Vapor Barrier Behind Pine Boards
I’m designing myself a little ski cabin in NW Maine (Zone 6). My contractor is willing to work with me but seems a bit reluctant to anything other than more traditional (2×6 with batt insulation) methods so I’m trying to make things as easy as I can for him.
Here are the details on the envelope…
– 11″ Double Stud Wall insulated with open cell spray foam. I asked for cellulose but his insulator informed us that dense pack cellulose is a lot more money in this area than open cell due to the added labor.
– Cedar clapboards on rain screen
– T&G Pine interior wall & ceiling
– Vaulted ceiling framed with 24″ deep parallel cord rafter trusses. Insulated with open cell spray foam.
– Zip sheathing, standing seam roof
– 1-1/2″ continuous air channel at underside of roof sheathing formed with 2″ rigid foam or plywood. Vented soffit and ridge cap
My primary question is, should I have him install a smart vapor barrier like Membrain behind the pine boards or am I keeping the wall/roof sheathing dry with only vapor diffusion to worry about and the rain screen on the wall and a vent channel at the underside of roof sheathing will allow the sheathing to dry
If I should install Membrain for cheap insurance, do I need him to be meticulous with caulking or gasketing at studs and rafters or don’t sweat a few staple and nail holes in the Membrain given that it’s not my air barrier, the open cell spray foam is?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
While open cell foam alone is code-compliant in ceilings because it's airtight, it's vapor-permeable, which can result in moisture migrating to the sheathing. With your vent channels that moisture shouldn't be a problem but I would spring for the Membrain to keep moisture out of the assembly altogether. It's hard to get a perfect seal around truss members so you'll probably have some air leakage through the foam edges as well. You need one really good air barrier somewhere in your assembly; since it won't be at the foam or the pine boards, it can either be at your vent baffles or at the Membrain.
In the walls, open-cell foam provides no benefit over well-installed fiberglass and a modicum of air-sealing, so if you're trying to keep costs and environmental impact down, I'd use fiberglass. I'm dubious that there aren't any cellulose installers in the area, though. I'm guessing you're in the Rangeley/Kingfield area?
@Michael Maines
Thanks for your reply! When you say "open-cell foam provides no benefit over well-installed fiberglass and a modicum of air-sealing" I assume you mean if one already has a very well done air barrier since the R-value is no higher than batt or cellulose?
If I were building this myself I would certainly just go with rock wool batts and be meticulous about the air sealing both the membrain and the exterior sheathing. However, I feel like the open cell spray foam covers up a lot of the sins of a builder who I am not confident will pay attention to those air sealing details when I'm not there checking. I really don't think he'll be meticulous with taping the membrain and the nail for the pine boards will also poke holes in so in my mind the benefit of open cell is that it will atleast stop or slow the moist interior air from reaching the sheathing and vapor that reaches the sheathing will dry with the presence of a rain screen. Am I still off base?
Its not that they can't do cellulose, they simply indicated that open cell was much cheaper as its less labor intensive in a double stud wall
Yes, fiberglass batts perform better than many greenies claim, they're easy to install (and not much harder to install well), and they're affordable, especially if you hire the work out--installers can often buy and install it for what you would pay for materials alone. Its R-value is very similar to cellulose or open cell foam. I prefer cellulose for other reasons but I would go with fiberglass over foam. Some air-sealing is required but it's also not difficult. If your builder won't do it, you can.
I strongly prefer cellulose in thick walls because it can safely store and distribute the moisture that will invariably build up in certain conditions. Its borate content protects everything it touches. With foam, fiberglass or mineral wool, the wood framing and sheathing is the most absorptive element. With high R-values there is little heat energy available to push moisture through, so assemblies are not as forgiving as they are in low-R assemblies. That doesn't mean we shouldn't build with high-R walls, just that it's important to do everything you can to help the wall perform well.
Mineral wool also performs well and is a little denser than most fiberglass batts, but it actually has higher levels of embodied carbon so I only recommend it when fire protection is a priority.
I would not try to use fiberglass in a trusses ceiling; cellulose would be best but open-cell foam is a good alternative, as getting tight fits around all of the webs is nearly impossible with fiberglass.
If you are already installing a smart membrane, which I would always recommend with tg, I’m not sure why cellulose would be any more labor than open cell. Maybe mention you are doing this barrier to the insulator and see if it changes cost. Personally I would not do open cell, just do batts if you have to, you don’t need it for air sealing.
You can't do a good dense-pack behind an airtight membrane without using special techniques, and even then it's tricky. When using a variable permeance membrane over a cellulose wall, the wall is usually netted and blown, then the membrane added afterwards.
Is this not accurate?
https://youtu.be/hOhE-UP72xg?si=eyEU4E_1Wb7n8sM-
Seems to me when the contractor tells you spray foam is the low cost option it is time to get a new contractor. I think this one is pissing on you and telling you it is raining.
Try getting a damp spray cellulose quote.
Walta
I would love to have other options. Unfortunately contractors don't grow on trees in this area. I was told directly from the insulation sub contractor that Open Cell was less expensive