Benefits of exterior rigid foam with additional insulation in cavity.
Back story: (if you could care less skip to the section below “Questions”;)
I’m in the early stages of planning our retirement home. The majority of which I plan on DIY’ing. I’ve decided to use 8″ ICF for my foundation walls (2.5″ insulation both sides) and would like to continue the insulation up the rest of my stick built house.
The engineer drawings for the ICF indicate you can build an exterior stick frame to be flush with the outside of the foam. I’m sure some engineer did a lot of math and said this would work. However, there’s just something about all the load be transfered on only about 1 1/2″ of wood that bothers me. After debating a few solution to support my exterior wall with something more than just a few inches of foam. Which included above grade ICF, using brick-ledge/tappered as the last layer of ICF, 6″ studs flush to the exterior, and even placing the wall on top of the concrete and tappering the ICF foam at a 45° angle. I’ve come up with this solution:
2×4 studs, 1/2″ OSB, 2 layers of 1″ polyiso rigid foam.
The OSB, while not required by code, provides structual benefits. Rigid foam provides vapor/wind barrier, using 2 layers decreases thermal bridging and increases air impermeablity more than a single 2″ layer. But best of all the total thickness amounts to more than 2.5″ (once I place fur strippings and siding) which means my studs will fully rest on the concrete core of the ICF. I could use 6″ studs but I’m attempting to keep the wall not much more than 6″ total purly for aesthetic reasons
Questions:
Cavity insulation.
I plan on insulating the wall cavity as well. Whats types are compatiable with rigid foam? I would like to use closed cell since it has the highest r value per inch. Yet I briefly remember reading that closed cell isn’t good with rigid foam but can’t remember why. My other two options would be open cell or blown cellulose are there any pros/cons to these with rigid foam?
Windows.
I’m planning on outtie windows. With the average depth of 4.5″ how would the windows be attached to the framing? Is some type of bucking required/recommended?
Doors.
Doors will be innies. Flush with the interior wall and the jambs built out. I plan on installing storm doors on the exterior. The storm doors will aid with water intrusion but are there any other “best methods” to waterproof such a door?
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Replies
Because of the thermal bridging of the studs, putting closed cell SPF into your walls is a waste. The overall wall R value is just slightly more than batts.
With your stackup with fiberglass batts you end up with around R26 whole assembly(if you go with cc SPF it ends up at R29, lot of money for not much improvment). You can also get pretty close to that with 2x8 walls 24OC with R30 batts. This would also simplify your window install. Going with 2x8, I would still install a rain screen.
For outie windows with 2x4 construction and exterior foam you need to make bucks. Search this site, it has a lot of details on how to construct and install them.
Best method to waterproof doors is a to build a roof over it. Storm doors also work great, I'm just not a big fan of the look.
There isn’t really a “compatibility” issue between cavity insulation and rigid foam. Everything has to do with vapor drive and drying ability. If you have a lot of rigid foam on the exterior, or something like polyiso that has a facer that is a vapor barrier, then you need a wall that can dry to the interior. Having poly sheeting on the interior would be a problem in this situation.
Spray foam in wall cavities doesn’t gain you much R value wise since the thermal bridging of the studs kills the whole-wall performance of the spray foam. If you were to use spray foam, at least use open cell here. Closed cell is a bit of a double whammy performance wise since it has the issue with thermal bridging AND won’t be installed to completely fill the wall cavities due to issues with trimming.
If I were building this, I’d use mineral wool in the stud cavities. Fiberglass would also work but with slightly less R value and much reduced fire resistance.
Bill
@Akos I appreciate you providing the R-values. Thats exactly what I've been attempting to find. I've done the math and your absolutly right about cost differance. Materials alone for SPF practically doubles the cost for a minor R-3 differances. Would take many years to save that in ustilities.
@Zephry7 I looked into mineral wool. The increased cost here doesn't provide anything other than a slight increase in R value. If I increase the cost I'd rather stick with SPF simply because it has alternate benefits such as increase wall strength, etc...
Believe that fiberglass batt will do just fine. Anything outside of that would make it more feasable just to use ICF walls throughout. Once I complete my budget I may revist the idea of open cell just because of the added benefits it has.
> If I increase the cost I'd rather stick with SPF simply because it has alternate benefits such as increase wall strength, etc...
Unless you're engineering the need for this wall strength into your design, I think this is a dubious benefit, at best. You already said you're using OSB for sheathing, and structural sheathing certainly does have benefits.
> Rigid foam provides vapor/wind barrier, using 2 layers decreases thermal bridging and increases air impermeablity more than a single 2″ layer.
I don't understand your comments about 2 layers being better than 1 in this case. As far an air barrier goes, I would tape/seal the sheathing and be done with it.
Around here, fiberglass batts and unfaced EPS foam are the best values for walls. Cellulose is close enough in price and has some advantages - I'd use it
I wouldn't give a second thought to the minor difference in whole house fire performance between mineral wool and fiberglass or cellulose. Apply fire-retardant paint if you are especially worried about fire.
>"Believe that fiberglass batt will do just fine. Anything outside of that would make it more feasable just to use ICF walls throughout. Once I complete my budget I may revist the idea of open cell just because of the added benefits it has."
R15s are preferable to R13s, R21s preferable to R20s for the higher air-retardency and greater ease of sculpting to a near-perfect fit with a batt knife, but you can get there with R13/R20s too. R11s & R19s are too low performance on air retardency to consider. An R19 only performs at R18 when compressed to 5.5" in a stud bay, and performs at R13 when compressed to 3.5" in a 2x4 stud bay. The weight per square foot is identical to an R13- it's essentially a "fluffed" R13, more of an air filter than an air retarder, with low enough density for convection losses to occur within the batt when the temperature difference is high enough.
With any batt (or blown fiber) solution it's imperative to air seal the sheathing to the framing the full perimeter of the stud bay with a polyurethane sealant/caulk, and tape any horizontal seams in the sheathing with an appropriate tape.
With an open cell foam solution the foam pretty much takes care of air sealing the studbays if the installer is competent (not all are).
With either solution it's also important to hit all of the seams that are NOT inside the stud bay with polyurethane caulk, such as jack studs, doubled up top plates & window headers, the seam between the bottom plate & subfloor, etc.
Doing the whole house with ICF makes it more hurricane-proof, but heavy used of concrete and polystyrene (or polyurethane spray foam) isn't exactly "green". The embodied carbon in concrete is huge, and the manufacturing processes of polymers have a variety of impacts that fiberglass & rock wool just don't have. Studs, plywood, and cellulose insulation are sequestered carbon, with a net negative carbon footprint when using sustainably harvested timber. It's not a subtle difference.
That's not to say that installing continuous foam on the exterior of the wood framed building isn't "worth it". R-for-R it's a bigger environmental hit, yes, but at least the performance isn't being undercut by thermally bridging framing, and if done right it's protecting the wood & cellulose.