knee wall insulation dilemma - foam board or fiberglass batts?

I've used both in different parts of my house, and honestly, foam board was worth the extra hassle for me. Fiberglass is definitely quicker and cheaper upfront, but like you mentioned:

"Fiberglass batts are cheaper and easier to handle, but I've heard they can sag over time or let drafts through if not done just right."

Exactly my experience - after a few years, I noticed some cold spots and had to redo sections. Foam board stayed tight and draft-free. Are you planning to seal the edges with spray foam or tape? That made a big difference for me.

That made a big difference for me.

Foam board's great, but honestly, fiberglass isn't that bad if you install it carefully. I've had batts in my attic for 8+ years - no sagging yet. Maybe your cold spots were more about air sealing than insulation type? Just a thought...

"Maybe your cold spots were more about air sealing than insulation type?"

Yeah, that's a really good point. Fiberglass can definitely work if you're meticulous about installation, but I've seen plenty of cases where even careful installs still left gaps or drafts. I think the real advantage of foam board is that it inherently helps with air sealing - especially around tricky knee wall areas where framing can be irregular or hard to seal perfectly.

I had fiberglass batts in my attic for years too, and while they didn't sag much, I still noticed some uneven temperatures. When I switched to rigid foam board (and spent extra time sealing edges and seams), the difference was noticeable almost immediately. Not saying fiberglass can't do the job, but foam board just seems more forgiving if your attic framing isn't perfect...which, let's face it, most aren't.

Also, have you considered combining both? Foam board as a base layer for air sealing and then fiberglass batts on top for extra R-value? Might be worth exploring if you're still having issues.

Combining foam board and fiberglass is a solid suggestion. I've managed properties where we did exactly that - foam board first for air sealing, then fiberglass batts for added insulation. It worked well, especially in older homes with uneven framing.

Yeah, combining foam board and fiberglass does seem like a pretty solid approach. I've seen it done a few times, and it usually turns out well. But I'm curious - did you run into any moisture issues with that setup? I've heard some folks mention concerns about condensation forming between the foam board and fiberglass layers, especially in colder climates.

Also, did you find it tricky to get the foam board cut accurately around uneven framing? I've helped out on a couple of older homes, and man, those studs can be all over the place... makes me wonder if spray foam might be easier in those situations. But then again, spray foam can get pricey fast.

Anyway, sounds like your method worked out nicely overall. Might have to suggest it next time we're tackling a knee wall.

I've inspected quite a few homes with that foam board/fiberglass combo, and moisture can definitely be an issue if the seams aren't sealed properly. Saw one place where condensation built up enough to cause mold spots - nothing major, but still something to keep an eye on. And yeah, uneven studs can be a real headache...spent an afternoon helping a buddy cut foam board around some wonky framing, and let's just say our measuring skills got creative real fast, haha. Spray foam's easier, but your wallet definitely feels it.

Had similar headaches with foam board myself. Helped my brother-in-law insulate his attic knee walls, and the framing was all over the place - felt like we were cutting puzzle pieces rather than insulation. Fiberglass was simpler, but then you worry about moisture and drafts. Curious if anyone's tried using house wrap or vapor barriers behind fiberglass to manage condensation? Seems like it might help, but haven't seen it done firsthand.