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Community and Q&A

Sudden attic moisture

MishaelCB | Posted in Green Building Techniques on

Hi Martin,

We live in an 1860s brick farmhouse in western Mass with 4 kids and a long list of DIY work ahead of us!  Last year we tackled a bunch of home improvement items on our checklist, including attic air sealing, adding blown-in cellulose to our attic (now 14″) and a new metal roof to replace rotting slate.  We also had a bath fan installed and routed it out one of the attic gable vents, closing that vent up, but added a ridge vent along the entire ridge.  Last week I went up to the attic and noticed a LOT of moisture collected on the underside of the roof deck, frost on old protruding nail heads, some spots of the deck wet to the touch, and droplets of water forming on the new ridge vent and dripping back into the attic.  We had had some small issues with moisture and mold in the past, but not this bad, and it made us really worried.  We are guessing it may be due to the new metal roof not letting moisture escape as the old slate did, the blocking off one gable vent, and the increased insulation contributing to a higher moisture content to the air that is getting through.  There is no vapor barrier anywhere (except the metal of our new roof), but we did make sure to seal and latex paint the ceilings below during renovations–there are few if any air leaks and the old plaster and lath ceilings are in excellent condition–leading us to suspect it is the slow movement of vapor through the ceiling materials themselves that has led to this.  No doubt using humidifiers in our bedrooms doesn’t help!  We are considering adding a plastic vapor barrier to the attic, positioning it under the blown in insulation, a few joist bays at a time, and taping sections together and around penetrations.  There is enough of a gap between joist bottoms to accomplish this, in theory.  Or we could drape the plastic over joists and add blown-in cellulose to make the insulation over the “joist-hills” deep enough.  Also adding soffit vents to work with the ridge vent to carry away moisture seems like a good idea, and possibly moving the bath exhaust away from the gable vent so that can work again.  As a side, our roofer did not want to run the bath exhaust out the roof, do you know of a good flashing method for doing this?  Many thanks for any input, happy to answer additional questions, happy winter to all.  – Misha & Nadya

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  1. MattJF | | #1

    Step one would be to stop regular use of humidifiers.

    Your attic is likely colder now that you have better insulated it. Soffit vents would be ideal, although properly sized gable vents should help.

    Was a blower door or similar used assist in air sealing? It can be very useful to assist sealing.

    Gable vents are usually much larger than what is required for a bath fan. Is the duct sealed to some sort of hooded damper or just against the old gable vent? You could move the bath fan duct exit to a new spot on the gable wall with a hooded damper. Also verify all the duct is sealed with mastic.

    I would expect you could skip the plastic until a number of other options are addressed.

    Edit: we had some odd weather here sat and Sun with sudden really warm temps (60-70f). A lot of things accumulated condensation as that weather pattern moved in that never typically would. Your issues sound prior to that event though.

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