Windows. The bane of the building scientist's existence. As we try to control the flow of moisture, air, and heat, a lot of our problems occur at windows. If the flashing's not done properly, they leak water. If they're not sealed around the edges, they leak air. And because they're windows, they leak heat.
Sean Lintow got me to thinking about this with his recent article on windows, the latest entry in his Building Science for Dummies series. He did a great job explaining the National Fenestration Rating Council (NFRC) window labels. I learned something new about window ratings from his article, too. I had no idea that there was a Condensation Resistance (CR) value for windows. I've never seen a window label that had it, but it does exist.
Anyway, back to the title of this article, a good window is a poor wall. It's a common saying in building science circles. Why? Because it's more likely to leak moisture, air, and heat than the wall sections surrounding it. Let's imagine the installer did a great job with the flashing and air-sealing, though, and just look at how much heat leaks in through windows.
Heat conduction through windows
Heat comes in through windows in two ways: conduction and radiation. Heat comes in through walls in one way: conduction. (Yeah, all three processes are going on, but we can describe the overall heat transfer process with the conduction equation, Q=UxAxΔT.) The NFRC window label (as shown here) tells you the two relevant numbers for conduction and radiation through windows: the U-value, which tells how conductive a material or assembly is, and the Solar Heat Gain Coefficient (SHGC), which tells...well, that's one obvious, right? It tells how much heat gain you get from the Sun shining directly through the window.
The U-value for a typical double-pane, low-e window is about 0.3. Since the U-value is the reciprocal of the R-value, that yields an R-value of about 3. In my part of the world (Southeastern US, IECC climate zone 3, and Georgia, specifically), I'd have to put in a window that has a U-value of 0.5 or better.
Better, you say? Why yes, I did say better, and that means lower. Since U-value measures how conductive a material or assembly is to heat flow, the lower the better.
OK, so a window with a U-value of 0.5 has an R-value of 1 ÷ 0.5 = 2. Yep. Here in almost the whole state of Georgia, we can put in windows that have an R-value of 2. And walls, you ask? Why, our Georgia energy code requires us to put R-13 insulation in our walls.
Up in what we affectionately call the "pig of Georgia,"* because of the shape of the counties that make up IECC climate zone 4, window U-values have to be 0.35 or lower. That's an R-value of ~2.9. Not a whole lot better than the 0.5 U-value required here in Atlanta when compared to the surrounding R-13 walls.
A really good window might have a U-value down around 0.2. That equates to an R-value of 5, still poor in comparison to R-13. Zola Windows, which makes European-style windows aimed at superinsulated buildings like Passive House projects, has windows with U-values as low as 0.123, which comes out to R-8.1. Serious Materials makes a window with U-0.11/R-9.1. In Passive House projects, those windows go into homes that have walls that are about R-60 or so.
Again, a good window is still a poor wall.
Let the Sun shine
The other type of heat that flows through windows is the direct solar gain of the Sun shining through. This can be a good thing or a bad thing. The Solar Heat Gain Coefficient is a number between 0 and 1. An SHGC of 0 means you get no solar heat gain, as with a wall. An SHGC of 1 is the most you can get through a window.
Unlike with U-values, however, lower doesn't always mean better. Sometimes you want solar gain. In a passive solar design, where you use overhangs to shade the windows in summer and allow the sunshine to come in during the heating season. That's why in a Passive House (or Passivhaus, to stick to its German origins), the window R-value being so much worse than the wall R-value won't necessarily be a bad thing. If the window helps with winter heating by capturing heat from the Sun, that can overcome the R-value deficiency. Martin Holladay wrote a great article at Green Building Advisor about windows that perform better than walls, and goes into the details of what kind of windows to use and which sides of the house to put them on.
In cold climates, you want all the solar heat gain you can get, so higher SHGC windows are good. The warmer the climate, though, the more important a low SHGC becomes. If you do a significant amount of air conditioning, you want low SHGC. In hot climates, that's even more important than low U-value.
The takeaway here is that solar gain may help, but you'll either have to be in a cold climate without much cooling load or have a good design to take advantage of high SHGC windows.
What should you do?
If you're an engineer and other energy efficiency extremist, it may seem that the only thing you can do that makes any sense, if you look solely at the liability windows present, is to eliminate all windows from a house.
Well, if you do a really nice passive solar design, get all the flashing details right, and do all your air-sealing, windows can actually help. The Passivhaus program is pushing the envelope here, so to speak. Even then, though, you don't want to overdo it on the windows. Keep the relative amount of window area low. A window-to-floor area ratio less than 15% is good. The green home that I built, the passive solar home shown above, has a window-to-floor area ratio of 12%.
For the majority of homes, though, I'll stick with my original statement:
A good window is still a poor wall.
*Actually, the only person I've heard call it that is Scott Lee, one of my former colleagues at Southface, but maybe we shouldn't call it that because of a certain movie reference** that the folks from Rabun County have a great distaste for.
**OK, if you really haven't figured it out, think Ned Beatty. Think Burt Reynolds and canoes and banjoes. Got it now? It's one of the most famous scenes in the movie Deliverance, a movie reviled by the good folks who live in the north Georgia mountains.
Photo of windowless house by dctim1 from flickr.com, used under a Creative Commons license.