Which holds up better over time: felt or synthetic underlayment?

I had no idea low slope would be such a pain until I bought this place. My inspector just kind of shrugged and said “keep an eye on it.” Fast forward to last spring - surprise, water in the attic. I swapped to synthetic too, but now I’m paranoid about ventilation. Anyone ever actually fix attic airflow themselves, or is that a pro job?

I get the paranoia - low slope is just a whole different beast. About attic airflow, I wouldn’t say it’s always a pro job. Depends how handy you are and what you’re working with. I’ve seen folks add baffles and extra vents themselves, but cutting into the roof or soffit can get sketchy if you’re not comfortable. Just be careful not to overdo it and mess up the balance. Sometimes too much venting can cause its own problems, especially with synthetic underlayment since it’s less breathable than felt. It’s a tradeoff, honestly.

Yeah, I hear you on the venting - too much of a good thing can backfire, especially with synthetic underlayment. I swapped out old felt for synthetic a few years back thinking it’d be a “set it and forget it” deal, but my attic got stuffier than expected. Had to tweak the vents after the fact. Felt breathes better, but man, it tears so easily if you’re not careful. Synthetic’s tougher, but you’ve gotta watch that airflow or you’ll cook your insulation. Always some tradeoff, huh?

Had a similar situation last summer - swapped out old 30# felt for synthetic on a steep asphalt shingle job. The durability was great, but I underestimated how much less vapor passed through. Ended up adding more soffit vents to balance things out. Synthetic’s definitely tougher, but it’s not always “install and forget” like the marketing says. Always seems like you’re trading breathability for strength.

That’s a really important point about vapor permeability. Synthetic underlayments are marketed as a cure-all, but there’s always a tradeoff, especially in older homes or roofs that weren’t designed with low-perm materials in mind. I’ve seen a few jobs where folks switched to synthetic for the durability - especially on steeper slopes or in high-wind areas - and then had to go back and rethink attic ventilation. It’s easy to overlook how much less moisture can escape through those synthetics compared to #30 felt.

Adding soffit vents was a smart move. I’ve had to recommend ridge vent upgrades in a couple of cases for similar reasons. The upside is, once the airflow’s balanced, the synthetic does hold up better against UV and foot traffic during install. But yeah, “install and forget” isn’t always realistic, especially if you’re dealing with older framing or insulation that can trap moisture.

It’s always a balancing act - durability versus breathability. I lean toward synthetics for most new builds, but on retrofits, I’m a lot more cautious. Good to hear you caught it before it became a bigger issue.