By Shawn Shouse

Ag Engineering Field Specialist

Iowa State University Extension

High energy prices and cold weather have us thinking about home heating costs. Housing experts say that 25 to 35 percent of home heat can be lost through doors and windows. Hardware and home stores sell storm windows and plastic sheet window insulating kits. But do these extra window layers pay for themselves?

Heat escapes windows by different means

One path is the conduction of heat through the glass panes and wood, vinyl or metal frames of the window. This conductive heat flow is estimated by the U factor (thermal transmittance) for the window. The U factor is the reciprocal of the thermal resistance factor (R factor) commonly cited for insulation and building materials, that is, U = 1/R. When you see a U factor or R factor for a window, be sure you know if the number quoted is for the entire window unit including the frame, or just for the window glass itself.

Glass is a poor insulator. But still air film at the surface of the glass and still air trapped between layers of glass makes good insulation. This explains higher R factors for windows with multiple layers of glass (insulated glass) or with added storm windows. Adding layers with extra glass or plastic film can increase the R factor of existing windows.

A second and sometimes larger path for heat escape from windows is leaking air. Air leaks between window sashes, between sashes and frames, and between frames and walls all allow warm air to leave and cold air to enter the house. Close attention to window seals and installation methods can minimize these losses. Added layers of plastic or glass can help reduce air leaks through window parts.

A third path for heat loss through windows is radiation. You feel radiation heat gain when sun shines in your windows. Smaller but similar radiation losses occur when warm surfaces inside your home radiate heat out through the window to cold surroundings. Special surface coatings (low emissivity, or low-e) can help control radiant losses and gains, but closing blinds and drapes at night and on all shaded windows can also help.

Saving money

If good control of air leaks exists, adding a second layer to a single layer window will double the R factor and cut conductive heat loss in half. Adding a third layer will increase the R factor by another 30 percent. Using a standard forced air furnace, Iowa winter, 2008 prices, and a window with 10 square feet of glass area, adding a second layer could save $10 to $20 per window. Adding a third layer could save three to six dollars per window. Savings could be higher if you significantly reduce air leaks as well. Adding storms or insulating films to windows can pay dividends in fuel savings and added comfort.

For more information on choosing windows, consult the Efficient Windows Collaborative

http://www.efficientwindows.org/

More information on home energy savings is available from the Iowa Energy Center

http://www.energy.iastate.edu/

And the US Department of Energy

http://www.energysavers.gov/

This article is from the December 2008 issue of Acreage Living.

Other articles in this month’s issue--

• Winter Power Outages
• There’s a Mouse in My House!

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