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Old 02-16-2019, 12:08 PM   #1
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venting and insulating ceiling with rigid foam


Good morning!

Writing to you from Vancouver, Canada, which I understand to be climate zone 4.

I've gutted my second floor bathroom. We're going to have the ceiling follow the roof deck (ie, the other side of this sloped ceiling is the roof). The rafters are a meagre 3.5 or 4 inches.

I am studying different ways to insulate the ceiling, and am thinking I'll use rigid foam to maximize the R-value, given the little room I have to work with.

Currently, the house has a ridge vent, but no soffit vents. I may install soffit vents in future, so I suppose I need to provide an air space between the ceiling insulation and the underside of the roof deck.

In the interest of providing as much insulation as possible, I thought -- rather than using insulation baffles (aka, rafter vents) AND insulation -- I will instead combine them into one by, first, gluing spacers made of rigid insulation to the underside of the roof deck, and then gluing rigid foam against these spacers and between the rafters. I would use Loctite PL300 as the glue. I would use spray foam (e.g., Great Stuff) to seal between the rigid foam and the rafters. Then I would install rigid foam against the underside of the rafters, as a continuous sheet, and then drywall against that (ie, there would be a continuous sheet of rigid foam between the rafters and the drywall).

Is that an ok idea? If not, where am I going wrong?

So to summarize, from roof to interior room:

1. Underside of roof
2. Spacers, made of rigid foam, glued to the roof's underside
3. Rigid foam board, cut to fit between rafters, sealed with spray foam
4. Rigid foam board, whole, laid against and across the rafters
5. Drywall

Further question:
I've never attached drywall through rigid foam to a stud/rafter. It seems a bit strange, but how else would it be done?

Thank you for your help!

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Old 02-16-2019, 12:19 PM   #2
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Re: venting and insulating ceiling with rigid foam


That is about as good a plan as you can have, Are you in the attic or are you removing ceiling joists? You really do need those soffit vents. What's the plan for bathroom fan?
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Old 02-16-2019, 02:43 PM   #3
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Re: venting and insulating ceiling with rigid foam


Thanks for the speedy reply, Nealtw!

So fastening drywall to the rafters THROUGH a sheet of rigid foam is a normal thing to do? You just use drywall screws to crew through the drywall and rigid foam into the rafters?
_____

To answer your questions:

Quote:
Originally Posted by Nealtw View Post
Are you in the attic or are you removing ceiling joists?
I have removed the ceiling joists inside this dormer so that I can raise the ceiling. The ceiling will follow the roof slope. Currently, the rafters are exposed.

Quote:
Originally Posted by Nealtw View Post
What's the plan for bathroom fan?
I was thinking:
The bathroom fan will be in a top corner of the room ("top" as in the highest point of the sloped ceiling inside the washroom). A rigid metal duct will exhaust bathroom air out.

But I am not sure where the duct should exit.

I've read that exhausting through the soffit is, at least in theory, not good because then you risk sucking moist air up through the soffit vents. But others say this is overly cautious, reasoning that bathroom air is not as moist as a dryer.

If I don't go out the soffit, then I would go out the side of the dormer. Now, I forgot to mention, for clarity, that my dormer is a "shed" style dormer, like this (that's just an image for reference--not my actual house!). If I go out "the side of the dormer" then I mean going out the wall that's the triangular portion of the dormer that's parallel to the house's gable (I don't know what you call it... the gable wall of the dormer? In the reference image, it's the blue triangular piece of wall of the dormer). This may be easier than going out the soffit, but I do like the fact that a soffit vent faces downward, so there's minimal possibility of water leakage--whereas going out the side horizontal wall, rain water would be a problem.

What's your opinion on where I should vent?
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Old 02-16-2019, 03:14 PM   #4
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Re: venting and insulating ceiling with rigid foam


Venting into the soffit is definitely a bad idea. The gable of the dormer would work.
I would sooner see a flat ceiling area at the top so you had a bit of a common attic with the fan up there with the vent out the roof, that way you could wrap insulation around the fan housing. But sealing lights and fans and switches is important to keep moisture from the room from getting into walls and ceiling.


They make 3" drywall screws for that.
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Old 02-16-2019, 03:30 PM   #5
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Re: venting and insulating ceiling with rigid foam


If I went out the gable of the dormer, would it be good to have the duct slope down as it exits, so that any condensation, or god forbid rainwater, slides away? Any reason not to?

Thanks again for your help!
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Old 02-16-2019, 03:57 PM   #6
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Re: venting and insulating ceiling with rigid foam


something like this
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Old 02-16-2019, 03:58 PM   #7
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Re: venting and insulating ceiling with rigid foam


This is venting to the soffit
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