Wall Assembly and transition points
Hello,
We are building a home in zone 7a. Our wall assembly into out is:
Poly
Drywall
R20 2×6
7/16 OSB
PSA WRB (Perm 33)
Rainscreen Gap
Exterior Rigid Foam (Perm 8-10)
(Stucco, Stone, Hardie)
1) My 2 questions are: at the exterior wall buildouts. We have a really interesting design, however the architects did not design the wall assembly with much thought in mind.
Where there is a buildout/chimeny/exterior wall with cold attic space behind, we are planning to do the peel and stick and exterior foam on the outside of the exterior wall.
We are NOT planning on putting rigid foam on the outside of all buildouts, on the ones that get EIFS. We are not wanting to create a warmer space inside of the “boxes”, where they will not be mechanically vented/heated.
2) Where the buildouts are fastened to the exterior wall, we were simply going to first do a continuosu WRB wrap of peel and stick and gap the exterior foam around the attachments, and then cover those with rigid foam on top in order to try prevent as much thermal bridging as possible.
Has anyone ever put the rigid foam continuous and sandwiched the buildouts with GRK’s? Halo Exterra has great compressive strength, but I don’t feel it will be solid, nor hold the load of the buildouts.
Thanks again
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Replies
Open fireplaces that need a chimney generally won't work in any reasonably well sealed new build. If you want fire, I would look at a stove instead. Even there you have to take a lot of care to avoid back drafting, commercial range hoods should be avoided.
The best way to get energy efficiency is to simplify your envelope. Complicated structures are very hard to air seal as you can have a lot of convoluted air flow paths. I would go back and see which buildouts are really needed. If you must have them, it should be part of the structure, the air barrier should be continuous over the buildout and wrap it with rigid as the rest.
Thanks for the reply. To clarify, there are no wood-burning fireplaces within the structure. Only outside.
As for the build outs, I want the exterior walls to have the thermal break of rigid foam as the buildouts are not mechanically ventilated or heated. If I only do that on the exterior of the build outs, then I will have cold air inside the buildouts against my exterior wall. Does that make sense?
I agree about wrapping the entire build out with peel and stick or roll on to ensure its water tight.
I'm assuming you will have poly BEHIND the drywall, and that's just a typo :-) Note that poly is not usually advised unless required by your local code. Smart vapor retarders (MemBrain, etc.) are safer.
You want the rigid foam directly against the OSB sheathing on the exterior, then you want the rainscreen on top of the rigid foam. If you put the rainscreen gap between the rigid foam and the sheathing, you effectively cancel out the insulating value of the rigid foam due to the air currents that circulate in the rainscreen gap.
I'm not sure I understand your design for the rest of the details. Can you post a drawing? Drawings are very helpful to see how things go together on any given design.
BTW, plywood has sometimes been cheaper than OSB lately,. so it's worth checking before you build. Plywood is generally considered superior to OSB, but OSB used to be cheaper. If there is no cost difference, or if plywood is cheaper, I'd use plywood.
Bill
Hi Bill,
Thanks for the reply. Yes. My mistake. Yeah, we do require interior air barriers also. Its a nightmare here to explain it and have a inspector sign off on anything other than interior poly. (unless we are are doing full ICF).
We did do the pricing analysis here with OSB and plywood. Plywood was much more expensive at the time of purchase. Client agreed. It was hard because I agree with you. Plywood has many more pro's. it also has a higher perm rating.
I will sketch up a wall assembly of what we have.
Thanks