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Thread: Plexiglass cover for inside of windows?

  1. #1
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    Plexiglass cover for inside of windows?

    I am thinking about ways to better insulate my shop, which is a 3-car garage. I have 5 windows, each double-pane, but no storms. I can feel a significant amount of cold air near the windows and am thinking about cutting some plexiglass to cover the windows, probably with some weather stripping around the edges to create a seal on the molding. My concern is that there might be a condensation buildup in there and I might get mold or otherwise damage the windows with moisture. I would probably attach the plexiglass with bolts that screw into t-nuts embedded in the molding so that I could take the plexiglass off in the warmer / moister months.

    Thoughts on doing this?

    Thanks.

  2. #2
    Why not try the shrink wrap window insulation first? It would be quick, easy and show you if there is merit from an insulation point of view. It would also be easy to see if you are creating a condensation problem.

  3. #3
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    Brian, I use 1/8" thick plastic to essentially do the same thing in the 250 year old portion of our home. I don't believe that the windows date back to the 1750s like that portion of the building does, but they are very, very old and I don't want to "replace" them. They don't open, either, so the plastic was a good solution to be able to provide additional sealing for air infiltration. I didn't use Plexiglas, however...I used Lexan. For your shop, I'd suggest the same because unlike Plexiglas, Lexan doesn't shatter. I attached with 5/8" screws and because the window frames are uneven, I did use a bead of clear silicone to adjust for that. It's worked for about a decade now to keep drafts non-existent from the glass.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  4. #4
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    i built 1x2 on edge wood frames for our skylights, and used foam weatherstrip along the edge to the drywall, 1/4 polycarb panel. drilled small holes in the drywall and used rivit to slid in the hole and hold the panel up. really reduced the cold air rolling down from them.

  5. #5
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    Sounds like a good idea, I wonder if adding some desiccant packs like these would help with the moisture concern.
    https://www.amazon.ca/Dry-Premium-In...esiccant+packs

    Last edited by Doug Garson; 12-16-2017 at 5:54 PM. Reason: added image

  6. #6
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    I have thought of this too. Except my issue is my regular double pane insulated windows I installed 20 years ago.. I am always getting build up of moisture on them that freezes, then I get the mold. It is a constant battle here in the upper Midwest. This year I am making mini storms using screen frame stock. I am going to put the shrink plastic on them and see how it works. If I get new windows or build I will skip the wood or vinyl and get the fiberglass windows and triple pane. Dan

  7. #7
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    I would check to insure that the gaps between the rough opening and the window frame is properly insulated. When I remodeled our family room I was amazed at the gaps around the window that were just covered up by the window casing. A little minimal expanding "Great Stuff" worked wonders on the drafts by the windows.
    Lee Schierer
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  8. #8
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    It's possible adding a piece of plexi, or better yet, Lexan as Jim suggested, will actually help eliminate condensation problems by creating the extra air space. Condensation occurs because you're hitting the dew point at that particular location. Much of the new superinsulation building technology deals with moving that location to a point where it isn't harmful. You could try Lexan, in some removable configuration on the inside of one window, and maybe the shrink wrap idea on another window to see where you get the best performance.

  9. #9
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    I have heard that adding such extra layers can overheat the glass and crack it in summer? Especially a problem with older stained glass windows. I do not know if this is really true or just a old story. Certainly in summer the windows interior space can easily be over 150F or higher which is just 30F over the air temp.
    I have no idea of the thermal expansion of wood vs glass or plastic.
    Bill

  10. #10
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    Quote Originally Posted by Lee Schierer View Post
    I would check to insure that the gaps between the rough opening and the window frame is properly insulated. When I remodeled our family room I was amazed at the gaps around the window that were just covered up by the window casing. A little minimal expanding "Great Stuff" worked wonders on the drafts by the windows.
    I put the casings up on these windows, so I know there is insulation in there. Thanks.


  11. #11
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    Quote Originally Posted by Dan Rude View Post
    I have thought of this too. Except my issue is my regular double pane insulated windows I installed 20 years ago.. I am always getting build up of moisture on them that freezes, then I get the mold. It is a constant battle here in the upper Midwest. This year I am making mini storms using screen frame stock. I am going to put the shrink plastic on them and see how it works. If I get new windows or build I will skip the wood or vinyl and get the fiberglass windows and triple pane. Dan
    I had the same problem. Went away when I paid better attention to the humidity in the house.

  12. #12
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    It looks like I'm going to try the shrink-wrap stuff this winter. I checked out the price of Lexan - $183 for a 4' x 8' sheet (1/8" thick) was the best I could find. I'll need three sheets, so that's a lot of cash, especially at Christmastime.

    Thanks for all of the input. I'll post results if I do end up doing something like this.


  13. #13
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    Brian, I bought the Lexan I used at Home Depot...they had pieces large enough for the windows I needed to cover. I don't remember the cost, however.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  14. #14
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    Brian, if you put the plexi on the outside of the window, it will reduce the likelihood of condensation since the vapor seal (the window seals) is closer to the warm inside. This is assuming your relative humidity inside the house is reasonable.
    Regardless, the air gaps will gain you insulation value (approx .5 R value in still air if I remember correctly, so your plastic sheet would gain you an additional 1 R value)
    Merry Christmas
    Sean

  15. #15
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    Old books about house building would describe ways to create dual pane windows. They recommended to seal the inner layer completely and let the outer layer vent to the outside. The reasoning is that colder outside air holds a lot less moisture than the warm inside air. There is a lot less chance for condensation this way.
    Steve

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