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Community and Q&A

Can I make interior thermal shutters for my windows?

PETER SCHWARTZ | Posted in Green Building Techniques on

I have single pane windows and would like to insulate them. I was thinking I could put some rigid insulation over them as a shutter and then hinge it at the top. I could lift them in the day and hook them to the ceiling and lower them at night. This would provide added value because it would really block out the light at night, provide sound insulation, and will double as a place to pin drawings that the kids make. When I went to buy this rigid insulation, I read a flammability advisory for R Max. Then I checked a piece at home, and yes, it does burn. Owen Corning’s Foamular F 150 seems to have a better rating, but the manufacture claims that you need some 15 minute wall covering over it. So I have some questions:
1) Is this a reasonable approach to insulating windows? Am I making a big mistake for reasons that I haven’t thought of?
2) Is there an ideal material to use for this? Is the pink F 150 “good enough” for inside use? Are there better products?
3) Any other advice?



  1. Ron Keagle | | #1


    Possible condensatin between the foam cover and window could be an issue. The inteior surfaces of the window glass and frame components will be colder with the insulating cover closed over them as opposed to when the covers are open. Therefore, whatever the dewpoint of your interior air is, the interior windw surfaces will be closer to it than they were prior to adding the covers and closing them.

    One remedy is to seal the window shutters against vapor transmission when they are closed, but that will be difficult to make 100% effective unless they are exceptionally well engineered and built. The only other option would be to reduce the indoor humidity to a level where no condensation occurs, and see if you can live with the air being that dry.

    I like the idea of operating insulating shutters, but it seems that they need to be on the exterior, and then operting them from the inside becomes an engineering challenge.

  2. PETER SCHWARTZ | | #2

    Thanks, Ron.
    The present windows don't seal as well as they could. Would it be possible to seal the indoor thermal shutters better than the windows, so any exchange of trapped air would be with the colder outside air? I've thought about using a gasket to seal the shutters. I've also thought about putting storm windows with double walled poly carbonate hurricane storm windows. Maybe I could seal them well enough on the inside? Or maybe I should just put storm windows in the exterior?

    In any case, is there a rigid insulator that I can use for the interior of the house? One that's not unreasonably flammable and is not toxic?


  3. Ron Keagle | | #3


    I would think it would be better to fix or rebuild the existing widows rather than let them leak and try to compensate by adding an insulating panel. Even good performing windows are relatively poor insulators, so closing insulated shutters can theoretically save a lot of energy by boosting the R-value of the window opening when not in use, which might be half the time. The challenge is making them practical.

    There is a book called, “Movable Insulation” by William K. Langdon that you might want to take a look at. I have been meaning to get a copy myself just to see what ideas are out there. One thing you could do is experiment with some type of insulated closure for just one window, and see how it works and if condensation is an problem.

  4. GBA Editor
    Martin Holladay | | #4

    Your approach to window insulation was all the rage in the 1970s. However, most homeowners soon tire of the chore of closing their shutters every night and opening them every morning, and most types of moveable insulation interfere with interior decoration or use of your walls, or look clunky.

    That said, I don't think that condensation will be a problem. After all, the shutters are opened during the day, allowing the space between the insulation and the window to dry out. The better the air seal at the perimeter of the shutters, the better they will perform.

    If you can afford them, you can buy commercial window quilts from a company called Window Quilt.

    For fire safety, I would choose Thermax foam. Encasing the foam in luaun plywood would probably improve fire safety. But that's a lot of work for a result that you may not want to use when you are all done.

    For more information, see:

    High-Performance Insulated Shades

    Plastic Film Kits, Insulated Shades, and Interior Storm Windows

  5. Michael Chelnov | | #5

    Roxul has a rigid insulation which is non flammable and has minimal outgassing.

  6. PETER SCHWARTZ | | #6

    Ron, Martin, and Michael,

    I chose not to be that patient. See, I was having some work done on my house, and in the process, the heater was taken out, then the contractor had some personal emergency that needed attending. I haven't seen him for over a month, and it got cold the night before last. After one cold night, I sealed the windows with that aluminum coated bubble wrap (Reflectix) and masking tape. This material is also flammable, so I consider it temporary. Besides, I'd like to have the windows transparent again sometime. So as I said above, I may put storm windows up, but for now, this will do. The sealed windows are all north-facing, so we don't receive much light through them. The house is small (650 sf) and we have lots of south-facing windows with low E glass. I documented the stuff I did to the house, if anyone is interested in seeing it:
    In any case, it's warmer inside today and I haven't seen any condensate. I'd like to put in interior thermal shutters and actually not get a heater. We live in San Luis Obispo and it gets below freezing only a few nights a year. In this way I want to use this difficulty as a an opportunity to try some changes. However, these changes will have to wait until I have time, and for now the windows will stay sealed (and hopefully dry).

    As I kind of explain a little in the above video, I think that the issues of energy use should become part of the every day conversation. This will likely happen as the (monetary and environmental) costs of energy consumption grow. So, I think that the rage of the 70s may come back as these issues return to be things we like to talk about.

    I greatly appreciate your attention and advice. Thanks for being community.

  7. Andy Alden | | #7

    Peter, has some great information on insulating window treatment projects.

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