Posted by: SherlockHomesSY | 5 Nov 2010

Solution Basics: Window Performance

Window Shading Options

Image by Florida Solar Energy Center


Increase thermal resistance
Minimize solar heat gain
Maximize light penetration
Minimize UV radiation
Reduce window condensation
Use window treatments

Windows are a significant source of heat loss in the cold climates and solar heat gain in warm ones.  We use windows to provide natural light, ventilation, and look outdoors.  These features make it difficult to limit heat transfer while preserving light and view.

The key to better window performance is window treatments.  Occasionally, replacement windows are recommended, but window treatments solve a majority of window problems.

Increase Thermal Resistance (U)

Cutting heat transfer through a window by half is easy—add a second pane of glass.  This can be done with dual-pane windows or storm windows. Storm windows can be installed inside or outside the house.

If you already have dual-pane windows, adding storm windows will help.

This works because the insulation value of a pane of glass is R-1.  Adding a second pane of glass increases it to R-2 right away.  Heat loss through the window is cut in half!

Window manufacturers increase the insulation value of a window by doing some other things, such as using gas between panes, spilt spaces, different kinds of materials, and other high-tech things.  The most important is the 2nd pane of glass.

U-factor is the inverse of insulation value (U = 1/R).  The building code dictates the maximum U-value for windows.  In Atlanta, GA, the requirement is U <= 0.40.

Minimize Solar Heat Gain (SHGC)

Solar heat gain is what happens when sun shines on objects inside the house.  The Solar Heat Gain Coefficient (SHGC) is the measure of heat gain by solar radiation when the sun shines directly at the window.

In the winter time, maximize solar heat gain.  In the summer time, minimize it.  Window manufacturers have high-tech ways reduce it.  Unfortunately, it comes at the expense of the intensity of visual light.  In Atlanta, GA, the required SHGC <= 0.40.

There are much better ways to reduce solar heat gain, but it requires knowledge of the angle of the sun throughout the year and orientation of your windows.  We’ll talk about it more in the discussion about Shading Coefficient (SC).

Maximize Light Penetration (VT)

Most of us expect sunlight when looking out the window.  We’re not impressed when looking outside as if through sunglasses.

When deciding among windows, get the lowest affordable U-factor and the most amount of Visual Transmittance (VT) for a given SHGC.

Minimize UV Radiation

A major destructive force on a house is ultraviolet (UV) radiation.  Sunshine in the house can fade whatever is in the room.  Better windows reduce UV radiation while letting maximizing visual transmittance and minimizing solar heat gain.

Reduce Window Condensation

The most significant benefit of going from single-pane to dual-pane windows is the increase in insulation value (R-1 ® R-2).  This increases the surface temperature of the window, which lowers the potential for condensation on the windows under normal conditions.

Window condensation is a moisture control issue, usually brought on by air leakage, lack of ventilation, and improperly insulated walls.  If condensation is still a problem after adding an additional window pane, the first thing to try is sealing the window frame to the wall covering to limit air leakage around the window.

Often, the next thing to try is raising the indoor temperature to greater than the dew point.  In California, people keep their houses cold (less than 68° F).  The problem is highly humid outdoor air coming in through air leaks.  Sometimes, the best recommendation is to raise the temperature (or at least run a dehumidifier).

Use Window Treatments

Window manufacturers, installers, and others promote better windows.  I say not so fast!  First, understand the physics of your windows, then use a combination of canopies, curtains, blinds, shutters, landscaping, and solar shades or screens.

Doing so is not as easy as replacing windows.  It’s certainly a lot cheaper.  Besides, once the windows are replaced, window treatments need to be considered anyway.


Diagnosing and correcting windows is on my home performance website.

Windows are a fascinating topic and there is a lot to think about.  The Florida Solar Heating Center discusses windows in depth.

In Conclusion

Before considering replacing window, consider other options.  Regardless of whether your current windows are replaced or not, window treatments need to be considered as part of a home energy improvement strategy.  Air sealing and insulation also have a significant impact on window performance.

Please leave your comments here on this blog for further consideration.  If you’d rather, please send your questions to

This concludes our series on Solution Basics.  Next we’ll explore Common Complaints.

As always, before making home performance improvements, consider our warning . . .

WARNING! Do not implement any of these solutions without considering the impact on IAQ, moisture control, & heat transfer. A combination of solutions are usually needed to improve health, building durability, comfort, and energy efficiency. Ignoring this warning may lead to disease, deterioration, high energy bills or worse!


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