Rigid foam insulation for attic floor
I saw an episode of This Old House recently where Tom Silva was insulating an attic floor. First the bays between the joists were filled with fiberglass insulation. Then 2″ rigid foam insulation was placed on top of the floor joists, followed by plywood flooring. It seems to me that the rigid foam should have more bearing surface than the 1.5″ afforded by the floor joists. Is there a potential for the foam to crush down over the joists? If needed, I figure I could sister some 2×3’s up to the joists to increase the bearing surface against the foam. I don’t recall if Tom glued the foam to the joists and/or the foam to the flooring; would this be recommended? This attic is used for dead storage, and I’ll be using 3/4″ plywood as flooring.
Thank you.
edit: damn keyboardo’s…
Replies
Adding two-bys makes sense...
Thank you, Dave. Your suggestion of adding two-bys on top of the joists makes much more sense than sistering to the joists to widen them. My current fiberglass insulation has the paper barrier against the ceiling drywall below. I'd imagine that as long as this is intact throughout the attic (probably unlikely) there shouldn't be a water vapor problem, so you raise a good point, because any interruption of the existing barrier could be a big problem. I agree with not using any glue for the same reasons you stated.
Thanks again. Has anyone else contemplated or done this?
What type of foam?
Rigid foam is rated for compressive strength. I think you could lay it right over the floor joists and then install plywood as long as you chose the right material. If you add 2x to the top of the joists, would you be leaving a 1-1/2" air space between the fiberglass and the foam? If so, you need to make sure this air space is entirely sealed at the edges.
Yes, there is an air space...
Thank you, David. The floor joists are 2x12’s, and it doesn’t appear that the installed fiberglass bats have a full 12” loft, which is why I wanted to supplement what’s there with the Styrofoam board. Another idea would be to cut the boards to fit in between the joists. This would fill some of the air space and permit the plywood flooring to rest directly on the joists. Sound like a plan? Is Styrofoam available in 32” widths?
Thanks again.
The big advantage
of running the foam over the top of the joists is that you eliminate thermal bridging and also get a very good air barrier at the same time.
Maybe the thing to do is add more fiberglass over the existing so that the bays are full, install 7/16" OSB over the joists, install foam over the OSB and tape all the seams, and then install another layer of OSB or plywood as your floor.
The foam is most likely in 4x8 sheets. I would use polyisocyanurate. You don't need the foil facing.
How much square footage are we talking about?
Time for hindsight...
Thanks, David. One point I hadn’t mentioned is that access to the attic is through pull-down stairs, so I’m pretty much limited to about 24” widths. I’ve already cut maybe eight sheets of ¾” plywood in half to fit them up there. Here comes the hindsight; 1) I don’t know what I was thinking when I decided I needed 3/4” for an attic; 5/8” should be fine (and cheaper and lighter), and 2) It would have made more sense (and ultimately less work with better end results) to cut a hole in a ceiling to move whole sheets up there instead of cutting them in half. In fact, the stairs themselves shouldn’t be that difficult to remove for this purpose; there’s my ready-made hole.
Anyway, the square footage up there is roughly 1,200. I’d thought about adding more fiberglass, but I’m concerned that I might be packing it down; I figured it’s better to leave what’s there alone and add something like the foam boards. However, adding more fiberglass alone might suffice for my climate (central NJ) and might be the most cost-effective solution right now (wife just got laid-off). I’ll have to get prices and R-values on these materials to see what makes the most sense. What kind of tape should be used to tape the foam board seams?
Thanks again.
Blow cellulose over the fiberglass until it's flush with the joists.
And since you aren't talking about a finished attic (pull down stairs), why not just elevate your plywood floor several inches and put several inches of cellulose over the FG? run 2x6s perpendicular to the current joists probably 24 oc and then blow level to their tops and lay down your plywood. blow the sides not covered by ply howerver deep you want after you put up a cardboard dam to hold the cellulose off the plywood. You won't have to worry about the vapor barrier issue at all then.
I thought you were talking about an attic to be finished.