Double Gypsum in an Unvented Roof Assembly – Climate Zone 4a
This group was very helpful with my last question, so I’m throwing another out there in hopes I can proceed with confidence:
As I finish up my plans for this roof, I’m struggling with something I haven’t seen address in other unvented cathedral ceiling threads, which is my used of double drywall for sake of soundproofing this studio room. Soundproofing is the reason for an unvented assembly to begin with: air leakage to the outside is an enemy of soundproofing.
In my quick illustration showing one end of the assembly, I have two layers of 5/8 gypsum mounted on decoupled furring strips. My drawing is crude, but the decoupled strips will create a 1″ gap between the rafters and the gypsum, which I plan on sealing with vapor barrier tape at the ends of the assembly. I was not planning on a vapor barrier behind the drywall since we already have 3″ CSF on the sheathing. Does this sound good? Any part I should reconsider from a vapor perspective?
I’m also unsure what I should do with the gypsum in terms of finish. I know that a vapor barrier will be provided by primer / paint, but with two layers of board, will this be too tight for any incidental moisture within the cavity to escape? Or should I be trying to make this as vapor tight as possible?
Or am I overthinking this? Any input is appreciated.
TIA
Steve
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Replies
Steve,
Your illustration looks almost identical to Assembly #5 in this article:
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Two layers of drywall with a painted interior finish are fine. 1/2" gypsum on its own is about 50 perms.
Thanks Malcolm!
Next question: what is the goal for the gypsum in this assembly? Is it to allow permeability for drying to the interior, or reduce permeability as much as possible?
-Steve
Good morning,
Bumping my question in case it got lost in the sauce --- is the goal of the interior gypsum to be permeable in an unvented cathedral ceiling? Or should I strive to reduce the inherent permeability of gypsum?
And being that gypsum is vapor permeable, is there any advantage to adding a smart membrane layer directly behind the ceiling gypsum? I know that the closed cell foam acts as a vapor barrier, but in theory, a smart vapor barrier would block vapor from traveling from the interior conditioned air to the roof cavity. Then again, it might end up trapping vapor that unintentionally infiltrated the cavity in times of low humidity.
I'm big on understanding the end goal, so I can make good decisions on all the little dilemmas that crop up during the build.
Thanks in advance,
Steve
Steve,
The CC foam is already a warm side vapour-barrier, you don't need or want another one at the ceiling as you want the assembly to be able to dry to the inside. The roof does benefit from a good air-barrier on the interior of the cavities which the drywall will provide.
A variable perm membrane is beneficial, but perhaps not necessary. It can't trap moisture in the cavity. If moisture accumulates the membrane will open up.
Thanks again for the input. It sounds like adding Membrain might be without much purpose. I was originally thinking that it would help prevent vapor intrusion through the gypsum coming from the conditioned interior, but it seems like that isn't much of a concern.
For a small well-air sealed space, you should be safe without a vapor retarder. For a larger space, or one that you can't be SURE is well air sealed, I would put in a cheap (MemBrain) smart vapor retarder for some added insurance. This is from past experience in my case, since the larger assemblies have SOMETIMES shown some moisture problems, usually when there are complex details and less than ideal air sealing. It's easier to just put in the vapor retarder in the beginning than to try to deal with problems down the road. The goal here is to try to keep moisture from getting into the cavity in the beginning, even though the CC sprayfoam should prevent it from condensing within the space.
How are you decoupling those furring strips? If they are parallel to the bottom edges of the rafters and tightly connected, then they aren't doing anything. Running them perpindicularly to the rafters would provide better isolation. Decoupling typically works by introducing some "bouncy and squishy" between two surfaces that would otherwise have a hard/rigid interface. Hard and rigid connections conduct sound. Bouncy/squishy surfaces do not transmit sound nearly as well. If you plan is to "decouple" with a layer of something like MLV between the rafters and furring strips, then I'd recommend you rethink your plans there.
Bill
Bill,
I'm in the middle of a protracted small recording studio build. Much of the sound-proofing is occurring inside the room, but for the roof we used two layers of drywall on resilient channels.
Thanks Bill. It is a small space without much complexity, so it should be straightforward to get a good air seal. I'm leaning towards skipping the Membrain, but I'm going to give it more thought.
The furring strips will be decoupled via Hushframe Rafts. I was originally going to use clips and hat channel, but Hushframe allows for easier installation, straightforward drywall, and possibly better decoupling. Basically the entire gypsum envelope meets up with a layer of silicon within the rafts.
Definitely worth checking out.
The more usual way to decouple is with hat channel and clips such as Akos mentioned. Malcolm is doing something similar. I've usually done it the same way. I have not used the Hushframe products. I actually have a small project with a section of floor coming up, and I'm going to try, as an experiment, "staggered joists", which is like a sideways staggered stud wall. Basically a seperate set of joists for the floor above the joists holding up the ceiling below. This way there are fewer rigid contact points between the floors, which should cut down on sound transfer. It will be interesting to see how well it does (it's not a critical assembly, so I can experiment a bit).
Note that some sound isolation products are better than others at different frequency ranges too. It's usually hardest to stop lower frequencies (bass) than higher frequencies. You want to use a product that will perform best in the frequency range you're most concerned with.
Bill
A hybrid roof assembly doesn't need any additional layers outside the spray foam and batts.
There is nothing really wrong with adding a vapor retarder over it (your two layers of painted drywall is a class III VR) and will perform even better.
The one you don't want is something impermeable over the batts such as 6mil poly.
Decoupling furring strips won't do much. If you want it quiet, use resilient clips with hat channel for the drywall and make sure to seal the edges and between sheets with something like GG sealant. I have not found that GG compound between sheets does all that much but maybe I was not careful enough with the rest of the details.
The extra gap the clips and channel creates between the drywall and the fluffy is not an issue.
Steve,
See Michael's reply in this discussion: https://www.greenbuildingadvisor.com/question/michael-maines
Thanks again. Now I'm thinking the Membrain can't hurt. There will be no additional cost
in my case since I will have enough left over from the walls. The agony of decision-making!