Can you stack foil faced Polyiso boards.

[3rdgendslmech]

Hoping to get my mini split installed by Thanksgiving. Have to finish insulating a 3-4 wall cavities before I can hang it.

I was hoping to get the tin and blown in done but I dont think that is going to happen till summer.
A friend came across a bunch of 1/2 foil faced polyiso boards. Can you stack or compound the boards to get a better R value or by the vapor barrier on each side would you just be causing an issue with thermal bridging?

 

[56Safari]

I wouldn’t be worried about thermal bridging but they do have a vapor barrier on them so I’m not sure if you’ll have issues with moisture trapping in between. I used 1 layer of 1” GAF polyiso with R-13 fiberglass in the 2x4 cavities on a room remodel and I’ve been very impressed with the insulation quality in that room.. By far the most comfortable room in my 1948 house, cools and heats very quickly.

If you’re thinking about stacking it inside the cavities, its a PITA to stack foam boards.. I stacked 2”XPS rigid foam in rafter cavities to get a R20 value while also maintaining a 1.5” air gap between the insulation and roof.. Took FOREVER, if it was warmer when I did this I would have just spray foamed it.


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[3rdgendslmech]

That's what I was hoping for....if I can get enough cheap enough to get me to R-13 would be nice. I heard a few people on here saying that just the vapor barrier alone on the 1/2" sheets will be a pretty noticeable difference since there isn't a lot of air movement between the 2 sides

 

[rburke65]

I think that the Tyvek idea is really good.