Where does the vapor barrier go on an exterior framed wall assembly?

When you frame wall assemblies in a basement, and this assumes the basement is more underground than not, you install a vapor barrier on the interior side of the framed wall assembly.

Not that it (the vapor barrier) would go anywhere else in a cold region but it could be a case that you would not need one, but that would be determined by the actual wall assemblies that exist, the materials they are made of, and how cold exactly is the climate you live in and for how many days a year it remains cold at what temperature.

But, to be fair and in short, you have to address these issues from the exterior first because what you are going to do in the interior side is basically capture the ability of condensation to move, and take it out of the environment.

In cold regions, the vapor barrier goes on the interior side of the living or heated environment directly attached to the studs. This is not up for negotiation, it is based on the principles of moisture diffusion, while very complicated for those that continue to study and up-date the phenomenon, there are standard considerations.

Building Science Digest: RR-0410 Vapor Barriers Wall Design
“Good design and practice involve controlling the wetting of building assemblies from both the exterior and interior and different climates require different approaches.”

RR-0412: Insulations, Sheathings and Vapor Retarders
“Two seemingly innocuous requirements for building enclosure assemblies bedevil builders and designers almost endlessly: keep water vapor out, let the water vapor out if it gets in. It gets complicated because, sometimes, the best strategies to keep water vapor out also trap water vapor in.”

Short answer is that in a climate with 8000 heating degree days (8000 / 72 degrees=109 days of cold weather) the vapor barrier is placed on the interior side of the room…no discussion. At 4000 heating degree days and depending on the materials that the wall was built from a vapor barrier might not, and mostly will not be used (this would be close to where I live in Grand Bay, Alabama).

As an aside, based on the perm rating of the materials I wouldn’t think that my home that is only 27ish years old (as of this writing) would have required a vapor barrier. But it does, and it is on the interior side of the framing.

We have about 3 or 4 weeks of cold weather, and this cold here isn’t below freezing cold like people experience in Northern climates. So why does it work? It works because the HVAC helps in the process. The ability of the wall assembly to dry from interior to exterior or from exterior to the interior would have been made possible by the number of heated days (by the Sun) or the HVAC. So while they got it wrong, they still got it right.

Now if we were in a tropical environment, the placement of a vapor barrier would be on the exterior of the framed assembly, often the vapor barrier will come in the form of the actual finish, like say Stucco.

But I guarantee you if you placed your vapor barrier on the exterior side of the wall assembly in a cold region, the vapor barrier would do what it is supposed to do, but the problem would be that the structure would suffer and rot, mold and decay would be the by product.

Also, if the vapor barrier is placed in the middle of a double wall assembly you are asking for trouble as well. I have to go back to my soft drink can analogy to perform this next magic trick.

Kid gets an aluminum can soft drink out of the refrigerator. Within seconds of being outside of the cold environment, the can starts to sweat. Now you think this sweat is part of the can don’t you? It isn’t, it is condensation in the air, the warm air moving towards the colder outside of the can.

Now take that analogy and place it on the inside of the middle of a wall assembly full of insulation and what happens? The gas that creates condensation when moving from the warm side to the cold side will “hit” the solid plastic sheeting, and stick to it.

When the house starts trying to dry out, the moisture will migrate back into the interior framed wall, but before it makes it inside it has to go through the insulation on this interior wall. It will diffuse on the insulation, wet the insulation and become a big problem and you will not know about it for months and months.

BSD-106: Understanding Vapor Barriers
“The function of a vapor barrier is to retard the migration of water vapor. Where it is located in an assembly and its permeability is a function of climate”

Basement moisture control

One of the main reasons a basement gets or stays wet is due to the walls being below ground level and subject to the moisture that the Earth contains, that’s one reason. Another is the fact that the Earth is cooler and if the temperature/dew point is elevated above the exterior wall then you have a classic example of “warm to the cold side” path when temperature seeks equilibrium. Still a third reason is the typical porous cinder blocks used in the construction that can retain water.

So you have two paths that condensation and moisture can travel on built into the basic basement, from the exterior and from the interior.

Seems like a losing battle already doesn’t it? The first line of defense is ALWAYS at the point of penetration, this means the exterior side of the structure. Where ever there is a wall underneath ground, then that wall should have been prepared at the time of construction.

This would include things like asphalt based sealing and rubber or felt membranes on any part of the wall that is below ground elevation. The builder would also consider and install a French drain around, if not the entire perimeter, at least at the areas of the basement that are below ground elevation. Does any of this seem likely to exist in your area?

Anything above ground elevation would at least be properly sealed and painted. Concrete block is a very porous material and can and will soak up moisture until the point of saturation and move right inside. You cannot stop exterior moisture or water problems by approaching a level of attack from the interior side it will not work.

If the basement moisture control problem is related to cracks or poor exterior wall preparation, deteriorated wall sealing or the often as likely event that the ground elevation has gotten higher and is now covering the, what was, unprotected exterior basement wall this has to be addressed from the exterior of the structure.

This is not uncommon especially after years of snow and water movement, owners modifying the grounds, flower beds being built up against the walls, etc., etc. But let’s entertain the notion that maybe all these barriers on the exterior are in place and are working properly. You still do not want to place insulation up against the concrete/blocks since the potential for the dew point to change with the climate still makes that path a two way street, it can come in and it (moisture/condensation) can go out.

So “warm side to the cold side” can change and just as easily be on the outside of the structure. Now if we add insulation properly, to the newly framed wall assemblies as required by acoustic law, then you are going to make the interior side of the room the warm side…if not always, at least in large part.

Then you add electronic gear, human bodies and you will be warm mostly all the time, and that leads you to HVAC. The ability of a properly installed and sized unit will be able to even out the moisture in the room(s) so cannot be over-stressed.

A recording studio in a basement?

Basements back in the day, the ones you have read horror stories on with leaking, had several things in common. One is that they were made out of cinder block. Why is cinder block not a good material for this you ask? Because it is porous, like a sponge and each block can soak up water. And the joints become the weak part of the structure against the Earth moving. And when the joints begin to loose the lateral battle, and they will, the joints crack. Direct moisture path at your service.

The inclusion of a sump pump in a basement recording studio anticipates that water will be present. I wouldn’t have one since the water proofing preparation starts from the outside of the structure. Like shingles on a roof only different.

You are at the beginning of a very, very long journey if you are going to include a basement as part of a recording studio. One filled with new concepts and ideas previously unknown to you and some others that will be reading your posts as you pursue your goal.

“Exterior walls of studio room: do I want insulation or air between drywall and exterior wall? Insulation seems logical, but I’ve read about the value of air.”

If you have read about the value of air it was in respect to interior treatments. When broadband panels are part of a treatment it is often recommended to affix the panels the same distance from the wall as the thickness of the panel. Say a 4 inch panel would be affixed with 4 inchs of air behind it. This increases the ability of the panel to reduce low frequency greater than if it where simply attached directly to the wall face.

This is not to say that a 2:1 ratio is the final word. 3:1 ratio has been used as well, it just depends on the frequency(s) in question and what it will take to attack it properly.

Studies have shown that to get the highest possible transmission loss from a double framed basement studio wall assembly that insulation, regular residential type, should be used in the air space. This helps to stop the resonating that happens when vibration passes thru the wall panels and enters the cavity and reduces the ability to excite the air if it is (and it often is) entrapped.

Understanding that a double framed assembly is just a hard panel attached to a frame with a decoupling mechanism and another frame with a hard panel let’s look at RC-1.

In a typical use RC is the decoupling mechanism that turns a single frame into a mass/air/mass assembly once panels are installed on one side of the wall assembly and directly to the Rc-1 on the other side.

The problem is this. When you reduce the air cavity with this type of wall assembly in your basement recording studio you raise the Natural frequency of the assembly so while you win, you lose. The small air cavity reduces the assemblies ability to contain low frequency, the area in music that each one of us has issues with. Or maybe the neighbors have issue with.

So taken that you are considering a basement studio without moisture let me recommend a few things after a little more discussion. An Earth damped concrete slab is considered one of the best starting places when trying to develop a high transmission loss recording or critical listening room. So having Earth damped walls should only increase the ability of this room to contain sound…high isolation. The weakest link in a typical basement is windows and door ways and the ceiling since the mass that it took to create the floor (concrete) and the walls (concrete) cannot be matched by light weight sheetrock/plywood and especially single pane glass.

Make the basement as tall as possible and as wide as possible and as long as possible and as post free as possible. This is not a sprint…it is a marathon.

In general it can take 3 to 4 months to get a handle on the simple concepts that you will be working with. In order to convey these concepts to the persons that will be developing your home you should have such a good understanding of these that you can explain to them, should they be the ones to do it, how it is and what it is that you refer to and why.

Trust me when I tell you this, what it is that house builders know about house building has nothing to do with what we are discussing here. I know because I have done both.

To represent how fast things can get out of hand…

The foundation of anything is always the most important so in this case you or someone in your position would have the ability to develop a fully decoupled slab that will have reduced flanking from the existing structure and from the surrounding low frequency that travels at ground level generated by trucks, nearby highways and byways and traffic in general.