It's for a greenhouse. The frame is going to be metal, the transparent parts glass, but metal conducts heat too well and the L shape of the metal doesn't lend itself well to attaching to the glass, also has potential rust problems in the high humidity interior of the greenhouse. Wood will mold and rot. This got me started investigating unconventional ways of gently holding the glass in place.

I'm thinking about cutting strips of XPS with hot wire and sandwich them over the seams in the glass, like this: enter image description here

The entire arrangement as little as 2 inches thick (width of the metal), just large enough to ensure the glass isn't in contact with anything inflexible.

I plan to give the exposed foam a protective cement based coating to keep the birds and rodents from eating it, but leave the foam touching the glass exposed. My main question/concern is whether the foam will survive outdoors like this: will it die of UV exposure from light passing through the glass, contract from the glass heating up, or any other weathering problems in this arrangement.

  • UV will kill it
    – Traveler
    Nov 22, 2022 at 0:28
  • @Ruskes Lets say I solve that with a layer of foil or something. Do you think the heat would also kill it?
    – gunfulker
    Nov 22, 2022 at 2:50
  • 1
    foam does not have the strength to keep the glass in place from wind pressure
    – Traveler
    Nov 22, 2022 at 3:39
  • that's not an issue when there is/can be metal on both sides.
    – gunfulker
    Nov 22, 2022 at 4:14
  • 1
    How will you fully enclose the foam to protect it from UV, especially at the critical glass/foam interface? I'd seek to use rubber or something else there.
    – isherwood
    Nov 22, 2022 at 14:20

1 Answer 1


XPS is not rated for this.

Why care?

It is easily dinged and cracked, and in the event of a fire you've supplied a perfect solid fuel so you can watch the sills melt as the glass panes smash sending out hot shards of glass.

XPS is not rated for lateral support, or any support for that matter. Pressing on the glass is quite possibly going to bend the XPS out of shape and cause it to loose its framing grip. The drawing does not show the metal bracket supporting the glass, but rather an extension of the XPS itself providing the support. The required test is (I believe) 50lbs over 1 square foot, and that should hold today and in 20 years.

Furthermore, that soft sheet stuffing is not a positive arrest to prevent lateral shifting of the window pane out of its frame.

Although this site contributes with creative and alternative approaches, reusing materials and making budgets work, this is not one I'd support.

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