Wall assembly OK?
Any issues with this wall assembly? Climate zone 7 Colorado. Foam is roofing polyiso felt both sides-new but stored for a couple of years. Price was right.
From outside in
Cedar siding
1.5″ air gap provided by 2x4s screwed to studs (vertical) with 8″ #12 exterior screws (Through foam below I can get 2x4s same price as 1x4s- seems a bit more robust than 1×4- OK?)
2 layers 15 lb felt
4″ rigid polyiso (two 2′ overlapping seams taped)
2×6 stud filled with fiberglass batt unfaced
7/16 OSB (inside studs)
1/2″ Gypsum board
Having the OSB on the inside allows me to seal it it all seams as a air block plus it will not be getting cold. Then I will overlap the vertical OSB seams with the gypsum board. Was planning to make the window boxes out of 2×6 with legs to the footer. 2×6 will bring the stud flush with the 4″ of foam + 1.5″ air gap. What am I missing?
Thank You
Ben
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Ben,
You'll need to (a) check with an engineer to make sure that your interior OSB provides enough bracing, since the interior OSB will be interrupted at partition intersections and at the rim joists, and (b) come up with a plan to provide a continuous air barrier at the partition intersections and rim joists.
From a moisture resilience point of view 4" of polyiso on the exterior is plenty of dew point control for OSB on the exterior side. With 2x6/R21 you'd only need R15, and even if you derated the polyiso to R4/inch for mid winter performance (it'll actually do better than that) you'd have at least R16.
If despite having sufficient exterior dew point control you're still concerned about interior moisture drives, installing a sheet of 2 mil nylon (eg Certainteed MemBrain) under the gypsum would be a lot less work to detail as a variable permeance air barrier than detailing interior OSB, and if moisture ever gets in it can leave more quickly through nyon than through OSB. Both are "smart" vapor retarders, but the moisture content of OSB has to be pretty high (high enough to support mold growth) to become more vapor open than interior latex paint, whereas 2-mil nylon crosses that threshold before the entrained air in the fiberglass hit ~50% relative humidity:
http://www.naturalspacesdomes.com/dome_store/dome_insulation_systems/images/Membrain3.jpg
If you're still going with interior side structural sheathing, using plywood instead of OSB would offer better drying capacity before it hits mold territory:
http://buildingscience.com/sites/default/files/migrate/jpg/BSI087_Figure_03_sheathing_perms_web.jpg
First, you should check with your local bldg. official to make sure you are allowed to have the rainscreen gap if you are in fire zone. Your 1.5" air gap is too wide IMO; using 1x4s should be good, and you need to install strips of METAL SCREENS at the top and bottom of the rainscreen to avoid insects and possible ambers from a fire.
Ben,
You wrote: "Was planning to make the window boxes out of 2x6 with legs to the footer."
Where this would run afoul of our code is that on load-bearing walls, the framing of window openings at the jambs are typically the jack studs supporting the headers above - and these are required to be continuous, so legs below the 2"x6" boxes wouldn't work.
I guess there are a few ways round this. You could use engineered connections for the headers on the king studs as they do with Advanced Framing, or move the headers up flush with the rim joists if there is a second floor. Either solution seems like a lot more work than simply applying the same 2x material you are using for the rain screen strapping around the windows.
Thanks for all the great information and thoughts!