Rigid insulation options
Skilak24
| Posted in General Questions on
Hi, I am hoping for some advice on insulation questions. I live in coastal (wet) South Central Alaska, zone 6, and am building a 1,200 sq ft, two-story house (one main room downstairs, 2 bedrooms and a bath upstairs). We’ve landed on heating with mini splits and a supplemental wood stove. So we want to make sure our insulation is up to the task, but on a tight budget.
We’re planning on doing blown-in fiberglass or blown in cellulose insulation, and want to do additional rigid insulation on top of that. We considered rigid foam outside but were warned about condensation issues/ratios, so now we’re debating between 2″ of rock wool on the exterior, which is a higher sq ft material, labor cost, and frankly a little out of budget, and 2″ of EPS rigid foam and/or rock wool on the interior inside the vapor barrier. But overall, we’re trying to build with low VOC materials and I’m very much on the “less off-gassing is better” train.
Do you have any thoughts about these comparative R values to cost, and/or indoor air quality concerns? (I am a little worried about the VOCs/fire retardants of the EPS foam on the interior inside the vapor barrier, and I haven’t been able to find a clear answer on that.)
We’ve also considered the idea of using 2×8 instead of 2×6 for the construction, and just doing blown in insulation, though you’d then still have some thermal bridging.
Any advice?
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Replies
In CZ6, as long as you have at least 33% of the total R-value on the exterior, your indoor humidity is not unusually high in the winter, and your assemblies are reasonably airtight, your walls will perform just fine. 2x8 walls would give you similar R-value but your sheathing will be damp in late winter/early spring, so you need to provide a generous rain screen gap to allow it to dry readily.
For an additional measure of safety, you can use a variable permeance membrane on the interior, such as Siga Majrex, Pro Clima Intello, or Certainteed Membrain. Membrain costs half as much as the others but is flimsier to work with. In fact, according to table R702.7(3), unless you have at least R-11.25 exterior insulation over a 2x6 wall insulated with fluffy insulation, you need to have a class 1 or class 2 vapor retarder on the interior, and variable permeance membranes are safer than poly sheeting. https://codes.iccsafe.org/content/IRC2021P1/chapter-7-wall-covering#IRC2021P1_Pt03_Ch07_SecR702.7
Your post mixes the term interior and exterior insulation a lot and makes it a hard to follow the assembly you are proposing. Why isn't EPS on the exterior one of the options, for example?
What will be your WRB on the outside of the sheathing, and what is its perm rating? If your WRB has a low perm rating already then adding low-perm rigid foam exterior insulation won't make things worse. And if it's on the exterior you won't need to worry about any off-gassing.
I'd throw Rothoblaas in here too. Their interior membrane and tapes are somewhere between Membrain and Intello on price.
On the exterior - take a look at local recyclers to save as an option. I just bought high density EPS, 1.5"x4'x8' boards for $8 each, and I'm pretty sure that supplier ships pretty far even.