Is it worse with OSB than plywood? Or just bad airflow? I’m in the Midwest too, so winter’s a beast...
Man, I hear you on Midwest winters. My place is rocking 1950s plywood and it still sweats like a gym sock some days. OSB seems to hold moisture longer for me—almost like it’s got a grudge against dry weather. But honestly, half the time it’s just lousy airflow. I had one spot over the kitchen where the underlayment was fine, but the wood still looked like it’d been through a sauna. Ended up cutting in an extra vent and that helped more than anything else I tried. Midwest roofs are just stubborn sometimes...
That’s interesting about the OSB holding moisture longer. I’ve always wondered if it’s just the material or if it’s more about how the roof is put together. My place has newer plywood, but I still get condensation in weird spots—especially above the bathroom, even though there’s a vent fan. Maybe it’s not moving enough air? Has anyone actually measured humidity up in their attic during winter? I’m tempted to try, but not sure if it’d tell me much or just confirm what I already suspect...
I’ve actually stuck a cheap humidity sensor up in my attic one winter, just to see if I was being paranoid. Turns out, I wasn’t—humidity spiked every time someone took a shower, even with the fan running. I’m convinced most vent fans just don’t move enough air, or they leak back into the attic somewhere along the duct. Plywood’s better than OSB for moisture, but if the bathroom air’s getting up there, it’ll find a way to condense. I wish more folks would talk about air sealing instead of just slapping on another underlayment layer...
Interesting point about the vent fans—I've seen a lot of folks blame the fan itself, but sometimes it's more about the ducting or where it actually terminates. Did you ever check if your vent is going all the way outside, or is it just dumping into the attic? I run into that a lot during inspections, and it's wild how common it is for the duct to stop short or have a loose connection right above the insulation.
I get what you're saying about air sealing, though. It's definitely underrated. But I wouldn't write off underlayment upgrades completely. In some older homes, even with decent air sealing, moisture still sneaks in from other spots—think leaky can lights or gaps around plumbing. Sometimes a beefier underlayment buys you time to figure out where the real leaks are coming from.
Curious if you've tried any of those newer self-sealing tapes or gaskets around your bath fan housing? I've seen them make a difference, but maybe that's just in my climate (pretty humid summers here).
- Had the same issue with my bath fan—turned out the duct was just sitting on top of the attic insulation, not even close to the roof vent. Wild how common that is.
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That’s exactly what happened here. I swapped in a heavier underlayment when I redid the bathroom. It definitely helped, but the real fix was sealing up around the can lights (those things leak air like crazy).“Sometimes a beefier underlayment buys you time to figure out where the real leaks are coming from.”
- Haven’t tried the self-sealing tapes yet. Do they hold up over time? I’m in a colder climate, so not sure if they’d peel or crack.
- Did you notice any difference in noise or airflow after using those gaskets? I’m thinking about adding something like that but don’t want to make the fan less effective.