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Is a 2-layer 3M window insulator worth the extra trouble?

x95 | Posted in Energy Efficiency and Durability on

I have 2005 Simonton Builder windows — dual pane, E-coated, Energy Start etc.

The builder put 2 ,side by side, single hung windows in a small bedroom. That’s a 6′ x 6′ opening. The bedroom has 3 exterior walls.

The room gets cold. The window seals leak air, and I’ve tried everything I can to plug the leaks.

I want to try the 3M window shrink film. I noticed some people make a wood frame and put the plastic on both sides, making a trapped air space. Is it worth the extra trouble to to have 2 films versus one?

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  1. GBA Editor
    Martin Holladay | | #1

    In almost all areas of the country, it is illegal to install single-pane windows. I'm surprised that your builder chose them.

    Installing two layers of window film is not recommended and won't provide good results. It you can afford the work, you should have new double-pane windows installed. There is probably some type of problem with the weatherstripping on your existing windows, but it probably isn't worth paying someone to fix the windows, especially if they have single-pane glass.

  2. Expert Member
    Dana Dorsett | | #2

    Methinks Martin mis-read "...single hung..." as "...single pane...".

    The U-factor of an circa 2005 Energy Star low-E double pane varies by region, but is probably between U0.28 and U0.33 for most. Adding the shrink-wrap would knock that back to about 0.25-0.30- figure on about a 10% lower heat loss from the windows, which may be half the total heat loss of the room not even including the air leakage.

    If the weatherstripping can't be fixed, tight fitting exterior low-E storm windows would be a cheaper and higher performance fix than a performance replacement window. Harvey has the tightest storm windows in the biz, and have a low-E glazing option (recommended), but the Larson low-E storms (distributed by the box store chains) don't suck if you pay to upgrade to the Silver or Gold versions. (The cheaper Bronze flavor are still pretty leaky, and have crummier hardware.)

  3. GBA Editor
    Martin Holladay | | #3

    You're right, of course. I must have been skimming instead of reading carefully. Sean is probably talking about single-hung double-pane windows. My mistake.

  4. x95 | | #4

    Thanks Dana and Martin. Is it possible to put exterior storm windows over vinyl windows? I checked the exterior of my windows, and it looks like it would be difficult make any attachment to the vinyl itself. Have you heard of anyone trying this on builder quality vinyl windows?

    I have two concerns with exterior storm windows. I don't know if these are valid concerns.

    1. I've heard that you should NOT make storm windows air tight, because of condensation problems. This makes me thing that interior, air tight storm windows might save more energy.

    2. Placing exterior storm windows over E-coated windows can cause very high heat between the two windows, and thus cause heat related problems. Again, I don't know if this is true.

    Thanks for listening.

  5. Expert Member
    Dana Dorsett | | #5

    Storm windows with hard-coat low-E coating rejects some of the solar gain (but not as much as soft coating) reducing the high-temp issues that comes with applying clear storms over sealed windows. If you are in a high-heat high summertime insolation climate, when the outdoor temps are up & solar intensity is higher, you can keep the storm open a crack at both top & bottom and let it convection-cool, or you can pull the storm sashes and store them (a bit of a pain, to be sure.)

    Interior storms don't block the wind or convection from bypassing the faulty weather stripping, but if you have ducted air heat blowing directly under the windows that can be an issue too.

    Local climate affects the amount of condensation you might have to worry about. I'm in climate zone 5A, and low-E storms over sealed windows aren't unheard of here. They slapped some Harvey's over the pre-existing low-E double panes on this deep energy retrofit project:

    Having leaky storms and leaky windows can create heavy condensation due to high exfiltration rates on the low-pressure side of the house. With tight storms the air transported moisture is limited to the convection leakage of the window you're trying to fix. In my own home with circa 1923 wood framed single panes and moderately tight clear storms (came with the house), the only storms with condensation issues are on the absolute leakiest warped-sash windows (slated for replacement with ~U 0.25 double panes when we do-over that room) in one of the upstairs dormers. YMMV.

    Storm windows are often installed on the trim casings (sometimes called "overlap mounting") when there isn't sufficient substance to the window unit to hang it on. I would assume that's how it would have to be done over most vinyl windows. See:

  6. x95 | | #6

    Thanks for your comprehensive answer Dana. It's nice to know someone tried exterior storm windows over dual pane low E windows. They were wood windows though, not vinyl.

    The first 35 seconds of this YouTube video:
    gave me the idea that exterior storms and VINYL windows may be problematic. It's a very old video, and low E storm glass may solve the heat trap problem.

    My biggest problem with exterior storm windows is that I don't think I can attach them to my vinyl windows. There's not enough overlap, especially between joined ganged window units.

    Pulte must have got a huge deal on 72" tall, 36" wide single hung windows. I've got them all over the house, and they ganged them together in 2 unit and 3 unit windows. The two unit window in my small bedroom is a 6 foot by 6 foot hole in the wall!

    Interior storms look much easier, something I can try (screw up) myself. I'm still wondering if two layer window film will make much of a difference versus one layer. I'm probably the only person in the world that is even thinking of that question! I doubt anyone has tested one against the other.

    Thanks again for your thoughtful response.

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