Measure window efficiency
Simple steps to tell if your windows really are energy efficient
You can easily measure window efficiency with a probe thermometer, or better yet, an infrared thermometer. By taking readings of the indoor and outdoor ambient and window glass temperatures of your windows - ideally on a very cold day - you can determine which of your windows are most in need of replacement.
This process gives you a relative view of window efficiency. It's hard to get an absolute view to determine how efficient your windows are unless you buy specialized equipment. But before you do the temperature readings to measure window efficiency, start by looking for obvious signs of inefficiency such as:
- A single paned window or one where the glass is held in place with putty
- A single paned window with a separate storm window
- Cracked glass, indicating that air may be leaking right through the glass
- Aluminium framed windows, since heat travels through the aluminum frame to the outside (vinyl or wood framed windows are generally more efficient)
- Any window whose condition suggests it is older than 20 years
- Excessive condensation on the glass or at the edges of the glass
Any of the above signs suggest you are losing a lot of energy through your windows. You should seriously consider replacing windows that have these problems. You can still measure window efficiency for these windows, by comparing the inside and outside glass temperatures, but the readings will almost certainly confirm the need for you to replace the windows.
Pen, paper and a thermometer
To measure window efficiency, have a pen and a pad of paper handy, and a digital thermometer or infrared heat gun.
Digital thermometer: A digital thermometer with a probe - either wireless or connected by a length of wire - is a more time-consuming way to measure window efficiency than an infrared heat gun, but it's also more accurate. First, tape the probe to the inside of the window, centered on the glass, and leave it in place long enough for the warmth from your hands to dissipate - a minute or two - then take a reading of the glass temperature. Take readings from all the windows you can (so you can determine which windows are least or most efficient). Also take a reading of the indoor air temperature, or the temperature of an interior wall or piece of furniture. Then go outside and do the same window temperature measurement on the outside of the glass, noting the temperature reading. Once the outdoor glass reading is done, take an outdoor ambient temperature reading, either of the air or of a part of your house that is isolated from a heat source (for example, the steps of a porch).
When you measure window efficiency with a probe thermometer you may have trouble getting the tape to stick, on the inside because of condensation on the glass, and on the outside because cold temperatures tend to make tape not work well. Another option is to hold the digital thermometer probe in place using a piece of blue or pink insulating foam, since this will insulate the probe from your body heat.
Infrared heat gun: An infrared heat gun makes it much simpler to measure window efficiency. I took some readings at my house last night, since it happened to be an extremely cold night, ahead of a major Groundhog Day snowstorm. I measured the indoor and outdoor temperatures of about ten windows on my ground floor and basement, in about 20 minutes. I pointed my infrared heat gun at each window indoors and wrote down the temperature reading, then also took a temperature reading of an interior wall (which gives an indication of ambient temperature). Then I went outside and took similar readings on the outside of each window, and readings of outdoor temperature on one of the brick posts that support my front porch.
Here are the results I found for each of the windows or doors I measured. I've excluded the upstairs windows, since I later determined that with a beam ratio of 1:12 (meaning, for every foot of distance, the infrared heat gun measures an extra inch of width) some of my outdoor upstairs measurements probably included a fair bit of wall as well as window glass, and were therefore incorrect readings. Also, I have consolidated a number of individual windows together; for example my 'front window' is actually five separate windows all in a row.
I have replaced all windows in my house since we bought the house in 1997; I've indicated the year of replacement for each one.
Ground floor:
Rear garden doors: +13°C to -17°C (2010)
Rear windows: +14°C to -19°C (2010)
West side window: +16°C to -20°C (2010)
East side window: +14°C to -18°C (2004)
Front windows: +13°C to -12°C (2004)Basement:
Basement windows: -20°C to +9°C (1999)
The outdoor temperature was about -23°C, and the indoor temperature on the ground floor was about +17°C for a total 40°C temperature spread. The basement was slightly cooler at 15°C.
The interesting point to note when you measure window efficiency is the relative positions of indoor ambient temperature, indoor glass, outdoor glass, and outdoor ambient temperature. The closer to the ambient temperature each side of the glass is, the more efficient the windows are. From the following chart you can see the temperature spread between the indoor and outdoor surfaces of window glass in green; the bigger the spread, the better the job the window is doing at keeping heat from leaking through the glass.
You'll notice that in most cases, the gap between indoor ambient and indoor glass is smaller than the gap between outdoor ambient and outdoor glass; in one case, the front windows, the outdoor glass temperature is a full 11°C warmer than the ambient outdoor temperature, while the indoor gap is only 4°C. I don't have a foolproof explanation for this, but it should be noted that my front porch is exposed to winds from the north as it faces northeast, and the wind may have been particularly effective at whisking heat away from that window that night. The side windows are somewhat sheltered by the neighbor's house, and the rear windows face southwest, again somewhat sheltered from the cold north wind. But that's just a guess.
Tackle the worst windows first
Once you have good readings on each of your windows, and you know which ones have the smallest temperature spread between indoor and outdoor glass temperatures, you know which windows you should order energy efficient replacement windows for first. Of course, if you don't have the budget to replace any windows this year, or too many of your windows have very poor insulating ability, you can use plastic window insulation to improve the insulation value of existing windows as well as cut down on drafts.
My rough guess is that plastic window insulation applied to the oak framing around my front window would increase the gap between the indoor glazing temperature (in this case, the plastic) and the outdoor glass temperature by an additional 4-6 degrees, enough to make that window at least on a par with the other windows I measured. But it looks like I'll be replacing these front windows soon, in spite of their being a mere 6-7 years old even now.
When replacing windows, I recommend buying the most energy efficient windows you can afford. It's better to replace a small number of windows at a time with very efficient ones, than to replace all your windows at once with only moderately efficient ones. Better yet, considering that R-5 rated windows can have a payback of under 3 years - meaning that within three years of buying them, you'll have cut your energy bills by more than the cost of the new windows - you should try if possible to limit your choice to R-5 windows. The Windows Volume Purchase program is a US Department of Energy sponsored program that you can use to obtain discounts of up to 20% on R-5 windows if you buy a certain volume at once (15 windows for a retrofit to an existing home, or 20 windows for new home construction). Either you or your general contractor can take advantage of this option.
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