Insulating Floor Above Unconditioned Space (layout attached)
Hi all,
I have gotten some conflicting information while searching forums and some articles about the proper way to insulate a floor above an unconditioned space. I am building a 16×12 room on a pier foundation with two beams and want to insulate it well enough to use during the winter time. My layers from top down would be:
1. Sub Floor
2. Vapor barrier
3. R-30 faced insulation
3a. 2×10 floor joist
4. 1″ thick rigid foam (to break thermal barrier. on bottom of floor joist)
5. exterior sheathing.
The conflicting information comes from the rigid foam being used. Would this create a second vapor barrier where moisture will now be trapped between this rigid foam and the vapor barrier below my subfloor? I have attached a pdf to show the layers more clear.
Thank you
Replies
I'm no building scientist, but it seems like you don't need the 6mm barrier. This question might get more traction over on Green Building Advisor.
I'd ditch the 6mm poly so you can get interior drying.
If you were using it as an air barrier you can do the following instead: caulk/tape/liquid WRB the seams of your subfloor or put something vapor permeable like red rosin paper down on top of the subfloor.
I hadn't considered this thank you
If you are putting poly over the floor joist, then you don't need the insulation to be faced. Kraft-faced insulation has to be in contact with gypsum board in order to not be considered a fire hazard, otherwise it would need to be FSK. I don't think you have have enough floor joist depth for R-30 batt insulation without compressing it, which degrades the R-value of the insulation.
Reverse the rigid insulation and the exterior sheathing, then tape all seams of the rigid insulation, so it acts as a WRB too. Or you could use Zip-R sheathing on the outside and tape the seams - this would be my preferred choice for your application.
I've attached some information for you to review and consider.