Adding drywall in between stud bays
Hi there,
I’m in the process of planning a music/recording studio build in a detached workshop. I’m trying to isolate sound from leaving and entering as much as possible.
I’ll be using double stud construction to essentially build a “room within a room” as is common practice for sound isolation. A commonly used construction technique to increase the mass of the outer shell, in this case the building exterior walls (OSB and wood siding), is to attach strips of drywall into the stud bays against the sheathing, held in place with wood cleats, and then caulked and sealed around all edges.
From reading some information on here and other general building sites, double stud walls seem to be inherently at risk of moisture problems in the sheathing. I’m worried that placing drywall against the sheathing could be a very bad idea.
So I guess my question is, would doing this be asking for trouble with moisture issues?
Extra information-
Location:
Ottawa Canada (very cold winters, hot humid summers)
Existing construction (detached, heated workshop):
(from inside out) – poly vapour barrier -> 2×6 wood studs (filled with R20 pink fluffy insulation) -> 7/16″ OSB sheathing ->1/2″ wood siding
Proposed new construction:(from inside out)
5/8″ drywall -> 5/8″ drywall -> poly vapour barrier -> 2×4″ wood studs (filled with R20 pink fluffy insulation) -> 4″ air gap -> 2×6″ wood studs (filled with R20 pink fluffy insulation) -> 5/8″ drywall in stud bays against sheathing -> 7/16″ OSB -> 1/2″ wood siding
TLDR – I want to attach drywall inside stud bays to exterior OSB sheathing. Is this a bad idea? (moisture problems?)
Thanks in advance!
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Replies
Cody, to be blunt, installing drywall to the interior of your exterior sheathing in a cold climate is a bad idea. Unless you have a thick layer of exterior insulation, that location is at risk of moisture accumulation and drywall is very sensitive to moisture. At least gypsum-based drywall (GWB) is. You might consider MgO board, a GWB alternative that is moisture resistant. Or use a high-mass insulation such as mineral wool or cellulose.
I'm often one to note the risk of double walls, but with good detailing they are usually fine. Check out the cover story in this month's Fine Homebuilding, by my friend (and frequent collaborator) Dan Kolbert: https://www.finehomebuilding.com/2020/04/08/a-case-for-double-stud-walls.
Hi Michael,
Thank you so much for your expert advice. That pretty much confirms what I was thinking. It seems like too risky of a thing to do. In the home studio building community, this is a very common recommendation and I've seen a lot of studios built this way. I guess climate zone really factors into the risk though.
From reading that article, it makes me concerned to build double stud walls at all, even though I know that's the only way I'm going to get the isolation I need. One problem is that I don't have a vented rainscreen. The siding is attached directly to the sheathing. I've never seen vented rainscreen around here actually and my wood siding house is also built without it. The detached building is wrapped with Tyvek though, I forgot to mention that. Does that lower the risk somewhat?
Another thing that I think will help is that, the interior shell is going to be completely airtight, or as close to it as I can possibly get, as air tightness is crucial in soundproofing. Am I correct in thinking that air penetration from the inside is the biggest risk factor in moisture accumulating on the interior of the sheathing? That if there was no air leaks from the inside at all, even drywall might hold up in the stud bays?
I've never heard of MgO board, thanks for pointing that out! I'm going to look into it around here. I have no idea where to source it or how it compares in cost to gypsum drywall.
So now I'm thinking just skip adding mass in stud bays and live with less isolation. So from inside out: 5/8 drywall -> 5/8 drywall -> poly vapour barrier -> 2x4 studs filled with pink fluffy insulation -> 4" air gap -> 2x6 wood stud wall filled with pink fluffy insulation -> 7/16" OSB sheathing ->Tyvek wrap -> wood siding.
Does this look right? Is my vapour barrier on the correct side here? Or should it remain on the inside of the exterior wall?
I would not build a double stud wall without a rainscreen. A good air control layer and thick walls filled with mineral wool or cellulose will block almost all sound, so I don't think a bit of extra mass is going to matter. Houses tend to be resilient and as a designer I have to think about worst-case scenarios, so even theoretically risky assemblies can be fine. But I would still keep GWB as an interior finish only.
You should not have an air gap between insulation layers. Pink fluffy insulation is ok but cellulose or mineral wool are better in most ways. I don't use poly sheeting because it allows no drying to the interior, but it might be ok (or required) in Ottawa. It is important for any vapor retarder to be near the interior, not the exterior, in heating-dominated climates.
It's 1/2" wood siding (cedar, I think) nailed directly to the sheathing.
Yes, vapour barrier on the warm side is required here. I wasn't sure if, with a double stud wall, the interior side of the exterior wall could still be considered the "warm side". Ok I will stick to keeping vapour barrier on interior side of inner shell.
Oh I see what you mean, install drywall at the interior of the exterior studs. Extrapolating from the IRC rules for walls, if you're using the same insulation throughout the assembly, you would want the drywall to be at least 44% of the way from the exterior to interior. Further out than that it could accumulate moisture. There are many variables that will affect the actual condensing plane location, but I would want drywall at the inner half of the wall to be safe.
You could use an exterior grade gypsum board like Densglass that is moisture tolerant here. What you’re really trying to do for soundproofing purposes is to add mass to the wall, so the exact type of material doesn’t matter as much as the density and Mass of the material. I wouldn’t actually bother with adding these strips, I’d build the wall as follows:
What I would do here is put two layers of 5/8” drywall on the inside of the outer wall, which would give you your extra mass in the middle of the wall. Use green glue between the sheets. I’d use mineral wool everywhere instead of fiberglass since mineral wool is slightly better than fiberglass for sound dampening (although not as much different as you sometimes see published). Use a double layer of 5/8” drywall with green glue between the sheets on the on inside of your room, and hang it on resilient channel.
If you want to add more soundproofing, use three layers of 5/8” drywall in the middle area of the wall. Using different thicknesses (three layers on one side and two on the other, for example) also helps reduce sound transmission by making walls with different resonance characteristics which tends to reduce the chances of any particular resonant frequency making it through the wall.
Using LSL studs might also help a little since they are “more dead” than conventional lumber for the purposes of sound transmission. I can’t say I’ve actually tried those before for this purpose but it might be something you’d like to try.
I can tell you that a 3 hour wall (three layers of 5/8” drywall on both sides) is very, very dead and sound pretty sound proof. I built a wall like that for a generator room (three 1.25 megawatt generators) when we couldn’t use a block wall. It was a MASSIVE wall, and completely dead when knocked on. I think if you use three layers in the middle of your assembly and two on the interior on resilient channel, combined with double studs and mineral wool, you’ll be pretty happy with the soundproofing levels you get.
Bill
Hi Bill,
Thanks for your advice. That's a good idea and I've considered using that type of moisture resistant drywall. I'm still concerned about the ability to hold up over the long run though.
As far as attaching the drywall to the inside of the exterior wall before building the interior wall, respectfully, I think this would actually make the isolation worse. What you're describing is a 3-leaf wall assembly, whereas the greatest low frequency isolation is achieved with a 2-leaf wall assembly. There have been studies done that show removing the drywall before building your interior shell greatly improves the ability of the wall to isolate low frequencies, which with a drummer and bass player both pounding away at 110dB + is crucial for what I'm trying to achieve. I have no doubt though that the walls you've built that way in the past still performed well for your application. I appreciate the input!
A place without interior moisture load will not cause issues with any double stud construction.
Since it is a studio, the amount of time in there will be limited and there will be no cooking or showers. This means that the interior dew point will be close to outside dew point so there is very little risk of condensation.
None the less, I would not use regular drywall in the stud bays.
For sound the only thing that matters is weight, OSB and typeX are similar density use that instead.
P.S. I find that metal studs work much better for double stud walls. On paper the STC is not that different but in practice it is noticeable, I think maybe because of low frequency transmission. If you can get some of the heavier gauge metal studs (20ga), I would build with that.
Agreed. After TONS of research into double stud walls and wet sheething, I've concluded the risk is minimal and I'm proceeding with my own double stud build. If the interior vapor barrier* is detailed properly and the house is very air tight overall, the risk should be minimal. The only** moisture source is interior humid air, so if that's kept out of the walls you should be good to go.
*I say "barrier" (instead of retarder) since I plan to use good old "Super-Six" polyethylene, which has an extremely low vapor permeability. As a material it is essentially a barrier to moisture, whereas the more permeable materials (or "smart" membranes) allow far more moisture transfer at all times. Whether a barrier or retarder, they all depend on proper detailing to perform.
**Some argue exterior water leaks cause concern, but that's an issue regardless of wall type.