Q&A: "Is My Attic Venting Working or Am I Just Slowly Growing Mold?"

We got an email from Wayne H, homeowner, self-taught building science sleuth, and builder of one seriously dialed-in roof retrofit. He’s doing the work, tracking temps and dew points, and still seeing mold in his attic. So he wrote in with one big question:

“I’ve redone my whole roof system, baffles, vents, air sealing, better sheathing, and I’m still seeing moisture and mold after a cold snap. Is my attic venting working properly? Should my sheathing ever be wet if the system is doing what it’s supposed to do?”

Wayne’s house is a 1989 rambler tucked into the woods north of Enumclaw, WA. Simple footprint, 26x66 ft, unconditioned attic, no raised heel trusses, and a 4/12 pitch with eave and ridge vents. From day one, he was getting mold on the underside of the OSB sheathing.

He brought in roofers, insulation pros, and mold folks. The verdict? Not enough venting at the eaves. Just two 2" screened holes every other bay—not close to cutting it. Their solution: more holes and a mold spray.

Wayne’s solution? Tear off the whole roof and do it right.

He pulled the sheathing, installed CDX instead of OSB, air-sealed the ceiling plane, and built custom baffles using polyiso and Cor-A-Vent RS-400 strip vents. The RS-400s provide 18.75 sq in of NFVA per bay—far more than those little round vents. He called Cor-A-Vent just to be sure and was told: “You can’t really overdo it at the eaves.” So every bay got vented.

Fast forward to July: new sheathing, new ridge vent, new blown-in insulation, and LEDs replacing the can lights. Job well done.

But then January 2025 hits. Several freezing days in a row. His trusty HOBO sensor shows attic air at 35°F… and roof sheathing even colder, below the dew point. Result? Condensation. In some places, actual frost. Then, as temps climbed again, black surface mold.

So what gives?

Let’s break it down.

First: Yes, Even “Good” Attics Get Wet Sometimes

This is the part most folks don’t want to hear, but it's true: moisture on your roof sheathing during freeze-thaw cycles isn’t always a sign of failure. It can happen when cold temps + high RH + low sheathing temp line up just right.

But a good attic dries out afterward. That’s the real test.

Second: Not All Ridge Vents Are Created Equal

Wayne's original ridge vent had fibrous insect screening underneath. Turns out, that mesh might’ve been limiting airflow just enough to create stagnation at the ridge. When mold showed up there, too, he swapped it for a Lomanco vent with no fibrous backing—but he’s not thrilled with the design.

He’s now eyeing Cor-A-Vent’s V600 product to complete the system. If his math holds up, he’ll end up with roughly 1,200 sq in of venting at both ridge and eaves—way more than the 1:150 sq ft ratio (and that’s a good thing).

Third: Air Movement Isn’t Always Obvious

Wayne notes that when he’s in the attic, he doesn’t feel much airflow. Smoke pencil tests show some movement, but not dramatic.

That’s normal. Passive venting—ridge and eave—relies on pressure and temperature differentials, not fans. You’re not going to feel a breeze unless wind or strong stack effect is at play. Still, it’s working in the background. Think slow and steady, not turbocharged.

So… Should Your Sheathing Ever Be Wet?

Sometimes, yes. Especially during those sharp cold snaps. The goal isn’t to eliminate all condensation—it’s to create a system that dries quickly and doesn’t let moisture linger long enough to grow stuff.

Persistent wetness? Problem.

Seasonal moisture that dries out in a day or two? Usually fine.

What About the Mold That’s Already There?

Wayne’s remediation crew wants to treat the mold and install a humidistat-controlled gable fan. But here’s where BS* folks tend to agree: don’t bring in fans or sprays until you know your system is working. Treating the mold without solving the airflow issue just restarts the clock.

Also, treating sheathing often ruins blown-in insulation, so you’re looking at redo costs. Instead, Wayne’s gut is right: dial in the physics first, then clean up.

Final Take: Is the System Working?

We’d say yes, mostly. The system Wayne’s built is solid: tons of venting, custom baffles, great air sealing,and better materials. The Lomanco ridge may not be perfect, but the drying seems to be happening now. RH and sheathing moisture have dropped. That’s a great sign.

If the V600 vent adds better ridge performance and maintains balance with those high-capacity eave vents, he might be looking at the final piece of the puzzle.

As for airflow “feel”—trust the HOBO data more than your skin. Passive venting is subtle.

To Wayne (and anyone else knee-deep in attic science): You’re not alone. And you’re asking the right questions.

You’ve built a system that’s smarter, tighter, and more transparent than 90% of attics out there. Don’t let a little frost spook you. Keep watching, keep testing.