Rigid foam on the attic floor
davidmeiland
| Posted in General Questions on
I’m going to insulate a small amount of attic floor with rigid foam. The joists are 2×6 @ 24″ O.C. Plan is to install 5-1/2″ between the joists and then another 2″ over, followed by 3/4″ plywood. The area is used for storage and the plywood surface will get occasional foot traffic. I want to use polyiso for this but I’m concerned about crushing at the joists. I could install a layer of plywood over the joists, then more foam, then more plywood. but would like to avoid that if I can. I’ve only installed polyiso in walls and have little sense of its crush resistance. Any comments?
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Replies
David,
If you want to put polyiso on top of the joists, you'll have to install plywood or OSB first.
As I'm sure you know, installing 5.5 inches of polyiso between joists is fussy, time-consuming work. I know that polyiso has a higher R-value per inch, but if I were you, I would fill the joist bays with cellulose. You could make up the R-value difference by using thicker polyiso in the site-built SIP that you are about to put together on top of the joists.
Martin, thanks for the info. I was pretty sure the foam would not be OK without plywood under it.
I take your point about the hassle of installing rigid foam in joist bays. This attic has about 1100 square feet. There is a variety of work to be done, stuff like installing a beam in one area to remove a wall below, then building up a catwalk over future blown-in areas. There is a plywood floor in about a third of the space now, and there is a fair amount of contents stored. There is also a water heater installed. What I want to do is establish a small area of floor that's "done", move the contents and water heater over, then do the structural work. Once that is complete, I can get cellulose blown in the rest of the attic.
It's not that easy for me to get someone to install cellulose. They have to travel to get here. It's not cheap like it might be elsewhere. If I could easily get someone to split the job, I would.
Another approach would be to tune up the fiberglass that's already between the joists and simply leave it there, with plywood/foam/plywood over the joists. It would be enclosed on all 6 sides. I'm trying to control my desire to remove it no matter what.
Just to add another factor, minimizing height buildup is desirable. There is barely headroom. The storage floor area should be as low as possible, and of course I want high R value.