Vapor Barriers In Soundproof Home Recording Studio Construction
May 20, 2024I get asked this question a lot: where should I put my vapor barrier in my soundproof wall? In this article I will teach you some best practices for installing vapor barriers and how they relate to your soundproof system.
1) When you need a vapor barrier
Vapor barriers are a common aspect of normal construction and are still needed in soundproof buildings with walls and roofs touching the outside environment. You do not need a vapor barrier with interior walls, you only need a vapor barrier around the entire envelope of your structure.
Let's take my studio for example, I have four walls touching the outside and a roof. My vapor barrier should be a continuous material that covers all the walls and ceiling and ideally never breaks a seal even where the walls and roof rafters meet.
2) What is a vapor barrier used for and why you need one?
The vapor barrier is meant to control condensation build up in your walls and roof and floor. It is needed because the build up of warm moisture inside your building in the winter will want to move to the outside cold air and will condensate on the cooler exterior surfaces of your building like the exterior sheathing leading to moisture damage. In reverse, in the summertime, the warm moist air from the outside will want to enter your cool air conditioned building leading to condensation on the outside surfaces and inside surfaces due to tiny air leaks in your building envelope.
You notice I say the moist air "wants to move." If you remember back to high school physics high energy states like to move to low energy states to create equilibrium. Hot moist air is high energy and cold dry air is low energy. The two will want to reach a state of equilibrium leading to moisture traveling to cooler surfaces and condensating which leads to mold and mildew growth.
3) The problem with polyethylene or "poly" vapor barriers
According to Pretty Good House: A Guide to Creating Better Homes, polyethylene or poly, as it is commonly known, is not an ideal vapor barrier in all but the coldest climates. The reason is that the membrane was usually rendered useless after many staples, nail holes and other tears were introduced during installation (Kolbert et al., 117).
What is the solution then? The answer depends on your type of construction and this is where things get interesting. If you have a double wall construction then the vapor variable membrane is ideal. if you are using single wall construction with acoustic clips and channels then you may want to use a different approach. I will go over all the options in the following section.
4) Vapor Variable Membranes
A great solution for your soundproof building is to use vapro variable membranes. "Vapor-variable membranes are designed to resist the passage of water vapor from the interior into wall and roof cavities in winter but allow any accumulated moisture to dry to the interior when the seasons change" (Kolbert et al., 118). You can buy vapor-variable membranes from brands like Pro Clima, CertainTeed, Siga, Delta and Rothoblaas. In climates with both cold and warm seasons the membranes should be installed between the drywall and the studs of the inner wall.
Compared to poly these membranes are more durable and will not rip or tear as easily. They still need tapes and seals at their seems and should always be installed according to the manufacturer's guidelines.
5) Spray Foam
Closed cell spray foam can work as a vapor control layer, however only if it is installed correctly. If any gaps or shrinkage occurs in the spray foam then water vapor can still get in and condense inside your walls. Make sure if you use spray foam that you create a continuous layer even over the studs themselves. This could mean increasing the gap in your double wall system because you don't want the spray foam to attach to your inner wall. The closed cell spray foam can couple the two walls leading to a failure in your isolation.
Also, spray foam does not help with acoustics. In fact, closed cell spray foam will hurt isolation because it is effectively shrinking the air space in your double wall system. Remember, spray foam does not replace fiberglass or mineral wool insulation. You still need an open insulation for sound control in your wall system. Second, spray foam has toxic off gassing and many brands increase global warming. For all of these reasons I don't recommend using spray foam unless if is used as a vapor barrier in a cathedral ceiling, but even then there are better methods, such as adding insulation outside the roof sheathing.
6) Vapor Open Assemblies
Another option if you are building your soundproof room from scratch is to use a vapor open assembly. The idea with this system is that the sheathing acts as the primary vapor and air barrier and the insulation is place on the outside of the wall not the inside. This method keeps the wood sheathing warm in the winter and allows the open insulation like cellulose to vent vapor to the outside. You can see in the diagram below an example vapor open wall assembly.
The real downside to this system is the added material and build costs. In soundproofing you would still need to insulate your interior walls anyways for sound, so you are essentially adding more cost without to much benefit, but this is an option and maybe one benefiting a clip channel design over a double wall assembly.
7) Outsulation Wall Assembly
Lastly, there is the outsulation wall assembly. This is similar to the vapor open assembly accept that it uses a vapor closed insulation layer rather than a vapor open layer. In this design the sheathing is still your primary air and vapor barrier with rigid polystyrene or polyiso insulation board layered over the outside of the sheathing. The insulation board becomes the primary thermal control layer and keeps the sheathing warm in the winter preventing moisture condensation.
Conclusion
Vapor control is an important aspect of any design and build. It enters the realm of green building design and building science. As you can see there is some overlap in green building design and soundproof building design. Air tight structures and double wall assemblies are both preferred. The main take away for use soundproofing designers is to look at our options and choose the best one for each specific job. If you are using a double wall assembly I would use the vapor-variable membrane method. If you are using acoustic clips and hat channels you may opt for a vapor open or outsulation wall assembly because the clips would penetrate your vapor variable membrane.
I would stay away from spray foam unless you feel it is your absolute only option. Remember, spray foam is hazardous to health, bad for the environment and is expensive. I hope this article has taught you more than you could have ever imagined about moisture control. As you are planning and designing your home studio talk with your builder about these methods and buy the book I referenced. It is a great resource for any building, but especially us home studio designers.
Works Cited:
Kolbert, Dan, et al. Pretty Good House : A Guide to Creating Better Homes. Newtown, CT, The Taunton Press, 2022.
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