They are everywhere, the full page ads with smiling people proud of how much money they saved and how they qualified for all those government grants. But what did they really save? Perhaps a bit of energy, but how much?
I thoroughly enjoyed reading David Owen's book Green Metropolis on the weekend, where he concludes that everything we are doing to go green is pretty much a drop in the bucket unless we live smaller, live closer and drive less.
One of the issues he covers is that of window replacement, a subject dear to my heart.
Windows are rated by their U-value; the lower the U, the better the window insulation. Owen quotes the National Fenestration Rating Council, explaining why U is used rather than R, the standard for insulation. The NFRC writes in their facts about windows and heat loss: (from PDF Here)
Windows are very different from insulation in walls and ceilings. Windows let the light in and allow people to see out, and they interact with their environment in ways that insulation does not. They react to outside air temperatures, sunlight, and wind, as well as indoor air temperatures and occupant use. Windows are strongly affected by solar radiation and the airflow around them. R-value does not accurately reflect this interaction. Therefore, the window industry measures the energy efficiency of their products in terms of thermal transmission, or U-factor.
Um, not really. R value measures exactly the same thing as U value.
The R value of a material is the multiple of the K factor (the number of BTUs that pass through a square foot of material, one inch thick, with a 1 degree difference in temperature between each side in one hour) divided by the thickness. When you calculate the R value of a wall, you total everything- the surface air layer, gaps, vapor barrier and insulation.
The U value is the reciprocal of the R value, period. Why use U instead of R? David Owen suggests that people understand R value, and "the R values of even the most energy efficient high-tech windows would seem pretty measly to people trying to decide whether such windows are worth the considerable extra cost." So the U value just confuses. But it does make it easier to actually calculate out the savings, which are, as Owen says, measly.
Lets do the math on a window; An Energy Star window for northern USA has U value of .30 or lower; that is an R value of 1/.30 or R 3.333. A single glazed window without a storm hits about R-1.2, and with a storm, about R-2. Let us look at a 3'x5' window and the heating season in Philadelphia, with a total heating degree days of 5126. (the number of degrees below 65, used in calculating total heating costs. More on degree days here)
The savings with a window that probably costs a thousand bucks: $12.40. Not much of a return on investment. (Note that I do not take cooling into account, so the number could be higher, particularly if the replacement windows have low transmission coatings) Putting on a storm window is a lot more effective (a modern interior magnetic storm window is probably a hundred bucks, a tenth the price.)
I am not writing this to say that people shouldn't replace their windows; I am just saying that in the end, do the easy stuff like caulking and insulating your attic first. Get a more efficient heating system. Turn down the thermostat. But in the end, David Owen is right; the only things that will really make a significant difference is if we live smaller, live closer and drive less. All of this is just, literally, window dressing.
More on Window Replacement
"Weatherization" Window Salesmen Overselling Savings And Wrecking Historic Buildings
Noooo Edinburgh, Don't Lift Ban on Changing Windows in Historic Structures
Spend your Weatherization Money Wisely
Obama To Announce "Cash For Caulkers" Program