Why is my new bath fan causing rot in the attic?

The yellow spots you're seeing are a huge red flag that your soffit vent is failing to actualy exhaust that humid air. When you vent trough a soffit, you're fighting against the natural physics of your attic's ventilation. Most attics are designed to pull cool air in through the soffits and push hot air out through the ridge or gable vents. By dumping steam right at the intake, you're essentially just recirculating that moisture back into your roof structure.

Upgrading to a high-CFM fan actually makes this problem worse if the exit point is restricted. If you're pushing 110 or 150 CFM through a small, louvered soffit cover, the backpressure forces that damp air out of the duct seams or right back through the perforation in the soffit panels. Cutting a larger hole might help slightly, but the real fix is to move the exhaust point to the roof. You want a dedicated roof vent with a proper damper and flashing so the steam is gone the second it leaves the pipe. While you're at it, check the ductwork itself. If you're still using that flexible white plastic or uninsulated foil stuff, you're likely dealing with internal condensation. That moisture can pool in the low spots and eventually leak out into your attic insulation or rot your rafters from the inside out. Switch to **insulated flexible ducting** and keep the run as short and straight as possible. This prevents the steam from turning back into liquid water before it reaches the exit. Make sure the connection at the fan housing and the exterior vent is sealed tight with foil tape, not duct tape. If you absolutely cannot go through the roof, you need a specialized **soffit termination cap** that directs the air outward and away from the house, rather than just blowing it against a screen. But honestly, for a high-power fan, a roof-mounted vent is the only way to ensure that moisture stays out of your wood framing for good.

I've been there and the roof vent is definately the way to go. If you're stuck using the soffit for now though, check if your duct's acutally sagging. I had a high-CFM unit basially blowing a 'trap' of water beacuse the hose wasn't taut. It'd just pool up and leak onto the drywall. Also, don't just trust the built-in damper on the fan itself; those cheap plastic flaps stick all the time. If it's stuck half-shut, that extra power you're paying for is just hitting a wall and blowing back into the attic seams. Give the flap a flick and make sure it's actually swinging free.

You're spot on about the roof vent being the ultimate fix. One thing people forget is that high-CFM fans pull a lot of lint from towels and toilet paper. If your exterior vent has a fine bug screen, it’ll clog up in a few months. That backpressure makes the humid air leak out of any tiny gap in the duct joints into your attic even faster. I’d suggest checking the screen on whatever cap you use and making sure it's a wide-mesh bird screen instead of a fine window-style screen. It makes a massive difference in how much air actually moves through that pipe.