What I learned putting foam board down on an attic floor

I went with spray foam on a small section of the main hallway because the rigid board cuts were seriously driving me nuts. Cutting around the electrical conduit and the recessed lighting was a nightmare, especially trying to maintain the required thickness along the edges. The cost for the cans was high, but the labor saving in trimming was worth the premium.

However, for the rest of the attic space, I settled on closed-cell foam sheets instead of the standard rigid stuff. It’s slightly pricier, sure, but the expansion is much more controlled and the edges feel way more robust than the big sheets I saw mentioned in the forum. I measured about 3 inches deep under the rafters and sealed the perimeter seams with high-expansion foam before laying the sheets.

My main gripe with the foam boards was the vapor barrier concern. Even if you get it installed perfectly, I worry about the heat buildup in winter since there's no airflow behind the panels. I had to leave a six inch gap between the drywall and the foam to manage that, wich looked a little uneven but kept the vapor issue at bay. If you are going all-in on boards, you really need to manage that thermal envelope correctly.

Did the blown cellulose route last fall and yeah, the air sealing part is where all the real work hides. Spent a whole weekend on just the top plates and the chase for the plumbing stack before any insulation went down. One thing I'd add for anyone blowing: don't forget the baffles at the eaves. I nearly plugged my soffit vents before a buddy clued me in, and once they're buried under a foot of fluffy stuff you're not fishing them out easy. Also wear real respirator gear, not just a dust mask, that stuff gets everywhere when you're working the hose.

The one place rigid foam acutally made sense for me was the kneewall at the back of the attic. Loose fill just slides off a vertical wall, so I screwed the sheets right to the studs, taped the joints and was done in an afternoon. For the flat ceiling though, I'm with you on the blown route. The thing that saved my sanity was a cheap pack of plastic depth rulers stapled up every few feet. Trying to eyeball thickness through all that fluff from the hatch is way harder than it looks, and those little sticks tell you instantly when you've got enough down.

Rigid foam found its real use at my place as an attic hatch cover. Glued two layers of 2-inch polyiso to the back of the lid and weatherstripped the edges, cut it a quarter inch oversized so it sits snug in the frame. That hatch was leaking air like crazy before and the difference in the room below was noticable within a week. Cheap little job too, one sheet did it with plenty left over for insulating the pull-down stairs hinges. So yeah, boards over the whole floor, no. Boards where you need a rigid thermal break, absolutely.