Using leftover R-Max
The leftover R-Max I have is either getting used this weekend or going to the landfill.
In the responses to my original question about using it as a radiant barrier, the alternate idea of using it as exterior insulation came up, which seems like a great idea to me. Is the below install safe and/or correct for the specific installation location, which is on the “outside”, lower half of a 2nd level south exterior wall, where the lower half of this wall is in an attic over the 1st level (ie 2nd level footprint is smaller than and set in from 1st level footprint). Thus, the portion of the wall onto which I’m proposing installing the R-Max is not subject to water (unless I have a roof leak); only to humidity.
Climate Zone: 3A.
Conditions: Attic temp regularly gets to 130 F; trying to reduce the temp in the conditioned space on the other side of this wall.
Install details: Wall onto which the RMax would be installed, from inside to outside: Latex painted gyp board; 2×6 wall insulated with damp applied cellulose R-21; Zip sheathing, taped.
– Can I apply the Rmax directly to the Zip sheathing, or do I need an air space between them?
– Apply the Rmax with the foil side facing the attic?
– What is the best way to fasten the Rmax to the zip? I’d rather NOT put holes into the sealed Zip sheathing with a bunch of nails.
Thanks!
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Replies
Foil should face an air gap and you should use furring strips or some sort of 3D mesh to make a drainage plane outside of the polyiso before you install your final siding. The drainage plane will act as your air gap to make the radiant barrier effective. You do not need a gap between the foam and zip, in fact doing so would allow convection currents to completely bypass your exterior insulation, rendering it useless.
The foam can be attached with short screws or nails with washers on them as a temporary holding mechanism but once you put the furring strips on those will be anchored into the studs with a penetration of at least 1.5” onto the studs and they will hold up the foam more permanently. Technically Zip does not require covering fasteners (at least on walls, not sure on roofs) unless they are overdriven so this should not be a concern.
@matthew25 - thanks for taking the time to reply.
Since the area I'm contemplating applying the RMax to is the lower portion of the exterior wall that is entirely in the attic, there will be no exterior siding, so no furring strip. Right now in the attic that part of the wall is just the exposed green side of the Zip. I'll just apply the Rmax directly to that. So based on your response
1. No air gap between the zip and the RMax (which sounds correct to me)
2. Foil side of the Rmax facing the attic (this will be exposed and the "finish" in the attic)
3. I'd rather not put holes in the zip. Any reason not to attach with some dabs of adhesive?
I think you're basically describing an attic knee wall area that happens to eventually project out of the roof plane. Please correct me if I'm wrong.
You should be able to adhere foam to the zip without a ton of fasteners. You'll likely either need a small amount of fasteners (really not a big deal to put into the Zip) or some kickers braced off of something else to hold the foam in place while the adhesive cures.
I've had success with the foam construction adhesives
Just a reminder that a lot of our unused building materials don't need to go to the landfill. There are a lot of resources to get them into the hands of people who will use them. Craigslist, Facebook Marketplace, Buy Nothing groups, a "Free" sign, etc.