Builder-Friendly Durable Wall and Roof Assembly Advice Needed
Hello,
We are currently designing our new house and will begin construction this fall in climate zone 6B. Our goal is to build an efficient and durable structure. Unfortunately, in our area many green building techniques are still uncommon. I am am looking for advice on our wall and ceiling assemblies. I have four questions outlined below.
Walls: Contractors in the area are not comfortable meeting the external continues insulation recommended thickness for our region (R11.25) and so we have moved to a traditional wall assembly with only cavity insulation. We want to avoid their interest in thin external continues insulation (R6) due to our moisture concerns. The compromise design we are considering is (from outside in) wood siding> 1x rainscreen gap furring strips with top and bottom vents and insect screens>air infil. Barrier/WRB> WD Sheathing> 2×6 studs 16” O.C.> R23 or R24 cavity batt or loose fill insulation> 6 mil poly vapor barrier> ½”GWB. Code in our location allows for R21 walls without continues insulation. Since we are not pursuing continues insulation, we want to beat that low bar with at least R23 cavity insulation. Our biggest goal is to create a durable and low risk wall assembly that our contractor is comfortable with. 1) Does the wall assembly being proposed accomplish this? Are there ways to improve it? 2) Is the 6 mil poly being the vapor barrier the best option? Our home will be air conditioned.
Ceiling/Roof: Additionally, the ceiling/roof plan currently does not have a specific vapor barrier/retarder. The roof assembly being proposed is Gypsum wall board> R60 cellulose loose fill insulation> trusses> synthetic underlayment on OSB> ice and water shield at eaves> steal roof. 3) Should there be an additional vapor retarder? 4) If so, would 6 mil poly work well or should it be a smart vapor barrier?
Other assemblies: Additionally, the home is being designed to have at least R19 in basement walls and R10 rigid foam under slab.
Thank you!
Nick
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Replies
Not high R value but at least better than code min. A decent R value bump would be to go up to 2x8" 24"OC with R30 batts. If you are dense packing the walls, the cost delta for the insulation is pretty much noise. This has about the same amount of lumber as a 2x6" 16" wall so not a huge cost adder except for not getting the lumber in pre-cut lengths.
Either 2x6 or 2x8 wall, the improvement would be to tape the seams of your sheathing as the main air barrier. Also tape this to the foundation and to your ceiling poly for air barrier continuity.
On the inside a small improvement is to use one of the variable perm membranes or use craft faced batts. 6 mil poly is also perfectly fine in cold climate.
One thing to make sure is your basement r19 is r5 rigid on the foundation (glued and taped seams) + r14 batts, no poly. You do not want all the basement insulation to batts as this can create moisture issues and mold.
"6 mil poly is also perfectly fine in cold climate."
is this still true with air conditioning?
Akos, thank you. This is very helpful. A couple questions. Is 6mil poly ok on the walls with air conditioning? I am also still wondering about the need of a vapor barrier in the ceiling, what do you recommend there?
Great basement advice, we are currently thinking 2 inch of rigid foam then 2 by 4 stud with fiberglass batts.
Poly is a bad idea in warmer climates. I'm in the north edge of zone 5 here with pretty hot and muggy summers, AC is the norm here. Code here has
been 6 mil poly for a very long time and has not been an issue.
You need a solid ceiling air barrier. This can be drywall but I think it is easier to do with poly. Make sure to tape the seams and tie it into the sheathing air barrier. This also lets you do a blower door test before any insulation and drywall goes on so you can easily fix any leaks. Using an actual blower door is the best for this but you can get pretty good results with a box fan in a window and some smoke sticks to chase down leaks.
For the basement 2" rigid plus bats will work well. Don't forget to also insulate the rim joist area.
Thanks for this helpful information Akos!
To clarify, it sounds like the following wall assembly will work well and be durable (from outside in):
- wood siding
- 1x rainscreen gap furring strips with top and bottom vents and insect screens
-air infiltration barrier/WRB
- WD Sheathing (Tape seams)
- 2×6 studs 16” O.C.
- R23 or R24 cavity batt or loose fill insulation
- 6 mil poly vapor barrier
- ½” GWB.
But it also sounds like a smart vapor barrier instead of the poly would be an improvement.
Additionally Akos, for the roof assembly your recommendation is that we should include 6 mil poly or a smart barrier above the ceiling drywall. If we use a smart vapor barrier in the walls should we also use it in the ceiling?
What do others think? I would greatly appreciate other perspectives on this. Thank you!