Window Performance
The insulating power of an ordinary single-pane window is mainly due to the insulating layers of air that rest against the inside and outside of the window pane. If the air is totally still and dry, these two air-films produce an insulating total value of about R-1.4, when a single layer of glazing is used. An R-value of 1.4 is not good, but rarely does a window provide even this much insulation. Wind erodes the air-film and reduces the insulating value. For example, an outdoor breeze of 15 mph reduces the insulating power of an air film on that side of the glass to about a fourth of its still-air value: from R-.7 to R-.17. And forced-air heating that produces a breeze half as brisk reduces the indoor layer’s insulating power to about R-.25. The R-value of the single-glazed window under these breezy conditions, then, is only R-.42 (R-.17 plus R-.25 equals R-.42). This compares to a modestly insulated wall’s value of about R-12. In such cases, each square foot of the window would lose about 29 times as much heat as each square foot of the wall insulated at R-12.
In practice, the thermal performance of windows usually is worse than indicated above– often much worse. The main problem in many windows is not the conduction of heat to the outdoors, which is detailed above, but the infiltration of cold air around window sashes and around the window framing. The cracks in modern casement and awning windows are pretty tight, testing (with a 25 mile-per-hour breeze) as low as .01 cubic feet of air infiltration per minute per linear foot of crack, but gliders and double-hung windows are much worse. These typically allow ten times as much air infiltration even if modern and relatively tight. Old windows are much worse, and metal casement windows typically are the worst of all. If you do nothing else, at least, lock windows. Locked windows admit less draft than unlocked windows. The insulating power of single-pane windows can be much improved by adding a storm window.
Indoor storm windows are much cheaper than outdoor storm windows. They also are much more effective, for they greatly reduce infiltration in addition to providing the necessary still air between the layers of glazing. (Most outdoor storm windows are far too loose to seal against infiltration.) And because they seal the window tightly against moisture inside the house (produced by bathing, cooking, etc.), they are much less likely to cause condensation problems (than tight outdoor storm windows that trap moist air in the cold space between the storm window and the primary window. Both condensation and infiltration can be further reduced by caulking around the window trim where it meets the wall. Unfortunately, indoor storm windows cannot be removed easily and there are several cautions to observe with the indoor storm window: (1) Leave at least one window in each bedroom easy to open so it can be used as a fire escape; (2) Single-glazed windows that lack storm windows dehumidify household air when they sweat in the winter (The indoor storm windows put an end to this dehumidification and will make the indoor air moister. The greater humidity is more comfortable, but it may cause hidden condensation concerns); and (3) Do not use any kind of storm window with primary windows that have thermal (multi-pane) glazing. You will void the warranty and ruin the window.
Most high efficiency replacement windows provide an R-5 to R-6 rating. Conduction around the edge of the glass generally reduces the best over-all performance to about R-5. Still that’s a significant improvement over single-glazed windows. Most upgrades for cold climates use “low-e” (low emissivity) glazing. This low-e means that radiant energy is not transmitted, thus preventing loss of energy during the winter. As a rule, low-e glazing also reflects summer heat before it enters the house. When choosing new windows, look for the label that shows the window’s U-value (the U-value is the inverse of the R-value, so the lower the U-value, the better; .3 is quite good). In the late 1990s, the fenestration heating ratio (FHR) was developed to denote winter performance. This ratio, based on solar heat gain coefficient (SHGC), visible transmittance (VT), and air leakage, compare the energy savings in percent compared to a typical single-pane window. An FHR of 40 is very good, meaning that the window will use 40% less energy. CAUTION: (1) Don’t mix brands of low-e glass. Such glass gives a tint to the light that passes through it, and the color of tint varies with the process used by the manufacturer. People find it very difficult to adjust psychologically to varied tints. (2) Wash low-e glass with vinegar-based cleaners. If a milky film has already fogged your windows, clean them with a cerium oxide cleaner , and (3) Contrary to some claims, low-e glass that blocks the sun’s UV radiation does little to prevent fading fabric colors. Fading is largely a consequence of visible light, not UV radiation.
Chet Riley is the Owner/Proprietor of CENTENNIAL Home Inspections. For any questions regarding home inspections, please call Chet at 585-293-2278. Or e-mail him at criley1@rochester.rr.com. Further information can be obtained at http://www.centennialhomeinspections.com/.