The insulation around the end of the duct was gray and heavy, like a bath mat someone left on the floor overnight. I pressed it with a gloved finger, and it stayed pressed.
That was last Wednesday evening, October 1, lying on my stomach across the joists of the knee-wall attic with a headlamp on. The energy assessor had found it the week before: our bathroom fan, in the ceiling of the downstairs bath, had a duct that ran up into the unheated space behind the upstairs knee wall and just stopped. No vent, no connection to the outside. Every shower, for however many years that fan has been there, pushed warm, wet air into a cold attic.
What the damp had done
Less than I feared, and more than I'd like. The fiberglass in about a three-foot circle around the duct end was damp and matted. The underside of the roof boards above it was darker than the boards around it. I'd brought the moisture meter up with me, and the wood near the duct end read 23 percent, against 11 or 12 a few feet away.
From what I've read, that's wet enough for mold and rot to get started if it stays that way. It hadn't gone soft. I poked it with a screwdriver to make sure.
Gary came over to look at the two contractor bags of wet insulation I'd dragged to the curb. He shook his head and said, "Water always wins." Then he went home, which is Gary's version of a long conversation.
The reroute
The plan was to send the fan's exhaust outside through the soffit at the low eave of the main roof, about six feet from the fan. A Cape's eaves are low enough to reach from my 6-foot stepladder, which was a big part of why this felt like a job I could do myself.
Here's what I bought, $64 in total:
- 8 feet of 4-inch insulated flexible duct: $22
- A soffit exhaust vent with a built-in damper: $24
- Foil tape: $9
- Two duct clamps: $5
- Exterior caulk: $4
I went with insulated duct because a bare one running through a cold attic collects condensation inside it, and that water runs right back down into the fan. I cut the duct as short as I could, pulled it straight instead of letting it sag and sloped it slightly down toward the soffit, so whatever condenses runs out instead of back.
The soffit vent is a proper exhaust fitting with a flap, not just a hole pointed at the perforated soffit panels. From what I read, damp air blown at ordinary soffit vents can get pulled right back into the attic, which would defeat the whole point.
I made one mistake outside. I measured from inside, transferred the measurement to the soffit, and cut my four-inch hole right into a wood nailer strip behind the soffit panel. I had to move over two inches and cut again. The vent's flange covers most of the first hole, and a dab of caulk covers the rest. You'd only notice from the top of a stepladder, looking for it.
Inside, the duct went onto the fan outlet with foil tape and a clamp, then out to the vent the same way. From opening the knee-wall door to closing it took about three hours, one of them spent threading the duct through a gap between two joists while trying not to put a knee through the bathroom ceiling.
The shower test
Tessa took the first shower after the job, and I stood in the driveway watching the soffit vent. A few seconds after the fan kicked on, the little damper flap lifted and a wisp of steam came out. Then she was done, and I was still standing there. She came out with a towel on her head and said, "You watched the soffit for fifteen minutes."
It was closer to ten.
The roof boards are drying. This morning, six days after the reroute, the meter read 17 percent where it had read 23. I'll keep checking weekly until it matches the rest of the attic. The insulation contractor comes October 21 for the attic and knee walls, and I'm going to ask the crew to look at my duct while they're up there. I'd rather hear it from somebody who does this every day.
The job came out of the house fund, and it's the best $64 that fund has spent so far.
