If underlayments had a race: which one would cross the finish line last?

Interesting point about the synthetic stuff peeling up in the heat. I’ve seen that happen a couple of times in my area too (I’m in central Texas, so we get those brutal summer days). I’ve always wondered if the real issue is the install timing or just the product not holding up to sustained high temps. My last roof was done with #30 felt and, despite a few storms and the occasional hail, it held up for nearly 18 years before I started seeing any issues.

That said, I’ve heard some folks say synthetics are better for steep pitches or metal roofs because they don’t wrinkle and they shed water faster during installation. But then again, if the attic isn’t ventilated well or you get a lot of humidity, maybe felt is a safer bet since it can “breathe” a bit more?

Curious if anyone’s actually measured attic temps or humidity differences with the two types of underlayment. Does synthetic trap more heat, or is that just a myth? I’d love to see some real numbers on that, especially for folks in hot climates.

I’ve seen synthetic underlayment bubble up on a few jobs here in south Texas, especially when it sits exposed for more than a week in August. I’m not convinced it’s just the product - sometimes crews rush the install or don’t fasten it right, and that makes a difference. Felt does seem to “breathe” a bit more, but I’ve never actually measured attic temps with both types. Has anyone ever tried putting temp sensors up there for a summer? I’d be curious if the difference is even noticeable once the shingles are on.

I’ve wondered about this too, especially after watching my neighbor’s roof job last summer. They left the synthetic underlayment exposed for a few days and it looked like a trampoline in spots - definitely not what you want. I’m with you that install matters a ton. I’ve always heard felt lets things “breathe” more, but honestly, once the shingles are on, I can’t say I’ve noticed much difference in attic heat. Would love to see some real numbers though... sometimes it feels like we’re all just guessing.

I’ve always heard felt lets things “breathe” more, but honestly, once the shingles are on, I can’t say I’ve noticed much difference in attic heat.

That’s what I keep coming back to - does the “breathability” of felt actually matter once everything’s sealed up? Or is it just marketing at this point? I’m also curious if anyone’s seen data on how these underlayments hold up after a few years, especially in humid climates.

I used to think felt’s breathability was a big deal, too, but after re-roofing my place in muggy south Georgia, I honestly couldn’t tell any difference in attic temps. What I did notice: the felt started curling and breaking down after about 7 years, especially where it got damp. The synthetics held up better, but they don’t “breathe” at all. At this point, I’m not convinced breathability matters much once the shingles are on and everything’s sealed tight. Maybe it’s more about how well the underlayment handles leaks or trapped moisture over time?