Soundproofing a Doorway
Jeff_McBride | Posted in General Questions on
I’m learning about soundproof design for a couple of projects I have in my adobe home that was built in 1865. The first project is filling in a doorway that currently connects two bedrooms. On one side will be a bed wall and the other a TV wall, a perfect situation for investing in soundproofing. The good news is since it is an old style adobe constructed home, there is plenty of room to work with. The doorway measures 20″ deep x 32″ wide x 82″ tall. The bad news is with so much thickness to work with, I’m not sure the best construction design to go with.
I’m considering using Roxul Safe and Sound batts for soft insulation, Roxul RockBoard for rigid insulation, and double drywall with Green Glue Noiseproofing compound plus their sealant for filling around everything. But I think my biggest decision is the type of framing to use and combining the Rockboard and drywall. Should I double frame with Safe and Sound in the middle and Rockboard on the inside of the studs, more Safe and Sound between the studs and then double drywall? Should I screw the studs directly into the adobe or build a jam and screw into that? Should I also consider noiseproofing / resilient clips? With the double drywall I don’t think mass loaded vinyl will add enough additional benefit to merit the cost?
Any suggestions or insight would be much appreciated.
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My goto buget but very effective sound isolation wall is:
-2x4 wood stud covered with 5/8" drywall, high density batts in the stud space
- 1/2" air gap
-1 5/8 metal stud wall, covered with 5/8" drywall
Glue the perimeter studs and top and bottom plate to the structure on both sides.
Seal any drywall seams and edges with Green Glue sealant before taping. Somewhere around STC 55. Add a 2nd layer of drywall to either side to put it a bit over STC 60.
Properly done, you can stand on one side and yell and it will come through as a whisper.
Thanks Akos for your reply,
Any reason for mixing wood and metal studs? Do you buy different sized batts to deal with metal studs being "hollow" or adjust spacing? Roxul sells Safe n Sound in either wood or metal sizes but the latter seems harder to find.
The thin metal stud wall is to save space. Since you have more than enough space, you can go with wood studs.
Insulating the 2nd stud space gets you around a 4dB improvement in STC value, doesn't hurt, but not very noticeable. If you combine this with an extra layer of drywall, that will get you near 7dB, which is noticeable.
The key to any high STC assembly is air sealing. Something that is 95% air tight, might only get you half the STC improvement. Focus your efforts on getting this right.
Here is a link to quite a few sound attenuation assemblies for walls from our building code. As Akos said, a lot of their effectiveness comes from making sure they are continuous with no small gaps.
I'll check out the assemblies you linked.
Malcolm, that PDF you linked to is great! Easy to see STC ratings for the different designs.
Including things like this in building codes makes so much sense. I don't know why more codes don't do it, and I'm glad ours does.
Where are you located Malcolm?
First, don't waste money on rockboard here, since you gain more from other things that cost less. I've never actually seen rockboard incorporated in any of the standard sound attenuating wall assemblies, either.
Sound attenuation in wall assemblies comes basically from three things: isolation, mass, and what I like to call 'squishy stuff'. Isolation is hardest to achieve, but works great, and ranges from full double stud walls at the top end to things like resilient channel at the low end. Isolation acts to prevent anything rigid from connecting one wall face to the other, so that sound can't easily travel between the two sides of the wall. The down sides here is that a full double stud wall makes for a thick wall, a staggered stud wall doesn't perform as well, and resilient channel complicates the installation of drywall and pretty much prevents you from mounting anything heavy (shelves, TVs, etc.) to the wall.
Mass is usually accomplished with extra layers of drywall. Mass basically adds inertia to the wall, making it harder for sound to resonante the wall assembly. The typical "soundproof" wall will use a double layer of 5/8" drywall on both sides.
"Squishy stuff" includes things like batts, but also green glue if used between drywall layers. Squishy stuff acts to absorb sound, essentially converting some of the sound energy to very low levels of heat.
When you use everything together, you get a good performing wall, but you add cost and you take up space. For some basic assemblies (BTW, soundproofingcompany.com has a lot of info about this sort of thing) are as follows:
Best: full double stud wall. This would be two studwalls with an inch or so of air gap between, and a double layer of 5/8" drywall on each exterior side (no drywall in the middle "between" the stud walls). You'd put batts in the stud bays, but you'd maintain an air gap in the middle between the two stud walls. This has the best acoustic performance, but it makes for a thick wall and uses a lot of additional framing lumber.
A staggered studwall is a cheaper version of the above. There are various ways to build this, but probably the most common is to use a 2x6 or 2x8 for the top and bottom plates, then align every other 2x4 stud with the opposite edge of the top/bottome plates. Use the usual double layer of 5/8" drywall on either side, and batts in the stud bays. Performance won't be as a good as a full double stud wall, but you can make the wall thinner and use less material.
Resilient channel on one side of a regular studwall doesn't perform as well as the above, but it's a lot thinner. The major downside here is the extra complexity of the resilient channel. You'd still use the double layer of drywall on at least one side of the wall, but ideally both. You'd still put batts in the stud bays.
Last is just doubling up the drywall on either side of a regular studwall and adding batts in the studbays. This is better than a regular wall, but not as good as the fancier assemblies listed above.
Any of these assemblies can benefit from green glue between the layers of the doubled drywall. You can always add more layers of drywall too, but two layers is the standard -- you typically use other methods when wanting more soundproofing instead of going to a third layer of drywall. I have myself built walls with a triple layer of drywall, but I was after a 3 hour fire rating (for a commercial generator room), it wasn't for sound alone. The resulting wall felt and sounded like a concrete block wall if you knocked on it with your knuckles. I don't recommend building such a wall just for sound attenuation though.
What I usually do myself is double up the drywall and add Safe'n'Sound mineral wool in the stud bays. This makes a noticeable improvement at minimal additional cost and no extra space required. I do use exclusively 5/8" drywall. If I need more performance, I'll step up to a staggered stud wall or double stud wall. I'm not a fan of resilient channel unless space is tight, except for ceilings where hat channel is a good option.
Note that mineral wool performs slightly better than fiberglass for the batts, but it's not a huge difference. I like mineral wool because it's easier to work with, but it is more expensive. Make sure to seal things too -- air leaks are also sound leaks and can 'short circuit' the rest of your assembly. It's common to use sealants around the permineter as Akos mentioned, and to use fire putty pads over the back of any electrical boxes too.
BTW, sealing a door is tricky. About the only good way to do it is to put in some kind of a sill that the door can seal against on the floor. You end up with an interior door built like an exterior door since you need the weatherstripping to seal out sound.
Thanks Bill for your in-depth reply,
The project is a doorway currently, but I'm removing the door and walling in the passage. On one side a bed will be against the wall and in the other a TV will be mounted to the new construction.
Good to know about resilient channels not being good for weight bearing since I'll be hanging a TV on one side. Since I have 20" of depth I'm considering a double or possibly a triple wall design. Any reason in a double wall design that you don't back each wall with drywall? Just the challenge of having to pre-build it instead of building it in-place?
The big issue with the TV wall mounts is that you can't mount them with drywall anchors, they have to bolt into studs. If you bolt a mount through drywall into studs when that drywall is hung on resilient channel, the bolts 'clamp' the drywall to the stud, and short-circuit the resilient channel. Resilient channel acts as a sort of spring to isolate vibrations in the drywall from the structure of the stud, so the bolts put a rigid connection in place that prevents the resilient channel from serving it's purpose.
If you need to mount a TV, you need to use one of the double stud wall options. If you want the best performance, build the real double stud wall, which means a second top and bottom plate and a second set of studs. Make sure you have the air gap between the two stud walls -- at least about 1/2". The air gap is to ensure the studs don't touch anywhere, since you want to avoid having any rigid connections between the two wall faces other than the top and bottom plates.
I suppose drywall in the middle wouldn't hurt, as long as you maintain the airgap, but it would greatly complicate construction of the wall, and likely wouldn't gain much. None of the standard assemblies include such a middle drywall layer. The standard "soundproof" wall is a double studwall with a double layer of 5/8" drywall on both sides and batts in the stud bays. Green glue between drywall layers is common too. There is little reason to go beyond this level of soundproofing.
So a 2x4 wall with double 5/8" drywall on both sides would be 6" deep. The doorway is 20" deep. If I do a double wall design, that is an air gap of 8". This seems like a lot of space. My goal is high STC rating over non-outrageous building cost, so it seems like I could do something with this space.
There is no reason to try to maximize the air space. Go with 1/2" or 1" and that's all you need.
I think Jeff may want to align the walls with the existing ones. I'd fill the 8" with batts.
Malcolm, that could make sense. I'm probably too used to trying to maximize leasable space :-)
If filling that extra space with batts, it's still helpful to leave an air gap in the middle. Just underfill a bit and you'd be good. Mineral wool would be good here since it's more self supporting than fiberglass.
Filling the whole thing with dense pack cellulose would probably work pretty well too, but I've never tried that nor I have I seen any testing numbers for how it would perform in terms of sound attenuation.
I suspect the 1" air gap is only there to ensure there is no continuity between the structural elements of the two walls, but I may be wrong.
Yep, that's exactly what it does -- the air gap is there to ensure the two sides of the wall are mechanically isolated from each other. You don't want any rigid connections. What I'm not so sure about is if you have a suffiently thick wall and you stuff it completely full of insulation, I don't know if that would perform better or worse compared to the same wall with a small air gap. I've never seen any test data either way. With thinner walls in the usual assemblies, the air gap is always specified, usually as "1 inch", but as long as there is enough of a gap to keep the two sides isolated from each other you get most of the benefit.
Malcolm, yes you are correct on my wanting to make the new drywall flush on both sides with existing walls. I should have stated that in my original post.
Bill, my previous real estate was all about leaseable space, metal studs, drop ceilings, and HVAC runs. I get it. But this adobe is an interesting new medium I'm learning. And with current lumber prices, mud bricks / earth construction is not bad ROI :)
With 8" of space I can add a whole third wall in the middle and still have 1" airspace between each wall. So I could drywall both sides of middle wall and just exterior of two outer walls for simplicity of the build, Safe n Sound inside all.
I'm not sure what the STC of a 20" adobe wall is, but I think the more STC increase I can afford would still be less than the original wall.
You have to be a bit careful. Sound attenuation doesn't always work in the way your intuition expects it to. So for instance if you add resilient channel to an existing wall with0ut removing the drywall, and then add a new layer over the channels, that extra enclosed cavity actually decreases the performance of the wall. That's why it's best to pick an assembly that has been tested as a whole, and build it as closely as possible to how it is described.
There is no existing drywall. The existing walls were built in 1865 out of Adobe brick and covered with chicken wire & plaster. So I'll be sanding all the walls in the room, closing in doorway with drywall on the outside, and then plastering it, finishing the whole room with a consistent trowel texture, then paint.
And this 20" doorway is in an interior wall, the exterior walls are 36" depth. I don't have to close those in luckily.
The best way to insulate an interior door if you don't want a step is to install an automatic door bottom. Something like that https://www.tmhardware.com/About-Automatic-Door-Bottoms.html.
Thanks @AlexPoi, but I'm doing a doorway not a door.