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Question DetailsAsked on 6/30/2013

need advice on Attic ventilation

My home has intake soffit vents and a outflow ridge vent. The north side of the attic has recurrent mold and I want to improve the attic ventilation so the mold problem will be eliminated for good.

We replaced the roof 2 years ago. At that time we had some mold on the one side of the house and also replaced the boards involved and the blown in insulation in that area. The bathroom is under the involved area and this was vented to the roof at the time the new roof was placed (previously was venting into the soffit region). The soffit vents appear to be open. The mold reoccurred.

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1 Answer


OK - you obviously have a moisture source unique to that area, since not occurring in rest of attic.

Possible sources:

1) bathroom vent not correctly hooked up or flashed, so bathroom moisture is getting into the attic still

2) other bathroom or kitchen fan venting into attic

3) HVAC return air duct, vent pipe, or furnace stack leaking into attic

4) significant air from house getting into attic, either through bad vapor barrier or around pipes, wiring, other penetrations into the attic.

5) ridge vent not working

6) The soffit vents may well be a major part of the problem, as vented soffit coves commonly do not let enough air in to ventilate the attic well. The industry "standard" is 1 SF vent per 300SF attic space - this should be more like 1SF per 50SF to work right. Therefore, soffit covers that are not largely open air space restrict the ventilation.

Things I would do in your place, in order (being careful not to step on and crunch or damage wires, vent ducts, etc buried in insulation, or stepping on ceiling drywall below - keep feet on joists. If you choose, to reduce the effort, you could have an energy consultant with thermal IR camera come in and locate the major air leaks into the attic first:

7) make sure ridge vent is working - during the heat of the day till about full darkness, you should be able to feel a distinct hot air flow out of the vent. If unable to feel it, hold a 1-2 foot long strip of saran wrap over it - it should bulge out from hot air pressure

8) uncover ALL pipe and duct penetrations into the attic from the house, and make sure all are airtight where they penetrate into the attic. If not, seal with vapor barrier and vapor barrier tape, with caulk for small gaps. Around furnace or other hot exhaust vents or chimney stacks, use unfaced fibreglass or rock wool. Bathroom fans and kitchen fans commonly have ann oversize hole cut for the fan installation and ducts, so you may have a 1" gap all around them into the attic. Seal the penetrations at the ceiling drywall.

9) make sure all the pipe joints are tight, ducts have ALL joints taped with water-resistant (NOT regular) duct tape, and that all actually fully go out the roof through vent pipes or hoods. A LOT of water can come from a broken (or attic-terminated) sewage vent pipe or furnace stack.

10) make sure all roof penetrations have a roof jack (rubber gasket that fits tight around pipe or duct and has a surrounding metal plate that tucks in under the shingles for its top 1/2, is exposed for bottom 1/3 to 1/2. Will be exposed on roof around pipe penetrations, will usually be covered with draft hoods for bathroom and kitchen vents. This is a common source of ductwork flaw - if they don't cheat and just terminate the duct at the bottom of the roof (letting house air circulate around in the draft hood and back down into the attic), then they just stick it up into the middle of the hood opening, leaving a 4-8" opening cut through the sheathing for the moist air to circulated back into the attic. There should be a tight-fitting rubber seal here - ideally installed from above by removing the hood, but there are retrofit ones that can be installed from inside the attic to the underside of the sheathing, though any moisture that accumulates in the hood pools around it, and then saturates the sheathing around the opening, promoting rot, so not the best solution.

11) check there is a tight-fitting seal (or caulk if it is very rarely used) on the trap door to the attic. If you intend to leave usable (without having to cut the caulk) then you can use thick foam weatherstrip to form an all-around airseal, and either put a plywood facing and 5# weight on top of the hatch, or put a plywood facing on the top, an eyebolt with washer through that sticking down from the center, and bungi cord it from the eyebolt to another set in the wall below, so it is tightly held down. Obviously this can work in a closet, not sort of unsightly if hatch is in a hallway - there you use the weight method. You would be amazed how much house air goes up through a typical drywall or plywood hatch just sitting on the joists.

12) If these do not solve the problem, then you may have to install a supplemental attic ventilation fan, though that partly negates the operation of the ridge vent.

Answered 7 years ago by LCD

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