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Is there any material that can be troweled on like plaster that is insulative? I am trying to finalize finish details for a window opening in a stone barn rehab. There is an area of exposed stonework between the window framing and interior walls. I would like to create a thermal break between the stone and interior heated space so the windows are not such cold spots. The best I can come up with is to sandwich minimal expansion polyurethane foam between the stone wall and cement backerboard. I can plaster over the cement board and the opening seems too narrow to use wood casing. A troweled-on medium might work, if it exists.
Picture shows windows spaces, but does not show 2×4 frame wall on inside of stone walls (will be DP) that is already in place.
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Gee, I don't see a lot of answers yet, but I think you're not going to find one. The reason I think so is that to get good insulating value in a thin material the R-value per inch would have to be very high. All the materials with a very high R-value per inch are light fluffy materials. If you could get a high R-value in a material only a 1/4 or 1/2 inch thick people would not be using several inches of insulation all over the place. I'd say your foam idea is the best option. Consider gluing it so you don't have little cold spots all over from metal fastneners. These discolor over time because their slight dampness attracts dirt.
*Thanks Wayne, I figured that the foam would hold everything in place sufficiently and no other fasteners would be needed, except perhaps if I overlap the backerboard over the 2x window framing, there it could be nailed or screwed in place.
*bill, you could look into perlite in a plaster mix.. but i don't know what depth you are thinking of..and then there's air krete by Palmer Industries..but small quantities are a problemhttp://www.airkrete.com/
*Aircrete looks like great stuff, but it also appears to be a foam in place product, applied by a specialized vendor. Bummer. This is a sample of the window. The outer oak frame is being replaced with new oak frames constructed the same way, but will be fitted with a single sash window, not louvers. 2x framing will be filled with cellulose, the wet-spray application which I haven't seen done before.
*Dear Bill,Try making mortar with cellulose instead of sand. Trowels nice, very buttery.It might work with plaster too. It's a cheap experiment. Try it and tell.Regards, Fred
*here's a wild idea for you to experiment with...use the cellulose with ice crystals. the ice melts and the water gets absorbed, leaving many tiny holes.the cellulose will serve as a "crosslinker" for the plaster.try it with a small sample, cut it in half and observe the profile.be sure to share and tell...brian