Wall insulation
I’m building a house zone 6 and care a lot about air quality. I think current code requires enough R value that I’m not as concerned about energy efficiency.
Talking with my builder – he recommend the following double stud wall assembly:
siding, tyvec rain screen, blue wrap, sheathing (plywood), rockwool, intello. Then another wall for electrical +/- more insulation if needed.
Would it be better to add insulation to outside? My only concern with this is condensation. With blue wrap and intello should be vapor open enough to dry? If we put a polypropylene rain screen over the blue wrap does that defeat the purpose?
If cost was not an issue I would do rockwool boards on the outside with furring but it’s getting expensive…
there is hardy wrap with polystyrene attached as another option I suppose.
thank you!
RIK
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Replies
You can just using furring strips to define a rain screen. I actually prefer an open gap-type rain screen anyway over using any kind of drain wrap, and an open gap is the cheapest way to go too. If you want exterior continuous insulation, I'd use polyiso here. Up to 2" isn't too difficult to work with, just make sure to keep the interior:exterior R value ratios right.
It's OK for the wall to dry in both directions, but it's not necassary. Inward drying is enough, and is how most walls get built that use exterior rigid foam. I personally don't see any point to "service cavities" in walls like this, so I'd not bother with the second interior wall, just put things in the "regular" wall and do a careful job of air sealing and you'll be fine. I do like using mineral wool in walls, since it's nice to work with, but if cost is a big factor, you can save some money by using high density fiberglass batts. Corning makes their "pure safety" insulation (R15 fiberglass in a 2x4 wall) specifically for people with allegies and other sensitivities.
Lastly, you can use a cheaper interior side vapor retarder like MemBrain. The main tradeoff here is that it's not as physically durable as Intello.
Bill
Personal "bias" -- Tyvek is at best "unforgiving" of bad installation, and at worst simply 💩
I've used and trust the SIGA membranes, such as self-adhering Majvest 500. There are other solutions that I think might be equally effective, but not the loose "Tyvek-look-alikes."
Simple PT furring strips for rainscreen work well. Be sure to use some form of bug screen at the bottom (the top to if accessible to bugs). I used 3/32" hole perforations in stainless steel Z-flashing.
You really have to be careful about where the dew point lands for your climate. Two strategies:
a) Really, really seal against vapor flow from the interior (at least in "heating" climates). SIGA membranes and tapes are very helpful; and/or
b) Use dense-pack cellulose or sheeps wool (possibly other materials) as "vapor reservoirs" in climates where one season is "drying" (we have two houses in Eugene, Ore, and one in Sunriver. Both have low humidity summers so the approach works.
Personally, I don't see that double-wall exterior construction would be preferable over rockwool rigid panels on the exterior.
If you're planning a stucco exterior you will want to use that Tyvek. Otherwise, I'm with Bill on an open gap type rain screen. Simple 1x4s or rips from plywood sheets work fine. Also the polypropylene meshes are an option, especially if you're using a siding (board & batten, cedar shingles) where you would need both vertical and horizontal furring.
Although this is definitely in conflict with lots of other good advice you'll get, personally I avoid continuous exterior insulation because it complicates door and window installations, adding both to cost and risk. With a double wall like you've described you can get all the insulation you want using fluffy stuff. A variation on the double wall is called the Mooney Wall. You might find it a bit cheaper than a double wall. Here's an article about it.
https://www.greenbuildingadvisor.com/article/furred-continuous-insulation-wall-assembly-to-meet-2021-codes