Ideal wall for 3C climate zone and in seismic zone
I am a homeowner interested in your opinions on ideal wall insulation for San Francisco in the 3C climate zone.
We are planning a new addition with 2 x 6 walls which I would like to fill with dense, wet spray cellulose insulation.
I have asked everyone I can locally about using one 1 inch polysio on the exterior and I always get – “not done in this climate”, “not seismic-safe” “will not pass inspections due to shear strength requirements”.
Can 1 inch polysio be put over the top of plywood and the waterproofing?
As often is the case in SF, we are on a very narrow lot and required to do 1 hour fire rating on the addition wall since they are at or 3’7″ from the property line and given the seismic concerns, it seems right to use plywood.
Also how important is the rain screen under fiber cement siding? We generally have a dry season of 6 – 9 months.
Thanks in advance!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Donovan,
Yes, you can install 1 inch of polyisocyanurate on the exterior side of plywood wall sheathing. As long as the wall sheathing meets your engineer's requirements (adequate thickness, proper nailing schedule), and as long as your wall includes all the necessary seismic clips and hold-downs, I don't see any reason why the polyiso would raise seismic concerns.
However, the final ruling on the matter rests with your engineer and local building inspector.
I think it's a good idea to include a rainscreen under your fiber-cement siding -- especially if the siding is being installed over rigid foam. Again, verify your planned details with your engineer and local building department.
Great, thanks for your response. I think they responses I got previously were thinking sheathing instead of plywood like shown in some examples.. Polysio over the top of plywood seems safe enough indeed. I will pursue.
Do you think 1 layer, 1 inch is appropriate for climate 3C? Would you do more?
Donovan,
In your mild climate, you don't have to worry about winter dew points or moisture accumulation in your sheathing, so 1 inch will work.
However, the usual insulation rule applies: more insulation is better than less insulation. If your budget allows you to install 2 inches of polyiso instead of one, your walls will leak less heat.
One last question - I think the inspector is going to require a house wrap for the waterproofing barrier. I think it should go under the polysio, but my architect is suggesting it should go on the outside? Most of the GBA videos and commentary seems like it should definitely go directly on the plywood before the polysio. Is this correct? Thanks,
Donovan
Donavan,
Here's an article that addresses your question: Where Does the Housewrap Go?
One additional question - I just learned that our north wall is too close to the property line to do exterior sheathing. I would like to add 1 inch polyiso under the interior drywall inside of a 2x6 cavity filled with spray in cellulose.
Two questions come up - do I need to do anything special when doing rigid foam on the interior for moisture control? Should all three exterior walls of this addition be consistent - that is it OK to do exterior rigid foam sheathing on east and west while north is interior rigid foam? Or since I have to do one wall with interior sheathing should they all be that way? Thanks in advance.
Above I meant to say that the 1 " polyiso would be on the interior side of the 2x6 framing, not inside it!
Donavan,
Q. "Do I need to do anything special when doing rigid foam on the interior for moisture control?"
A. No.
Q. "Should all three exterior walls of this addition be consistent?"
A. They don't have to be consistent.
Q. "Is it OK to do exterior rigid foam sheathing on east and west while north is interior rigid foam?"
A. Yes.