window
Figure 1. Profile of a high performance window (above)

Figure 2. Principles of radiant energy - light and heat for LSG.

Make note of the natural convection current which occurs between the panes of glass.

Figure 3. Characteristics for a high performance window. Compare the U values, SHGC and VT with the windows below.

Figure 4. Characteristics for various other windows.

Figure 5. Radiant energy spectrum showing short wave and long wave energy.

Figure 6. Clear glass performance showing blocked, transmitted and reflected energy.

As you can see (pun intended), compared to HSG and LSG, clear glass is ineffective at blocking short and long wave radiation

Figure 7. HSG performance showing blocked, transmitted and reflected energy.

HSG blocks out most of the long wave radiant energy and UV but lets in the visible light and near infrared.

Figure 8 LSG performance showing blocked, transmitted and reflected energy.

LSG blocks out most of the long wave, a significant portion of the UV and IR while retaining visible light.

Figure 9. Window coating location comparison between HSG and LSG.

Figure 10. Summer surface temperature comparison between HSG and LSG.

Figure 11. Winter surface temperature comparison between HSG and LSG.

Figure 12. Surface temperature comparison between winter and summer with HSG.

Figure 13. Surface temp. performance between LSG and HSG for summer and winter.

Figure 15. Transmitted energy performance between LSG and HSG for summer and winter.

Figure 16. Cold climate regions which benefit from HSG.

Figure 17. Conclusions.

Keeping summertime shortwave energy off your building (below)
To maintain control over solar gains follow the simple strategies below.

external shading

Use adjustable external blinds


external shading using landscaping Use seasonal external shading
precooling ventilation air Consider pre-cooling of  ventilation
passive cooling principles Use night time passive cooling
prevent use of internal loads Eliminate internal daytime loads
geothermal cooling Use direct connect geothermal cooling

Window installation in an R40 Wall - courtesy of the folks at

window installation
Window installation is critical. Even the best windows if installed poorly will leak.

1. Huizenga, C, Zhang, H., Mattelaer, P., Yu, T., Arens, Edward A, & Lyons, P.(2006). Window Performance for Human Thermal Comfort. UC Berkeley: Center for the Built Environment.
2. Swinton, M.C., Manning, M.M., Elmahdy, A.H., Parekh, A., Barry, C., Szadkowski, F., "Field assessment of the effect of different spectrally selective low emissivity glass coatings on the energy consumption in residential application in cold climates," 11th Canadian Building Science and Technology Conference (Banff, Alberta, March 22, 2007), pp. 1-16, March 22, 2007 (NRCC-49481).
3. NASA, Clouds and Earth's Radiant Energy System, CERES

4. The Efficient Windows Collaborative (EWC)

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