How To Pick An Energy Efficient Window for a Detroit Home

You're living in Detroit, or in the Detroit area, and you're wondering, "How do I pick a good, energy efficient window?" The answer isn't terribly complicated, but it will require you to get serious about understanding the one standardized and reliable resource for energy efficiency information about the products that local window manufacturers and resellers have on offer: the National Fenestration Rating Council (NFRC) certification label. With this label, you can learn all that you need to know about an window's energy performance.

The NFRC Label: What It Means

The NFRC label (shown to the right) gives a few metrics that are important:

1. U-Factor
   U-Factor measures the effectiveness of the window in keeping warmth or coolness inside your home; it's simply a measurement of the window's overall effectiveness as an insulator. This is the most important value to consult; the lower the number the better. Mathematically, U-Factor is simply the multiplicative inverse of R-value (R-value of 2.0 equals U-factor of .5 (that is, 2/1 becomes 1/2).
   
   
2. Solar Heat Gain Coefficient (SHGC)
   This number tells you how much of the sun's heat is allowed to pass through the glass (both as visible light and as long and short wave radiation) and into your home. In some climates, you want high numbers and in some climates you want low numbers---it depends (and we'll get to the issue for Detroit below).
   
   
3. Visible Transmittance
   This indicates how much visible light passes through the glass, and is meant to indicate whether or not the window will seem clear or slightly dimmer. In some cases, it's good not only to filter out invisible light, but invisible light as well, to keep your interior from taking on too much solar energy (low visible transmittance numbers typically correspond to low SHGC). Also, if you pick a window with a particular spectral shift---a window that tints incoming light toward a particular color---it will reduce your visible transmittance, since you're filtering out some of the gamut. This might well be desirable if you need to match other windows, or if you want a particular mood or feel.
   
   
4. Condensation Resistance 
   Condensation resistance is mostly a function of the window's effectiveness as an insulator (given by U-factor), so it doesn't really help in selecting an energy efficient window. But it's a no-brainer that no one dislikes condensation resistance, so the higher the better.

Energy Efficiency for Detroit's Climate

First, low U-factor is always desirable; it doesn't matter where you live (unless it's always 72 and low humidity where you are ... but that wouldn't be anywhere near metro Detroit). However, the proper SHGC is less obvious. You might be thinking, "Well, I want to gain heat from the sun during the winter, so that my heater doesn't consumer as much energy." Well, that's a perfectly rational point of view, at least prima facie, but it really doesn't work out insofar as we're blessed with very little real sun exposure in the winter. It's always that gross winter gray, and only very occasionally does the sun really shine (and even then, we're far enough north of the equator that the sunlight is very indirect during the winter, and therefore doesn't produce much warmth even when it's beaming). Now, summers in the D are intensely sunny and warm (why'd the French settle here again?), and that means that a high SHGC window will make your home hot during the summer, and will cost you in terms of AC. So the question is, do the savings from the heat gain during the winter offset the cost of the heat gain during the summer? The answer is no. You don't want a high SHGC window in Michigan. You want:

• The lowest possible U-factor for your money (lower than .29)
• Low SHGC (lower than .3)
• Whatever visible transmittance you prefer given the way you use a room, or the color shifts you might like, etc. (approx. 0.5 is typical)
• High condensation resistance is always good, I suppose...

Nota bene: if the window isn't NFRC rated, don't buy it, because you don't know what on Earth you're getting (no matter what your salesperson tells you).

Energy Star Doesn't Matter (For Windows)

Before leaving this topic, it definitely bears pointing out that Energy Star ratings don't help a window manufacturer's case any--Energy Star certification is based on the NFRC's ratings of the window, so the Energy Star label doesn't certify anything above and beyond what you're already getting on the NFRC label. And on top of that, Energy Star certification costs the manufacturer or reseller money both in the certification process and in the application of the Energy Star logo to every window. This results in a higher price (if only marginally) for the consumer. It's a waste, if you ask me. The NFRC does a fine job and Energy Star is adding no value for consumers that---crucially---know what the NFRC label means. And that's you!