Vapor barrier?
I’m renovating a (1982 era) 2×4 stud modular house in Climate Zone 6 (upstate NY). I have the interior all gutted and I’ve furred out the walls with two layers of 2×2’s so that I get real insulation depth of 6.5″ inside. I’m thinking to insulate with either spray foam or blown-in (wet) fill insulation. Because of the furring at 1.5″ of the 6.5″ is “continuous”.
I’m wondering if I need a vapor barrier on the inside if I do the blown-in insulation? I prefer blown-in because I hear it is cheaper and is more environmentally friendly, but I want to be confident of the building science.
Other relevant info: House is clad in vinyl which is set over 3/4″ blue foam. The sheathing is plywood, not OSB. I assume there is some building wrap on the outside, but I have not demoed under the vinyl so I can’t say what it is. I have not demoed the ceilings, and the attic insulation is pink batts which are in decent shape. I will augment the attic insulation with loose fill.
Thoughts?
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Replies
In zone 6 with ~1.5 perm ~R3-R4 foam on the exterior you will DEFINTELY need an interior side vapor retarder and take care to build it air-tight.
A "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain) or Intello Plus is preferable to 4-6 mil polyethylene. Smart vapor retarders behave as vapor barriers when the exterior sheathing is cold, lowering the humidity of the entrained air in the insulation, but become vapor open if/when moisture levels are high enough to support mold.
Cellulose will be more protective of the structural wood than fiberglass or foam, but slightly lower R (though higher mass) than sprayed fiberglass, and comparable R to half-pound density open cell foam. Damp sprayed cellulose may have shrinkage issues if not installed a high enough density, dense-packed celluose can be either dry or damp sprayed. Damp sprayed fiberglass (JM Spider) does not have this issue at any density.
Hi Dana - Thank you, I am familiar with Intello-Plus (from 475 Building Supply). How does that stuff work? Also - since I have an existing ceiling is it ok to install that only on the walls? The ceiling (as described in my previous email) is batt insulation in a vented attic. I will add insulation to what was originally installed... Thank you
Intello Plus works fine, and may be the preferred product in your stackup. It's more rugged than 2-mil nylon and a bit more vapor tight, but it's still sufficiently vapor open when the humidity is high enough to allow drying toward the interior if needed.
Intello Plus is also tough enough to dense pack cellulose into in lieu of blowing mesh.
https://foursevenfive.com/blog/how-to-install-dense-pack-cellulose-with-intello-plus-airtight-membrane/
Tim,
Issue #1 is that your exterior rigid foam layer is too thin, according to building science principles. That fact is no reason to panic, however, especially if you are able to install a smart vapor retarder on the interior. More info here: "The Exterior Rigid Foam is Too Thin!"
Since your exterior rigid foam is so thin that it allows a little bit of (very slow) outward drying, you might want to consider insulating your walls with closed-cell spray foam (or perhaps the flash-and-fill method, a variation of flash-and-batt). For more information on flash-and-batt, see "Flash-and-Batt Insulation."
Needless to say, it would have been better to have an insulation plan finalized before you installed the two layers of 2x2s.
If you decide to use closed-cell spray foam or the flash-and-fill method, you don't need an interior vapor barrier or vapor retarder.