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Thick Walls – 6″ Plexiglass Interior Window Covering For Thermal Benefits? Any Downsides?

briancornwell | Posted in General Questions on

We’ve got thick walls, leaving us 6-8″ of window depth on the interior.

We’ve got double glazed windows that are massive.

I’d like to add 6″ thick, layered, plexiglass panels inside of a frame to cover the windows from the interior.

Are there any downsides? I’ve heard window coverings from the interior can raise temps and cause melting of frames/etc. But since these still transmit light, I wonder…

Condensation isn’t a major concern yet.

Just curious. I looked up as much as I could but the whole thermal shutters thing died off in the 70-80s.

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  1. Expert Member
    BILL WICHERS | | #1

    If you're trying to get more insulating value, you'll need good air spaces between acrylic panes. It's the air that gives you insulating value in a multi-pane window assembly, the glazing material itself doesn't contribute much in that regard.

    I would use acrylic (generic name for "plexiglas") inner panes and polycarbonate outer panes. The reason for this is that polycarbonate is far, far more durable than acrylic, so if you use it for the outer panes, it will protect the inner panes and make your assembly more durable overall.

    I would fill the "air" spaces with dry nitrogen and solvent weld the perimeter. This ensures you don't have moisture trapped between panes where you could get hazing issues under certain conditions, and the solvent welded perimeter won't leak.

    I think at the end of the day though, this is a lot of work and expense for little real benefit. You'd be better off just putting the money towards a good full-depth (1-3/8") triple pane commercial window assembly, which will almost certainly be cheaper than the large DIY assembly you're considering.


  2. briancornwell | | #2

    Thank you!!

  3. wastl | | #3

    You would also have problems with condensation if you go internal. PMMA and PC are somewhat permeable and some vapor will find its way to the glas.
    If you go external then you would be save but thats not what you want.
    (I use 2 twin walled Plexiglas sheets in front of a glass brick wall (so a triple glazed unit in a way) - that works out to app. R 7.5 or so). Did not want to remove the glass brick wall for a new triple window - also a cost issue..

    edit: actually the above was not correct. You have no real condensation if you are really airtight with your inside placement when framing the Plexiglas window. If air can pass the frame then it will be worse. It depends also a bit on the quality of the old window. An oldfashioned double pane window (no coating) with R2 will have some condensate, a modern one not so much.
    What the old window frame will do is something I do not know. With the old internal shutters (opaque insulated) the heat staid in between. Here it would not! Light/IR energy can pass, the insulation is not that good either.

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