Floor Trusses, OSB Sheathing, Rigid Foam, and Masonry Finish
Wow, I have spent countless hours reading your articles and blogs. Much to my husbands chagrin! We have already built a garage with an apartment above it in WNC, zone 4. We used certainteed cement board siding, tyvek, OSB, 2×6’s, 5.5″ open cell in walls. The roof deck was sprayed with 8″ of open cell and is unvented. (The Hvac is up there and from what I have read we should probably have supply and return ducts up there to keep moisture from becoming an issue… We are considering adding more insulation on the attic floor. But that will be another post .)
Now we are building the house. See photo for husbands mock-up. The basement is superior wall with a walk out on one side where the grade permitted (already in, decked, and back filled). After we ordered the houses floor trusses we decided to avoid the thermal bridging that is so evident on our garage by adding rigid foam to the exterior of our house. As a result of the last minute decision, the trusses were not designed to allow for OSB and foam to be applied without them being proud of the superior wall basement. The framer was able to gain 1/2″ of setback on three sides of the trusses by setting them closer together but one side is flush with the superior wall.
Upon consulting with our framer, we are considering using OSB to sheath the walls but not the first and second floor trusses. The walls would be set back 1/2″ from the trusses to allow OSB to sheath the walls but stop flush on top of the trusses. Then a continuous layer of rigid foam would be applied over OSB sheathed walls and bare trusses. This would leave the foam proud of the superior wall.
To further complicate things, we are applying cultured ledgestone to the first three feet of the first floor and continuing down the wall as the grade falls away to the walkout basement. Potentially, we can make up the difference by building up the superior wall with mortar before applying the stone.
Would that work?
I have read here that in zone 4 any exterior foam thickness is enough. Could we use as little as 1/2″ or 3/4″ EPS to minimize the problem but not cause issues with sorption (condensation)?
Boral specs applying two layers of wrb under the metal lath. They also say applying stone to rigid foam thicker than 1/2″ may require special fasteners. Any masons out there have suggestions?
After applying rigid foam and tyvek (both taped) we would fill the 2×6 walls with blown cellulose in the hopes of saving money, good moisture management on the interior side, and yet getting good r value. All at less cost to our health than using spray foam. Then drywall and latex paint.
I have tried finding an insulation contractor to speak with on these issues but it seems they are only able to promote their method of insulating.
Sorry for being so long winded but we are frustrated at this point.
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