Passive house exterior wall
I am volunteering on a Habitat for Humanity project in a criminate zone 5 on the East coast. This chapter is building its first Passive house and I wanted to get some opinions on the exterior walls from the experts here.
The walls as shown on the drawings by the Architect are as fallows from inside to out 1/2” drywall, Intello Plus as primary air barrier, 2×6 framing 16”OC with dense pack celluloses, 5/8 CDX sheathing, Mento 1000, 2 layers of 2” foam, strapping and vinyl siding. The windows are outies.
The construction supervisor made the decision to change the second layer of foam to Zip-R12 providing a continuous layer for the vinyl siding. The plan is to install the Zip with 6” #10 deck screws 6” at the perimeter and 12” in the field.
Any concerns or comments on this system will be greatly appreciated.
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Replies
With two possible WRB layers--the face of the Zip-R, and the Mento 1000--there might be confusion about which one to use for flashings etc.. I'd suggest using the Zip-R as the WRB and eliminating the Mento.
Intello makes a good air control layer but its position will leave it with a lot of holes from wires, pipes etc.. I have had better luck locating the air control layer at the sheathing. In your case, I would probably make the Zip-R the air control layer, or possibly taping the CDX, or keeping the Mento as you planned and using that as the air control layer.
All of the materials are on site, so my plan is to bring the Mento over and into the window bucks and then flash the windows to the Zip. This way both will work as an air barrier. I understand that making the Intello the air barrier is difficult. It is needed to control the dense pack cellulose so might as well install as an air barrier. With the work being done by volunteers 3 tries may work.
Any concern with the 6" screws and the Zip-R12
The Zip only really makes sense when it's being used as structural sheathing. Otherwise it provides no benefit over foam and strapping. The sheets are heavy so it's more work to install and the material costs are quite a bit higher.
When Zip is being used as structural sheathing, you have to install it exactly as specified -- nailed directly to the framing, following their nailing pattern and fastener schedule. The reason is it's really marginal structurally, the foam layer puts a lot of leverage on the fasteners. But it's cheaper than having structural sheathing, continuous exterior insulation and a nailing surface for the siding. Ordinary sheathing over continuous insulation isn't approved.
That's a long way of saying I think you're going to be fine so long as you're not trying to get shear resistance from the Zip.
Thanks for the reply. You have reenforced my original thoughts. Since we have started down this path with the Zip it makes sense to continue. It is more expensive but as long as I know it works, I feel more comfortable letting the process continue.
I question how an assembly that has 1 layer of foam insulation + foam from ZIPr12 and ZIP sheeting taped at the seams could be expected to dry to the outside. If someone can screw the zip with 6 inch screws, then it would also be possible to screw regular strapping with 6 inch screws. While ZIP is tough/robust material, even if I keep being told that the vinyl siding provides sufficient venting. My own perspective is that the assembly could have been less expensive to build and with better drying capacity. Those are my comments from the sidelines.
To state the obvious, I would agree with the other comments above.
“The construction supervisor made the decision to change the second layer of foam to Zip-R12 providing a continuous layer for the vinyl siding.”
That is some deep stuff the construction supervisor has decided to jump into.
Did Architect draw new plans?
Did code enforcement approve the new plans?
Has Habitat’s board given this supervisor the authority to make such a change?
How likely is code enforcement to notice it is not built to the plans?
Will the supervisor pay out of his pocket if the next hurricane flattens the house that was not built to plan?
It seems apparent you would not be asking your question if it was an approved plan.
I have never been a fan of Zip+R any place where a hurricane, earth quake or tornado is even a remote possibility.
Walta