Hey guys I have access to some well priced Polyiso Insulation.
Do you think it would be a good Idea to fill a 2x6 wall 24 o.c. with one 3 inch and one 1.5 inch sheet of polyiso?
This will give me a total R value of 27.5 I can rip them to the right size with a table saw. Will polyiso hold up well?
What do you guys think?
thanks, Matt in Wyoming

I think it would work in theory but I am not sure if it will really work. Will you be able to get a perfect fit and make sure there are no gaps at all? Ifyou have small gaps here and there it kinda defeats the purpose. I was told one time by an insulation guy air movement is just as important as R factor. So you could have a high R factor but it if leaks it probably would not work. I am interested in others opinions though.

Posted By pechan on 20 Sep 2011 12:46 AM
Hey guys I have access to some well priced Polyiso Insulation.
Do you think it would be a good Idea to fill a 2x6 wall 24 o.c. with one 3 inch and one 1.5 inch sheet of polyiso?
This will give me a total R value of 27.5 I can rip them to the right size with a table saw. Will polyiso hold up well?
What do you guys think?
thanks, Matt in Wyoming

No matter what the center-cavity R is, the thermal bridging of the ~R3 studs kills performance. The "whole wall" R of what you described would be about R12-13.  If you filled it with cheap cellulose or R13 batts you'd be at about R10-11.

But if you put 1.5" iso on the exterior of the sheathing & studs you'd get  whole-wall of  R20+ out of it, and the structural wood would all be warmer- above the dew point of interior air all winter, and you wouldn't need an interior vapor retarder of any kind (and it would be best not to put one in, since the foil facer on the iso is a vapor barrier.