7.      Windows and Doors

Even the best windows and doors on the market perform much worse than even an average wall, but they can also allow solar energy into the house, and of course light and views are important. Doors can provide an excellent way to go in as well as out of the house. Three numbers to look at when considering windows and glazed doors are:

U-factor (or U-value). As previously noted, the inverse of R-value. (In fact, U-factor is what we use in energy modeling; R-value was created as a marketing effort to help consumers understand heat flow.) A code-minimum window in climate zone 6 is U-0.32, equivalent to R-3.1. Pretty lousy, but better than the single-glazed windows in older homes. A Pretty Good window should be U-0.2 or lower, equivalent to R-5.0 or higher. The best windows on the market today are around U-0.09, or R-11. Generally speaking, triple glazed windows are necessary in a zone 6 PGH, but some double-glazed windows may work for some houses.

SHGC is the next most important number, the solar heat gain coefficient. This measures how much solar energy a glazing allows into a house. Insulating blinds help slightly but are vastly less effective than using better windows. Sometimes high-SHGC, or high-gain, windows are desirable, typically in south-facing walls. Other times low-SHGC, or low-gain, windows are better, especially on east- and west-facing windows. The range is from about 0.25 SHGC to 0.8 SHGC. The Energy Star program only recognized low-SHGC windows, but often higher SHGC is desirable.

VT is visible transmission. Sometimes noted as VLT, or visible light transmission, but that term is redundant. Below about VT 0.40, glazing will appear tinted.

There are a variety of coatings and glazing configurations available, but choosing glazing is a process of balancing perfect numbers with what is available in the style and price range you want. Different numbers are desirable in different climates.

Window Options. Operation type is the first big thing to think about. For highest performance, European (or European-style) windows with tilt/turn operation generally offer the highest performance. They have big, beefy frames that sometimes include internal insulation, and the operation is different from what most Americans are familiar with—turn the handle one way and they swing into the room, like a door; turn the handle the other way and the top tilts into the room a few inches, like a hopper. They have multiple gaskets and usually feel like a bank vault door. For American-style windows, there are the traditional double hungs, common since colonial times, and not greatly improved since then—they tend to leak air, especially over time, and while double-glazing is now common, only a few brands offer triple glazing. The window police won’t come after you if you use these on your Pretty Good House, but in general they are best avoided or left to auxiliary spaces like garages and sheds. (Horizontally sliding windows are essentially the same thing, turned on edge, and no better when it comes to efficiency.) Out-swinging windows, sometimes called crank-outs, can be a good alternative, in the form of casements (which hinge outward, like a door) or awnings (hinged at the top, opening out at the bottom) because they don’t impact interior space but seal better than sliding windows because they compress against gaskets when they close, instead of sliding up and down on a track. Only a few brands approach the performance of Euro-style tilt/turns. Since the glazing typically performs better than the frame, one large window performs better than multiple small windows. Consider using fixed windows where possible—not every window needs to open.

Exterior doors can match the windows, or they can be a different brand and type. Often the front entry and garage doors will be different from the windows, but patio, deck or balcony doors will match the windows. Good glazing performs better than even a good frame, so perhaps paradoxically, a fully glazed door is usually better than a solid door. Exterior doors of all types are prone to leaking and should be protected with a roof of some sort.