The Anatomy of A Residential Window

Since I'm a cheerleader for energy efficiency, I thought I'd go ahead and create a post about the parts of a window, the way that each part works, and how all of it comes together to either succeed or fail in delivering serious thermal efficiency.

Part 1: The Wall

The first thing to understand is how the window sits in your wall; if you'll be installing the window yourself, the graphic below may prove helpful. The most important parts of the framing process concern the forming of the "rough opening" and the "finish opening" within which the new window will sit (see below).

Remember to seal it up right.

These are the primary areas of concern for air leakage, and if there is any doubt at all about the seal, spray these gaps full of foam insulation and cut off the excess. The image to the right shows foam insulation (prior to trimming) in the finish opening, up against the right-side trimmer stud.

If your windows have already been installed, but you've still got an air leakage problem, it is probably worth attempting to get at these openings with a can of spray foam wall insulation to seal up the area around the window.

Part 2: The Window Frame

Next, the window frame itself; this is less important than the sash for energy efficiency's sake, but can still figure in a significant way in the overall thermal effectiveness of your window. The design of the frame is probably the more important than the materials, since airtight, structurally sound frames can be built from uPVC (vinyl), wood, and aluminum. The question is whether the joints are sound and impervious to cracking and breaking. Once a corner joint has cracked, it will very likely become a source of air leakage and cost you month-to-month in your heating bill. There's not an obvious way to tell a good quality joint in a wooden or aluminum window from a bad quality joint, since the joints are mechanical (i.e., they're screwed/bolted/nailed together). However, with a vinyl window, you can tell easily: just see whether the corner joints are welded or mechanically joined. Welded corner joints are just about impossible to break, so you don't have to worry about the stress from the weight of your house forcing the joints apart and letting in the cold (or heat) from outside.

It's also frames made up of multiple closed chambers, which each act as a layer of insulation, since they contain a sealed pocket of air. This is much more effective insulation than a solid piece of uPVC.

Part 3: The Sash

The sash is the most important part of the window where energy efficiency is concerned. The sash contains the pane(s) of glass ("glazing(s)"), and thus ultimately controls energy input from the sun as well as most of the energy loss through glass and air leakage. First, you (obviously) want double or triple glazing windows---although the argument for three panes isn't as strong as it might at first seem, where energy efficiency-per-dollar is concerned.

Holding the two panes apart is a spacer, which must create a powerful seal to keep the gas between the panes (usually argon) of the window isolated so that it acts as insulation, and so that it stays free of moisture (otherwise, you'll get condensation between the glass, which is a bummer).

Next, you want to make sure you're getting the right sort of film coatings (or suspended film layers) for our heating climate, here in Metro Detroit. You want a Low-E layer, which helps to reflect radiant heat energy back into your home when it encounters the glass. This substantially improves the window's U-factor. Solar heat gain (SHGC) is not a number you need to worry about in this climate, since any heat gain from the sun during the summer that you have to counteract with AC will be offset by solar heat gain during the fall, winter and spring which reduce the need for heating.

Now, notice that I said you don't need to worry about solar control; I didn't say that you wouldn't want to fuss about it, since you can use solar control layers to achieve (or match) particular color temperatures of incoming light. If you have windows that dim incoming light somewhat, it will also affect the color temperature of the light, if only slightly. Having one window which looks greenish blue, and a number of others than lean yellow can create a lousy aesthetic. Make sure it's consistent.

Last---use good weatherstripping, to ensure that air isn't leaking in around the sash when it's closed.