Home interior air/vapor barrier selection
Hello, new member here, I’ve read a lot of good information off this site. I’m building a post frame / pole barn style home and am looking for guidance on proper air/vapor barrier selection for the exterior walls. We actually purchased the home in an uncompleted state (structure is complete and dried in, no interior finishing has been completed). While there are a few things I would have done differently, overall it is well built. The home is located in climate zone 5 (West Virginia, right near the tip of the Maryland panhandle). The home has exterior metal siding and roof (agri panel) and traditional building wrap. The roof also has styrofoam insulation under the metal, I’m guessing ~1/2″ thick. I am insulating the exterior walls with 1.5″ thick rockwool boards cut to fit in between the horizontal wood girts and then 5.5″ thick rockwool batts to fill the remainder of the wall cavity. This should be ~R29 with minimal thermal bridging. I intend to put an air/vapor barrier over this on the interior side, from pole to pole (~8′ spacing). I will then install interior horizontal girts and then drywall. My utilities (electric, etc.) will be ran in the 1.5″ cavity between the back of the drwyall and the air/vapor barrier. I’m struggling with determining the appropriate barrier here and could use some advice. I believe since my exterior metal siding is practically impermeable, I should allow for some drying potential inward. I’m currently leaning towards Certainteed Membrain as I believe it will serve as an air barrier and prevent most vapor diffusion, but still allow inward drying. It’s also much more affordable and easier to purchase. I’ve also seen Intello plus and Siga Mijrax, etc. but the differences start to blend together and it gets confusing. What’s the right option here?
Additionally, I’m not sure what to do on the ceiling. It is a slightly vaulted scissor truss ceiling, with traditional attic space and soffit/ridge ventilation. Currently planning to use two layers of R30 rockwool batts, but that may change depending on how difficult install goes. Either way, it will be a high R value, just might end up being something else like blown cellulose. We will have tongue and groove pine for all ceiling. I know air leakage through the T&G will be an issue if not addressed. Can I use Certainteed Membrain (or one of the other smart air/vapor barrier products) on the ceiling behind the T&G pine to serve as an air barrier? Or something else? I don’t want to install a drywall or plywood backer if at all possible as the ceiling is already fairly low and the additional cost/labor. I don’t think I need to worry about inward drying for the ceiling being a traditionally vented attic, but certainly do need a good air barrier.
Thanks in advance for any guidance.
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Don't worry too much about the permeability of the metal siding. Any condensation on the back of the siding should run down the corrugations and exit at the bottom of the system, or it will if the siding is properly installed. In your climate, an interior vapor retarder is less important than in colder climates, but MemBrain will provide some additional safety margin. Your walls should work fine as you have described. The challenge in any post frame building is making the interior air barrier continuous at the large frame members, and especially where those members pass from interior to exterior to support roofs and other such details.
Using an air barrier like MemBrain on the ceiling will certainly help to make it airtight. You will have to install the T&G ceiling before insulating, though, as the staples fastening the MemBrain to the ceiling won't support the weight of the insulation above without support from the wood ceiling.
Thanks for this feedback, it is helpful. I agree that the ceiling install will be tricky. Insulating after the T&G is installed will be difficult due to the scissor trusses. I will have very limited space to be able to access the lower portions near the eaves from the attic side alone. My hope was that the rockwool batts are rigid enough to hold their own weight when pushed in between the trusses (at least long enough to get the rest of the ceiling assembly in), but I have no prior experience using rockwool so this may not be the case. I may be able to do it in phasese/sections as well, installing a few feet of certaineed membrain / T&G pine / insulation, then moving a few more feet at a time up the vault towards the ridge. This way I have better access to the eaves for insulating off of scaffold. But this will be slow and I'm sure also not be as simple as it sounds. The good thing is I do not have a timeline and can focus on doing install the best way that I can vs rushing to meet a deadline.
I feel pretty good about the wall assumbly and installation details. Still need to determine the best approach for the ceiling. I think what has been described would work well in the end, but I'm not sure the practicality of installation. I could take an easier approach and go to blown in cellulose which solves my issue of accessing the eaves, though I think I prefer the rockwool product for a few reasons, even though it will be more difficult to install and likely more expensive, etc.
The Rockwool batts may be rigid enough to hold up their own weight, at least for the brief period during construction. Make sure your batts are the right size. For most attics I recommend loose cellulose because it's cheaper and it provides some air sealing benefit as well, but in your case, the friction fit of the Rockwool is more important. Good Luck.