Mixing vapor barrier techniques in unvented cathedral roof and soundproof walls?
Hi all, long time reader who is starting the build of ‘soundproof’ rehearsal space in climate zone 4a. Hoping this great community can help!
I have a 10’x16′ well-built shed on new concrete in my yard that I plan on building out for use as a music rehearsal space, and I’m trying assess the best configuration of vapor barriers.
Due to my soundproofing requirement, there are no vents in the building. No ridge, soffits, gable, etc. Ventilation will be provided using an ERV with airflow passing through a baffle box. The desire is to build an unvented cathedral roof (2×8 rafters), and I believe the only decent option for vapor barrier is 2″ of closed cell HFO foam on the roof sheathing, with the remaining depth filled in mineral wool behind decoupled sheetrock ceiling.
The walls are 2×6 and would be layered as follows from the outside in: Hardie Plank siding, Tyvek, 5/8 plywood sheathing, mineral wool, decoupled furring strips mounted on studs, and smart vapor barrier behind the first layer of sheetrock.
This is where I have a dilemma: I’ll have a roof assembly where the vapor barrier is closed cell foam on the roof sheathing, and a wall assembly where the vapor barrier is just under the sheetrock.
Since all of the sheetrock will be decoupled, there is a small air gap between the wall studs and decoupled furring strips that leads up to the roof assembly. I cannot fill this gap with much anything solid, since it would short circuit the decoupling.
Can I simply attach the wall’s smart vapor membrane to the top sill, and overlap it with the closed cell foam? Is that sufficient for keeping moisture from the wall cavity from travelling to the roof? Maybe this is actually an advantage, since it give the unvented roof a place for any accidental moisture to go?
Would love some thoughts if this isn’t too confusing. Attached is a crude side profile of the walls and roof.
Thanks!
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Replies
I think the easiest option here (going from your drawing), is to continue your smart vapor retarder membrane up from the wall and along the inside edges of the rafters to the ridge. This will make a sort of "room bag" supported on the interior of the framing. That will make sealing at the edges easier since everything will be flat in those areas as seen from the interior side of the mineral wool and rafters/studs.
Regarding sound proofing, I suggest you look into two things:
1- Consider building your exterior walls as staggered stud walls. This is more compact that a double stud wall, but offers performance almost as good. This avoids the need for furring or any fancy clips/channels on the interior too, which simplifies hanging drywall.
2- For the ceiling, I'd use hat channel and isolation clips. This is a more common assembly for ceilings than doing anything fancy with the framing.
You could also use resilient channel on the inside of a "regular" studwall, but a staggered stud wall should perform better. I would NOT use furring strips here -- I don't think you'd get a good support for the drywall that way, and you'd have a lot of rigid connections where the drywall, furring strips, and framing cross. You want to avoid rigid connections for soundproofing, especially in the middle areas of walls where things act somewhat like a big membrane. You want to minimize rigid connections as much as possible, and keep any that you have to have out at the edges/corners and not in the middle of big, flat, surfaces.
Bill
Great idea, thanks Bill. Sealing the barrier at the corners is an anticipated problem if I'm rolling the barrier onto the sill, and I think your suggestion may help keep the corners airtight. The direction of the rafters may be an issue, but I think I can get around that.
So what I'd be looking at would be a wall cavity that would extend all the way to the top of the roof trusses, with the outer 6" perimeter of the roof (plus overhang) included with that wall assembly. This would essentially 'join' the two vapor barrier methods, since the closed cell foam would be at the top of the roof trusses, and the smart barrier was be attached to the side of the truss (or perpendicular 2x's if needed for a flat surface).
Will the smart vapor barrier provide the seal needed for an unvented roof? Any concern about the barrier 'opening' under high humidity, and my roof assembly becoming exposed to vapor from the wall?
I was trying keep my post concise, so I didn't go too far into the soundproofing plans. I had initially considered staggered studs, but space / STC tradeoff just isn't there on paper. Double wall studs would have been best, but again, space is limited.
That led me to clips and channel all around, but a newer product has gotten a lot of attention called Hushframe Rafts. This is what I would mount the furring strips onto, which is the manufacturer's suggestion. If you search them out, you can see that the decoupling is introduced by way of a silicon layer built into the raft. Plus there are many advantages in terms of attaching drywall and a mini-split vs. hat channel. I like the idea and am going to try them on my small project --- I'll report back to the thread to let you know how it goes.
Thanks again for taking the time to response, really appreciate it. Let me know what you think about that smart membrane configuration...
You could potentially just leave the smart membrane poking up into the rafter channels, then have your spray foam contractor sprayfoam the membrane when they do the roof. This will embed the edge of the membrane into the spray foam, creating a seal at the edges. The downside (speaking from experience here) is that the spray foam won't always stay where you want it when you're spraying against something floppy like a loose edge of a vapor barrier membrain. You're likely to have voids and sags. You could try stapling the edge of the membrane up so that it will stay put when the spray foam covers the edge, but the spray foam will add weight to he membrane that will tend to pull the membrane away from things while the foam cures (i.e. before the foam gets rigid), and that's where the sags and voids form.
If you use closed cell spray foam of sufficient thickness for your assembly in your climate zone, then you don't need an interior side vapor barrier over it, but you DO need to seal the edges of the walls into the spray foam layer to keep a continuous air barrier for your structure as a whole.
Bill
Great insights, thanks Bill!
The EIFS system is also known as the ETICS system. They are energy efficient and versatile designs for your insulated wall systems.
https://wotaichem.com/eifs-system/