Funny timing, I just went through this whole synthetic vs. felt debate last fall when I bought my place. I ended up with synthetic underlayment too (asphalt shingles, southeast PA, so pretty similar climate to you). My roofer was all about it—said it’s lighter, tougher, and “breathes better,” but honestly, I’m still not 100% convinced it’s a miracle product.
I didn’t add any extra vents, just kept what was already there (ridge vent and a handful of soffits). My attic fan is ancient and sounds like a jet engine, but I leave it running on the hottest days. Not sure if it’s actually helping or just making the electric bill sad. I did notice the attic felt a bit more muggy this summer, but it’s hard to say if that’s the underlayment or just the weather being extra gross. I picked up one of those cheap humidity sensors from Amazon—turns out my attic was hitting 70% on some days, which seems high? But I don’t have “before” numbers to compare.
As for the warranty stuff, my installer swore up and down that everything was fine as long as the manufacturer’s venting specs were met. But when I looked at the shingle paperwork, it was all “must have X square inches of ventilation per square foot” and a bunch of other fine print. I get the feeling if there’s ever a problem, the manufacturer will try to blame it on ventilation no matter what you do. Kind of feels like a lose-lose.
My neighbor across the street did his roof with synthetic too, but he went with black shingles and says his attic is an oven now. I think color makes a bigger difference than underlayment, honestly. I went with a lighter gray and my upstairs is still tolerable, even on those 90-degree days.
If I had to do it again, I’d probably still go synthetic just for the peace of mind (and because everyone seems to be moving that way), but I’d pay more attention to venting. If you’re worried, those humidity sensors are cheap insurance. At least then you’ll know if things are getting out of hand before you see mold or anything nasty.
I get the appeal of synthetic, but I’m still not totally sold on it being “better” in every way. I went with felt on my last roof (mid-Atlantic, similar weather), and honestly, it held up fine for 18 years—no leaks, no weird attic issues. The synthetic stuff my neighbor used seemed to make his attic hotter, not cooler, even with decent venting. I do think shingle color matters a lot, but I’d argue venting is just as critical. Sometimes the old-school options aren’t as outdated as they seem.
Sometimes the old-school options aren’t as outdated as they seem.
I hear you on felt holding up—18 years without issues is nothing to sneeze at. Still, I’ve seen a lot of roofs in the mid-Atlantic where the felt started to wrinkle or tear after a few years, especially if the shingles took longer to go on or there was a lot of foot traffic during install. Synthetics do have their quirks, but I’ve found they’re a lot more forgiving if you get a sudden downpour mid-job. They don’t absorb water or break down from UV nearly as fast.
About attic temps, I haven’t noticed a big difference between felt and synthetic myself, but I agree venting and shingle color play a bigger role. Venting’s huge—seen plenty of “hot” attics with both types of underlayment just because the airflow wasn’t right. I’d say the main edge with synthetic is durability during install and if the roof ever gets exposed for a while. But yeah, felt’s still a solid option if it’s installed right and covered up quick.
I get where you’re coming from about synthetics holding up better during install, especially with unpredictable weather. But I’ve actually seen some synthetics get super slick when wet, which made things a bit dicey for the crew. Felt isn’t perfect, but at least you know what you’re dealing with—sometimes the “quirks” of synthetic catch folks off guard. I guess it really comes down to what you’re used to and the specific job conditions.
That’s a good point about synthetics getting slick—my neighbor’s crew had a close call last spring when it rained unexpectedly. I’ve always stuck with felt just because it’s what I know, but I keep hearing about these “grippy” synthetics now. Has anyone tried those on a steep roof? Wondering if they really make a difference or if it’s just marketing.
