Styrofoam Insulation and the Vapor Barrier for a 2 X 4 Interior Stud Wall Assembly
This seems like such a straightforward question, but I have yet to get a straight answer so I now offer it to the Green Building Adviser community. My gambrel-roofed garage, like most garages, has never had HVAC. And the garage’s second floor has always been a place of storage. But no longer: most of the “storage” has left this space and the stairway to the second floor has an insulated door. The space is now ready to insulate and climate control. The ceiling has its own set of issues, but the building’s two end walls, framed with 2 X 4 studs, are the next matter. There’s not much room for insulation, and this has led me to consider Dow Extruded Polystyrene Foam Insulation. 2” of insulation = R-10, another 1.5” = R-7.5, for a total of R-17.5. That is a bit less than the required R-20, so if I must add an I” of wood to increase the interior stud cavity from 3.5” to 4.5”, and add an additional 1” layer of Styrofoam (R-5) this would bring the total R-value of this wall assembly to 22.5. Those are the raw mathematical elements. What I am unable to understand is how to handle the critical vapor barrier aspect of this simple project. I understand dry wall will be needed to cover the Styrofoam. That will be attached to the studs, either with or without the additional 1” of wood to deepen the stud cavity. But what is the best way to add a reliable vapor barrier to this wall assembly? I’ve seen where 6 mil plastic could go under the dry wall, carefully stapled to the wood to cover the Styrofoam. But is it possible this knowledgable GBA community has any better ideas? Any assistance would be greatly appreciated. Also note: The Climate Zone of this building is 5.
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Replies
Be green and use polyiso instead of XPS. You will have less thermal bridging if you put the additional 1" of foam over the studs (no 1"extensions). Tape it well to serve as an interior side air barrier and a vapor barrier/retarder.
Around here polyiso is sold by the pallet only. Grrr. But before I move on this I’m looking into going that better route.
This article is worth reviewing: https://www.greenbuildingadvisor.com/article/walls-with-interior-rigid-foam
I don't think you need (or want) poly, but you do need a vapor retarder. You should be able to achieve that by taping and caulking your foam layers and installing drywall in an airtight fashion.
But let's see what the experts advise.
Thank you for that GBA link, which I will definitely look at ASAP.
And Bill’s post will get ALL of my attention, ASAP
"Extruded styrofoam" is what we call XPS, eXtruded PolyStyrene. It's the "least green" of the three common types of rigid foam used in building projects. The other two types are EPS (Expanded PolyStyrene), and polyiso (polyisocyanurate), both of which are greener options. EPS has slightly lower R per inch (about R4.2) compared to XPS, but XPS will gradually degrade in terms of R value to get to about the same R number as EPS. Polyiso is about R6 per inch, so it gets you the best R per inch. I personally prefer polyiso since it's easier to work with, but EPS is cheaper.
The "6 mil plastic" you're probably thinking of is a polyethylene sheet. This is a true vapor BARRIER, and you probably don't want that unless it's required by code in your area. Most areas won't require that (unless you're in Canada, there are others on GBA more familiar with Canadian codes than I am that can help you if you're up there). You ESPECIALLY don't want a vapor barrier on the interior if you have an exterior that doesn't allow any drying (such as foil faced polyiso, which is sometimes used as an exterior insulating sheathing).
Your plan seems to be to cut rigid foam to fit between the studs. We call that "Cut and Cobble", and it's pretty labor intensive. What I would recommend is using R15 mineral wool batts between those studs, and a smart vapor RETARDER (not a vapor BARRIER) on the interior, such as CertainTeed Membrain.
If you want R20 or better walls, you have two potential options if you can only work from the interior side of the wall:
1- Fur out the studs 1.5" to allow you to use R23 mineral wool batts that are made to fit 2x6 stud walls. You'd still need that vapor retarder on the interior in this case.
2- If you have an EXTERIOR that allows drying (vinyl siding, etc.), then you could put 1" polyiso over the interior of the studs to get you up to R21 with R15 mineral wool batts between the studs. You wouldn't need any furring this way, since the polyiso can just cover the studwall and you can anchor your drywall to the studs through the polyiso. This would also cut down on thermal bridging to get you a better performing wall compared to option 1 above. With foil faced polyiso on the interior, which is a vapor BARRIER, you don't need anything else but you really need to be sure you have a vapor open exterior if you go this route.
Note that these aren't your only options, but they're some of the easiest options to build.
Bill
Bill:
Thank you for your response. And, as usual your analysis and recommendations are spot on. Because it was easily available where I live I was considering XPS. Lousy reason, with an even worse impact relative to sustainability and the environment. Your mineral wool batts between-the-studs and polyiso over-the-interior-of-the-studs concept to obtain a healthy R-value makes all the sense in the world. And it's way beyond time to bite polyiso's expensive-shipping "bullet" to get this project moving! Your willingness to share your knowledge with stone amateurs like me is greatly appreciated!