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Interior storms on newer (1980) casement windows worth it?

Solitas | Posted in Energy Efficiency and Durability on


My wife and I moved into a ~1980 house in September 2019 in the Twin Cities, MN.  The house has double-pane casement windows that appear to be original.

I’ve spent hours looking for information on whether it’s still a good idea (i.e. “worth it” to do something like interior storm windows… all the articles are addressing the ~100 year-old single pane double-hung windows.

Generally speaking, is there a reasonable ROI for installing interior or exterior storm windows on a more recent window?

As far as we can tell, the house is well constructed using best practices and quality materials available at the time.

If there’s other information I can/should provide please just let me know.

Thanks much!

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  1. 730d | | #1

    Hi Timothy, I am in the Twin Cities also. I don't know if they would pay back. When I was a young man in the business forty + years ago I was always trying to sell energy upgrades. I could never sell them with a financial return argument. Offered them below cost. Frustrating.
    Than one day I emphasized they would be more comfortable. Did it all the time after that.
    I can say you will be more comfortable.

    1. Solitas | | #2

      Hi Mike! Great to "meet" someone on here from the same area.

      I really appreciate the insight, and it's a good point. It was needing to move a space heater into my daughter's cold bedroom that originally got me thinking of doing interior storms. Maybe it is worth scaling the project back from "every window in the house" to "windows in the rooms that tend to get too cold / hot."

      I'm thinking of DIY'ing it by purchasing 1/8" continuous cast acrylic sheets and a custom run of white silicone bulb gasket around the outer edge (wife's requirement is that it is white... can't find this standard anywhere). Cut everything to size and put the gasket on the edge. Press into the frame for picture windows and for casement windows replace the screens with the storms. See any issues there? I'll need to watch for condensation and potentially deal with that.

  2. walta100 | | #3

    Let admit there is some risk with your interior storm plan. Depending on your indoor humidity and how well your storms seal. If your storm window idea works the glass of the original window will be cooler and more likely to fall below the dew point of the air in your home. Should any air get past the storm the original windows will get wet and likely have water running off the glass. Where will the water go? What will the water damage? Will it be wet and warm enough long enough for mold to grow?

    I am not saying not to try just to consider and understand the risk.

    If the real problem is a cold bedroom get an energy audit with infrared photography and a blower door test. My guess is finding and fixing missing insulation and drafts will make the room more comfortable than storm windows will.


  3. GBA Editor
    Brian Pontolilo | | #4

    Hi Timothy,

    Storm windows, interior or exterior, can improve performance, if built and installed well. But I tend to agree with Walter: I would start with an energy audit. If your house is leaky or thermally deficient, you may not improve comfort much with the storms until first taking care of those issues, particularly if you have a leaky lid. How's the attic air sealing and insulation?

  4. PAUL KUENN | | #5

    Hate to say, there were No good practices in the 80s. Go with interior storms. It will be the best $80 you'll spend. That's 12-14 good beers. We love ours. You can sit next to the windows at below zero (F) temps and never feel a thing. Our favorite is with Eugene in SW Wisconsin at:
    We did it to our big bay window and patio doors. No air leakage so no moisture issues. In my deep energy retrofits, I've talked many customers into trying and they all love me for it. Life should be comfortable.
    Don't talk about "payback" unless you can explain why you buy a car that loses 500% of it's value when it leaves the lot.

    1. Expert Member
      Dana Dorsett | | #6

      >"Don't talk about "payback" unless you can explain why you buy a car that loses 500% of it's value when it leaves the lot."

      It's still worth a positive value when it leaves the lot, unless you run over somebody in your haste to get away from the car sales-droid. To lose 500% of it's value you would be owing somebody 4x the price paid, only negative residual value.

      Maybe that was meant to read ".. loses 50% of it's value..." ?

      Payback on windows is almost never financial but they often "pay back" in comfort.

      The upcharge for low-E glass on an exterior storm window "pays back", in reasonably short years financially but not the full installed cost of the storm window- only the cost of the glass upgrade.

  5. Expert Member
    KOHTA UENO | | #7

    If you had an energy model to play with, you can check out the effect of storm windows (either low E or not low E) on window U-factors in the table below; I believe it is data from LBNL. From that you can get a handle on the cost effectiveness.

    I have been playing with a very similar situation--circa 1990s vinyl-frame double glazed (clear) replacement windows in a 1910 building, and adding an interior storm in a window that is seldom used (in the pantry, over the washing machine). The unit is a low E storm from Innerglass ( It experiences condensation on cold days--the frosting you see in the second image is on a 2 F day in the Boston area. I've been keeping an eye on the window for mold growth, damage, failing IGU seals... nothing so far, and it has been in service since 2017.

    BuildingGreen (specifically, Peter Yost) did a long-form article on upgrading existing windows. One point I remember is concerns that an interior storm will increase the glazing unit temperature, increasing risks of damaging the seals. The article here is behind a paywall, though. A relevant excerpt here:

    Interior “storm” windows: Fixed interior window panels (widely called “interior storm windows” even though they do not protect windows from the weather) offer unobtrusive high performance starting at a relatively low price and are especially popular in places where exterior storm windows won’t work—in historic homes, condominiums, long-term rental properties, and on dangerously high windows. They are made of plastic or glass—sometimes even low-e glass, which costs more—and come with different framing materials (most commonly aluminum). Some have operable parts and can remain in place year-round, but most have to be removed seasonally; this is the main drawback of interior panels, as they can block emergency exits through the window and also require space for safe storage. Plexiglas or Lexan panels can also scratch or fog more easily than glass, although manufacturers have largely addressed these problems—if you follow their cleaning instructions. According to LBNL research, low-e interior window panels offer a number of benefits when properly installed. They improve R-value by about 2, increase comfort by creating a barrier between the cold window glass and the room, and do a pretty good job of sealing air leaks; the tight seal on the interior also helps reduce condensation by preventing warm, moist indoor air from entering the window assembly and being chilled by the air near the cold glass. These panels also block outdoor noise very effectively, if they are airtight. Low-e versions have a radiant barrier that further reduces heat loss, bringing the entire window assembly almost up to par with low-e double-pane replacement windows.

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