I'm finally tackling the attic space behind the knee walls upstairs, and I'm stuck between rigid foam board and fiberglass batts. Foam board seems like it'd seal things up tighter, but it's pricier and a bit more of a hassle to cut and fit. 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. Um...anyone have experience with either? Curious which you'd pick and why.
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.
