Where do I put the vapor barrier in a double stud wall with pluming penetrations?
So if you have been following the site for a few weeks you may have seen some of my previous questions. Some have sparked a lot of debate and some I left out too many details to get any good input. However, I am back with another question. First, my wall assembly. I’ll have a double stud wall with OSB sheathing (except north wall where I’ll use plywood). Outer wall will be the structural wall and since it will be two stories I’ll be using 16″ centers with 2×4’s. I’ll have a 3.5″ gap and then another 2×4 wall. I may do the second wall on 24″ centers instead of 16″ centers, but that doesn’t really impact the question. Insulation will be fiberglass (yes, yes I know I’d rather use roxul, but I’ve got a tight budget…I know, I know blown in cellulose…I’m just not a fan and it’s still more expensive than fiberglass batts…I know, I know, but I’ll be doing a lot of air sealing so it will be okay…really, it will be okay, no I’m not trying something else…this is my wall and I’ll do it the way my budget and research dictate….zip it).
So (now that all that is over) my kitchen sink will likely be on an exterior wall and my plan is to have the drain go through the floor (no surprise there) and the supply lines come through the wall. So where does the vapor barrier go? I’ll have three layers of batts. My thought is that on every other wall the first two layers of batts will be unfaced and to use a kraft faced batt on the third layer just behind the sheetrock like in 90% of the houses out there. On this particular exterior kitchen wall I’m thinking I’ll do the same thing in most of the framed bays, but where the plumbing is do something different so the wall is sealed and the plumbing penetrations don’t break that seal.
At framed bays with plumbing penetrations use a smart vapor barrier after the first two layers then use roxul batts around whatever plumbing I need to. Roxul won’t have an issue with moisture and I think can be cut and fit better around plumbing penetrations.
One of my design criteria is to use really basic components and methods. Would standard tyvek work as a vapor barrier in those bays mentioned? Thank you
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I've read a couple of Martin's articles on vapor barriers and air barriers since posting this just a few minutes ago and I'm thinking that I don't need anything. If I use the roxul to fit snug around the few pipe penetrations (and I'm planning on using acoustical caulk on my studs so the sheetrock will be "air tight") I don't think I need to worry about any vapor barrier there or anywhere for that matter (though I'll keep the third layer of fiberglass batts with the kraft face). Everyone in agreement?
Andrew, Put your sink drain in the wall too.
If you do a good job of air-sealing your drywall, remembering in addition to the studs to seal the base plate to the floor and all interior wall top plates, you are probably in good shape. However that aside, it might be a good idea to see what the building code you are under says about your plan to not include a vapour barrier.
According to my local county building and codes enforcement website they use the 2012 editions of the International Building Code, International Plumbing Code, International Mechanical Code, International Residential Code. I am in climate zone 4 in east TN.
Also in my original question I misspelled pluming. Per East TN vernacular it should be plumbin'.
Andrew, Maybe someone familiar with your code can comment.
I've got to say it's fairly bemusing that people make design and construction decisions without knowing what their code requires.
Andrew,
There is no code in the U.S. that requires an interior vapor barrier. Some codes require an interior vapor retarder, which is a different type of barrier. Examples of code-accepted vapor retarders include the kraft facing on fiberglass batts and vapor-retarder paint. For more information on this issue, see:
Do I Need a Vapor Retarder?
Vapor Retarders and Vapor Barriers
2. I'm not a fan of including plumbing in insulated walls. In general, if you are going to the trouble of building a double-stud wall, you should keep your plumbing on the interior side of your interior air barrier. For more information on this topic, see Service Cavities for Wiring and Plumbing.
3. You suggestion that it makes sense to install Roxul near plumbing penetrations confuses me. In most physical respects, mineral wool insulation and fiberglass insulation behave the same. What you need to worry about at plumbing penetrations is air leakage. Roxul won't help you with air leakage.
4. Tyvek will never work as a vapor barrier because Tyvek is deliberately manufactured to be vapor-permeable.
So I guess I was/am still a little confused on the whole air barrier vs vapor barrier issue. Though I wrote vapor barrier I was thinking Air barrier. It looks to me like Intello is made of stuff very similar to tyvek. If I use airtight drywall I don'the need both, but there seems to be a strong argument for putting The air barrier on the Exterior side of the service cavity. So to ask my question more correctly...should I put an air barrier on the exterior side of the stud bays that effectively act as a service cavity?
Andrew,
Intello Plus is not similar to Tyvek.
While both can be used as air barriers, Tyvek is designed and manufactured to have a high vapor permeance.
Intello Plus is a smart vapor retarder. It is designed to have a low vapor permeance under most conditions, to prevent vapor from passing through. However, in moist conditions, when the relative humidity (RH) adjacent to the Intello Plus membrane gets very high, the vapor permeance of the membrane increases, so that some of the moisture on the humid side of the membrane can diffuse through the membrane. This property can help a damp assembly dry out.
Tyvek's most important function is to act as a water-resistive barrier (WRB), meaning that its usual location is between the back of the siding and the exterior sheathing.
Intello Plus is designed to be installed on the interior side of a wall assembly.
Andrew,
Q. "Should I put an air barrier on the exterior side of the stud bays that effectively act as a service cavity?"
A. Yes. A service cavity should be entirely on the inside of a house.
For more information on air barrier placement, see One Air Barrier or Two?
Andrew: in my double stud wall, we put the primary air barrier on the outside of the interior stud wall. Almost all plumbing and wiring is inside the air barrier. We also taped the Advantech sheathing at all joints and ta ped the typar wrb as well. We used a lot of tape! We hit PH level airtightness, even though my (excellent) GC was new to the airtightness concept.
More info in my house blog: stephen1147.wordpress.com