I just had another window salesman knock on my door and tell me that switching out my windows can save me 40% to 50% on my energy bills. When I pressed him, he said that's for switching out metal double pane windows with his high efficiency triple pane ones here in Texas.  He promised much higher savings when it comes to switching out my single pane metal ones.


I've modeled a lot of buildings for the Weatherization Assistance Program using the NEAT tool and windows have never ranked.  Even Krigger and Dorsi say that windows have a long pay back period and are probably the last things you should do.


I have this conversation all the time with my wealthier client base.  They always seem to have tight, double paned windows and want to switch them out before doing any air sealing or insulation. I end up talking them down.  And it seems like the windows guys have an evangelical zeal to convert energy auditors to their way of thinking.


I'd like to see the research.  Do you know of any studies on switching out windows?  I'm specifically looking for switching out double paned metal windows that are fairly tight.

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The unfounded and excessive claims of window salesmen/women are responsible for the removal of the historic fabric of buildings around the country.  The payback period for window replacement can be many decades in moderate climates.  There are many tools for improving energy efficiency of historic buildings without removing original windows - including repair, caulk and paint, interior window coverings, storm windows and doors, and selective window sash replacement.


Hi Joel,

First, I hear ya...(eyes rolling :>) Amazingly, I heard through the grapevine similar about a state weatherization program doing just that too (it was NOT in Vermont however).  About a year ago I was at a house conducting IR training that had brand new exterior doors, weather stripping and all, installed as part of "energy upgrades" and yet the building leaked like a sieve.  Air sealing had not been addressed.  The doors looked great, though!


Building Science Corporation might have some data on this.  I'd recommend taking a moment to view their site, www.buildingscience.com


A great resource with many articles available for download.  You might find something there.


It seems to me, almost everyone has this all wrong. The energy savings claimed by window salesmen is crap. Their claims are loosely based upon the enegry loss for the area of the window only not the building as a whole and the payback is longer than the life of the windows. Replacement of good historic windows is a horrible thing to do. However, no one seems to address the reality that the payback, should be affected by the value improvement of the home. A house with great new windows is worth more than a house with old crappy windows. That helps pay for the new windows and they will often help with energy efficiancy as well. I try to look at the whole picture and the home value is part of that picture.
Wouldn’t it be great if home energy geeks were as good at sales as window salesmen? The fact is that they are in sales, not efficiency, and may not even know what U-factor is. They may not even know how best to install a window for durability (perimeter flashing, etc) and efficiency (air sealing is critical).

Math is the best proof: U factor x Area x heating degree days x 24 for heating season losses. SHGC is just as critical to reduce AC for cooling climates and to turn a net energy loser into a net gain for cold climates. Tune window specs by orientation – high SHGC on south and west, low on north and east sides (generally – of course it all depends).

Here’s an example: U.40 x 200 sq ft x 5,000 HDD x 24 = 8.4 MMBtu/yr lost through windows
Now upgrade to an R5 window:
U.20 x 200 sq ft x 5,000 HDD x 24 =4.8 MMBtu/yr lost through windows
43% savings from window upgrade, for windows only, NOT whole house energy savings (used as a confusion tactic)
Cost: $12,000
Lifetime: 25 years
Lifetime savings: 90MMBtu
If you’re heating with gas at $20/mmbtu, you’ve saved $1800 over 25 years.
Yes, you save energy. Is it the best place to put your efficiency dollars? Probably not, at least not the first $12k you spend.

That said, I put in a triple pane window in my own home where I knew it would never pay financially, but the comfort payback was immediate. Money isn’t the only reason to do things.
it is important to remember that although we all know that changing out windows rarely provides a reasonable payback in energy savings, it is extremely cost effective to upgrade to high-performance windows if you are going be replacing the windows anyway.  Often in older residential buildings, the windows have failed; cracked or otherwise leaky sashes invite air leakage, after many years sub standard installations relying on caulk start allowing water intrusion, irreplaceable weather-stripping between the frame and sash has deteriorated.  These are valid reasons to replace windows.  When doing so it is virtually always cost-effective to go to a much higher-performing fenestration product than the norm (or code minimum).  This is not an argument to replace the windows for energy savings, but to upgrade to really good windows when you are replacing them for other reasons.
Thank you so much for posting this. I understand it, but, had a really difficult time explaining it to a client who was told by a window salesman that she would be stupid not to replace all of her windows. Thankfully she thought to call me before spending a ton of money with him. I told her if she was doing it for aesthetics, go for it, but as far as payback, she probably wouldn't see it for a very long time. I just sent her your math and she was thrilled! THANK YOU! She is going to get new windows that look good and are efficient, but not the crap ones the salesman wanted her to buy. AND she is having them installed properly.

There are a lot of things about windows that need to be considered before you replace them. If you are looking for a quick payback, immediate savings, you won't get it with windows. Paybacks are on the order of 20 - 30 years (depending on costs - windows and utilities) and if you use air conditioning. These are not the paybacks investment conscious folks are looking for, it is even hard to sell them a 5-year payback because that is too long. However windows will pay for themselves over their lives, unless you believe, as I was told by DOE that windows don't last that long, a 20-year EUL is max for WAP ARRA calcs apparently. Anyone know how long a window lasts? AAMA says 50 - 60 years if maintained properly - yet DOE says 20 years max. When pressed they told me that is because that is what the NEAT audit software uses, 20 years. Good reasoning, not building science though! Apparently the thermally-inept, BTU-sucking, moisture-condensing, mold-growing, sill-rotting, moderate-leaky, single-pane, 50-150 year old windows I see all the time don't last that long. There are ways to replace really old windows with high performance windows and still maintain the historic integrity of a house and not force.

In energy auditing programs like NEAT only the U-value and maybe the SHGC counts in the energy savings calculations. New windows are also much tighter and reduce infiltration which needs to be included in the SIR calculations to get the full energy benefits yet I've never seen a study as to how much infiltration they reduce, a lot of estimates and no hard studies. And who does pre and post blower door (pressure diagnostics) on a window replacement job to determine the infiltration reduction value new windows have. So give windows their due and add the infiltration reduction potential into your NEAT calcs and see what you get. All the non-energy benefits are great too, but those sell windows too.

If you check out the LBL Home Energy Pro on-line energy audit program they show that replacing old windows with high-performance windows is very cost-effective (I guess the folks who put that together didn't talk to the window guys) and very misleading. They based the savings calcs and the payback on the incremental cost of a high performance window versus a standard replacement window. Which is fine if you are building a new house or if you replacing your windows anyway, which I guess is what people do when their windows die at 20 years.

Nehemiah do you have any money for a study on infiltration reduction and window longevity? I'd love to follow window replacement crews around for a while doing pre and post pressure diagnostics. Darn, I forgot, window replacement sales have dropped off because folks can't borrow money easily anymore for their $1,000 per window replacements. 

Windows will pencil out in many cases if DOE actually follows the congressional mandate for weatherization funds which state that measures have to pay for themselves over their lives and not their imposed 20-year max for measures in the NEAT audit software. In the 60 years my windows have existed they would have done that at least twice.


How much a window leaks is dependant on how leaky the home is. If you have not sealed the rest of the house then the windows will leak more....affected by stack affect.


Also the air that leaks in the windows is cleaner than air leaking through other places so it is not the first leak to get rid of.


Tuning up a window can greatly reduce the air lekage through the window and is mouch more cost effective.


If you are talking about comfort and energy savings that $12,000 will go a lot farther if a priority list is created and those steps are followed. An old house with minimal insulation can be air sealed and insulated for much less than the $12,000.


Like one of the post said we are salesmen. We need to selll people on the immediate benefit of comfort AND the increased vallue of the home. Buyers will pay more when they see it is cheeper to operate.


As far are replacing windows I tell people it is done for something other than energy efficiency. 

DOE models lots of weatherization measures (for financial cost-benefit analysis) at 20 years life ONLY because of the peculiar mathematics of calculating Savings-to-Investment Ratios and discount factors. When you use a 3% discount rate, there's hardly any financial NPV return 21 years down the road, so they don't count it. The window could last another 30 years, but the financial return is negligible (in the peculiar world of cost-benefit analysis.)

The problem with this is that windows are just so freakin' expensive, because we expact so darn much from them. Light, air, security, operability -- it's a nightmare. It would be VERY ciost-effective to jerk out the window and build solid, insulated wall. But who'd pay for that??

So, the window guy is just a sleaze when he says the utility bills will go down enough to cover the whole cost. But windows do lots more that control energy, so people need to price the other functions they are buying. 

As for infiltration -- we (Wisconsin MF Wx program) have done a little work on measuring window leakage directly (a la ASTM E783) with poly sheeting and a Duct Blaster. I've only done this in a few apartment buildings, but it's not technically beyond the capability of a relatively smart HERS rater. Just gotta have the willingness to invest the time and experiment on something new and interesting -- and you know how we all hate that!!  

But if you measure some existing windows, and compare that number to the maximum 0.30 CFM50 allowed for new Energy Star windows, you can make a reasonable guess at the real energy savings from a window replace. They are better than 30 to 50 year simple payback, but not by much -- maybe 20 to 30 years!

All I can say is -- in the last four years, we've put $10K in energy improvements into our 100 year-old farmhouse. The original windows (at least 50 years old??) are still there, and we'll do lots more work on other things before we get that far down the list. There's lots of other things to do that will save more energy and money. Mama didn't raise no fool.

I agree that windows should usually be the last thing done unless there are other reasons for doing them such as asthetics or to increase functionality.  Many new window inserts are coming on the market like the ones from Indow Windows: http://indowwindows.com/ which are much more cost effective for older windows especially with the Lead abatement problems we have now.
I haven't had a chance to talk with you since the 1990's. Good to know that you are still providing advice on energy savings options in fenestration. For those who don't know him, in the late 1980's and early 1990's, Steve taught lots of us how to understand heat flows through window systems, and was critical to the launch of the National Fenestration Rating Council.

Does anyone have any data or info (anecdotal or otherwise) on the benefits of adding storms to existing windows? I've heard anecdotally that well-sealed storms provide a bigger air gap, and thus a higher R-value for the whole assembly.


I've seen some beautiful wood storms with excellent gaskets which are an attractive retro for older homes, at a fraction of the cost of all-new windows.


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