Replacement Windows Redux

by Erik North on May 22, 2011

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Replacement windows are on many homeowners’ minds. But should you replace your windows? What if your windows are brand-new? What if your windows are older? Do repalcement windows help save money on your home’s heating costs? Does window replacement contribute to energy efficiency?

One of my first posts was on replacement windows. Should I replace my windows? This is the MOST COMMON question I get from customers, followed closely by “When I replace my windows, that will save money, right?’ and “we’ve decided to replace our windows and just need advice on the color.”

There are many reasons to replace your windows. Rotted sills and framing that would be more expensive to fix than a new window or making an aesthetic improvement to the house. Or hey, maybe you just have $15000 to blow.

However there’s a persistent belief that as a customer said, “once my windows are replaced, I’ll save about half my heating bill”. And that’s simply not true.

Yes, windows are the weakest point in the thermal envelope. But they are just one piece of your home’s efficiency and there are more effective uses of your money.

The Perception of Replacement Windows

Why do many folks have this foggy idea that replacing windows will fix all your heat loss problems? I’d say a few reasons; homeowner intuition, misconceptions about their house and marketing taking advantage of both.

Most people understand that big holes in the side of your house, covered with glass, are going to lose a lot of heat. A giant bay window, no matter how gorgeous the view, intuitively seems like an enormous source of heat loss.

Drafts also make windows seem like an exaggerated source of heat loss. With a stiff nor-easter rattling the glass panes, it is like a neon ‘FIX ME’ sign is affixed to each window.

Next is what I call the ‘Lemon House’ misconception: Everyone thinks they bought the ‘lemon’ house. The heating system must be shot, the insulation is terrible and, yes, the windows MUST be awful and need replacing.

Marketing drives the final point. Replacement windows are very high ticket items that generate high profit margins for the companies that distribute and install them. Ever noticed that there aren’t national advertising campaigns featuring Jessica Simpson expounding the virtues of caulking. Why is that? Because $6 of caulking would cover most of your house while $6 worth of replacement window would cover the cost of driving the delivery truck 500 feet.

Poor heating system efficiency, poor attic and wall insulation or warm air leaking out of the house aren’t OBVIOUS problems. Leaky windows are obvious problems.

Some Boring Math

Why do energy auditors rarely recommend replacement windows? Simple…because it’s a bad return on investment. Let’s break down the numbers (ignoring the collective groan).

The formula for annualized heat loss across a buildings thermal barrier is:

Heat loss (in BTUs) = Total surface area (square feet) * Heating Degree Days * 24 / R-Value

Simple enough…assume a 5′ by 3′ window with an R-Value of 2 in Portland, Maine (so Heating Degree Days around 6500). As an aside, window heat resistance is labeled as U-Value. U-Value is simply the inverse of R-Value. If the U-Value is .33, the window’s equivalent R-Value would be 3. Why do windows report thermal resistance this way? Probably because it would tick homeowners off if they had hard numbers showing that windows were letting out 5 times as much heat as the walls they are installed in.

The window heat loss calculation is 15 sq ft * 6500 HDD * 24 / 2 R-Value = 1170000 BTUs. Let’s make that easier to understand, dividing it by the BTU content of a gallon of oil (138000 BTUs). We arrive at 8.5 gallons of oil heat loss equivalents. We’ll just ignore heating system efficiency for the moment, meaning at $4 per gallon the window is losing around $35 per year. Not exactly worth breaking the bank.

But wait! My windows are really leaky, I feel cold drafts all winter….I must be losing a lot of heat through air leaks, right? Not as much as you may think. Blower doors help auditors measure exactly how much heat is being lost from your home by air leakage. Assume a 500 CFM (cubic feet per minute of air moving through the fan) reduction of home air leakage when we test the house after installing 10 new windows. I’ll spare you the calculations (they’re much more complicated this time)…500 CFM is equal to approximately 4790000 BTUs or 34.7 gallons of oil heat equivalents.

So You’ve Installed Replacement Windows

You’ve installed fantastic fiberglass framed, triple-pane argon-filled replacement windows with an R-5 rating. Because you ‘know a guy’, the price was a bargain basement $600 per window and they did such a phenomenal job with the installation it completely eliminated all air leaks. Each window’s thermal heat loss is reduced to 3.4 gallons of oil (just recalculate the solid surface formula above with an R-Value of 5) and air leakage is reduced to 0.

For your investment, you’ve saved a bit over 8.5 gallons of oil per window (8.5 gallons – 3.4 gallons + 34.7 gallons for the whole house divided by 10 windows).


You’ve invested $600 in save $35! Your investment will pay back in just over 17 years.

What to do about your windows?

If your windows are truly in rough shape, then replace them. Also if you have $15000 laying around that would be a perfectly passable reason to replace your windows.

Like I noted in the previous post, if you do replace your windows, don’t use vinyl framed ones. The framing for vinyl replacement windows is a petroleum derived plastic and the energy, oil and pollutants generated by their manufacture outweigh any savings benefits. Wood framed windows have much lower embodied energy than their vinyl equivalents.

Windows are a significant source of heat loss but the standard replacement windows only produce marginal improvements. More advanced windows achieve better results but the cost lowers the return on investment. Alternatives like interior storm inserts or, yes, the cheapo plastic films are a much better return on your money.

The cost of significantly improving window efficiency is prohibitive and should be evaluated carefully. There are excellent windows available but unless your current windows are in dire condition, there are better investments in efficiency to be found in your home.

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