Venting bathroom exhaust part II (not yet started!)

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Flyover

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To vent my bathroom exhausts I'll have to install a couple vent hoods in my roof, which means cutting holes in my roof and sealing the vents in place with roof cement. I bought the kind you see pictured.

On the tube it says how long the cement takes to dry at 70˚ (24 hrs) but doesn't say anything about whether the stuff even works in 47˚, which is the average temperature here over the next few days. Or if it does work, how long it takes to dry. Anyone here know?

If it doesn't work unless it's warm out, is there another product that works in the cold? Or do I have to put this project off until spring?

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billshack

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is there anyway that you can go out of the attic on a end wall ? that is what would make sense to me.
 

oldognewtrick

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Always use the caulk under the shingles, you shouldn't see any of the sealant. Never caulk the bottom of the vent hoods, only the sides and top. Keep the sealant inside until you're ready to use it and you'll be fine. Once it's applied don't think again about cure time.
 

Flyover

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Thanks for the comments, everyone.

is there anyway that you can go out of the attic on a end wall ? that is what would make sense to me.
There's probably a way...but to me it makes more sense to go out the roof. That seems to be what everyone recommends anyway.

Never caulk the bottom of the vent hoods
I'm not clear what you mean. The vent hoods have a square flange that spreads out around the base. The "This Old House" video that @nealtw shared in the other thread showed the guy putting the caulk along the bottom of that flange. He also put some under the shingles that lay back down over top of the flange. Made sense to me.

Once it's applied don't think again about cure time.
Even if it only hits 50˚F during the day and dips to 25˚F at night? And it rains the next day?
 

oldognewtrick

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Don't worry about the temp. It will seal fine. I've hand sealed shingles many times when it's been below freezing. It's not a problem.

Any type of flashing should never be sealed at the bottom. If any water should enter around the flange, you need a pathway to daylight. You don't want to trap moisture. Yes, seal the top course of shingles. Yes, seal the sides, but leave the bottom unsealed. You're not waterproofing a bathtub, just creating new shingle seal strips.

But, it's your house, use your best judgement.
 

Flyover

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Oh I see what you're talking about: not sealing the side of the flange that's downslope. So you get an upside-down U of caulk, with the vent hole in the middle of the U. For some reason I thought you were talking about "top" and "bottom" in terms of which side of the flange is facing the house or up at the sky.
 

oldognewtrick

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That's a good video on how to install a bath vent Neal, thanks for posting!
 

Flyover

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I followed the This Old House instructions to a tee and just finished one vent today. The hardest part was hooking up my insulated duct (4") to the metal duct coming out of the exhaust (3") in the dark and cramped quarters of my attic. But in the end everything was taped and zip-tied securely, and one of my bathroom exhausts -- the one that gets the most use, unlike the other one that hardly ever gets used -- is now being vented through the roof rather than directly into the attic.

I'll do the other one later, either when it's warmer or when I'm feeling up to it, whichever comes first. This has been an exhausting (but satisfying!) morning.
 

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