Window Theory: Seal Windows for a More Efficient Home

• windows
• DIY
• how it works
• energy
• heating
• cooling
• home improvement

Windows are complex. A new model may consist of wood, metal, plastic, glass, mineral oxides and perhaps an inert gas. Yet even a well-insulated unit delivers an R-value no better than an uninsulated wood-frame wall’s (roughly R-4 to R-5). And a house can lose nearly 12,000 Btu per hour, or about 10 percent of a heating system’s output, just through these leaky openings. That means there’s a lot at stake when it comes to choosing, installing and sealing windows.

Adding storm windows cuts heating losses from drafty windows in half, and replacing them with an insulated product pares the losses still further. Finally, a modern unit holds in heat, reflecting it from you and your heating system back into the room. It seals out drafts and fends off summer sun. Here’s what you need to know.



In winter, the inner surface of window glass can be 5 to 20 F cooler than the adjacent indoor air, depending on the window’s thermal efficiency and how cold it is outside. Indoor air cools as it passes over the window and sinks, producing an uncomfortable drafty zone. Insulated glass with a low U-value is warmer and improves comfort near windows.

Also, the larger the glass surface, the more it needs a low-E coating, a microscopically thin layer of metal oxide that reflects back infrared energy. Without the coating, the heat flowing out from your body would flow to and through the glass, increasing your discomfort. But a low-E coating bounces the infrared energy back, so you feel warmer.

The tendency of a window to let heat pass is described as its U-factor—lower is more energy-efficient. It’s the inverse of R-value, which describes insulation’s resistance to heat flow; higher is better. Adding a second piece of glass dramatically improves the window’s U-factor. While a single-pane window is rated at 1.1, a second pane cuts the U to the range of 0.47 to 0.63. Filling the space between the panes with argon instead of air drops it to 0.37. Argon is a better insulator than air, a distinct advantage of insulated glass compared to a storm window installed on the outside of the house.

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