Building tight
Hey everyone, thanks for being a part of this great community. I getting ready to build a house and I’m catching some slack for contractors which are telling me that I’m crazy ( sometimes I think I’m crazy, I just don’t tell anyone) lol. Anyway the issue is because of my wall and roof system. I’m in climate zone 4 (Northern NJ) Now to get started, from the inside out…paint, 1/2 drywall, maybe Certainteed MemBrain vapor barrier ( any suggestions yay or nay), 2×4, rockwool insulation r-15, 5/8 plywood sheathing, Bluskin vp100 water and air barrier, thermal-buck around windows, Anderson 400 series windows, 2” Rockwool Comfort Board 80 insulation on the outside, 3/4” furing strips for the siding and acts as a rain screen, will have bug screen at bottom and top, and finally the siding. The wall will extend past the roof line to form a parapet wall. The roof will be as follows… flat roof trusses, two layers of 6” r-23 rockwool (for a total r-value of r-46) 3/4” sheathing, 2 layers of staggered joints of 2”EPS insulation board, 3/4” sheathing as the final roof deck, a 75# felt, and app burn down cool roof membrane by John Manville. The attic space under the roof will be unvented, it will also store the mechanics for the house two HVAC units and most importantly an HRV. The contractors I had looking at the job (4) don’t understand why I’m wanting to build it this way, they tell me it not the right way to build and that I’ll have major mold and moisture problem. I believe I’ll have a dry 100+ year sound house for other to appropriate the building science. Please tell me I’m not losing my mind. Please help. Thanks again and sorry I know this one is long.
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Replies
Manny, you might be a little loco here...
I suspect your ratio of exterior insulation to interior, air permeable insulation is too low. The under-sheathing layer will be prone to moisture accumulation on cold days as the exterior foam isn't sufficiently thick enough to warm the sheathing above it's dew point. A white roof may actually make this worse.
Although type I EPS (commonly used on flat roofs) is somewhat vapor open, 2-3 perms per inch, two inches will still constitute a vapor retarder. The accumulated moisture will be trapped and forced to dry inward. Fine for walls perhaps but I wouldn't trust it for a roof.
You could replace the EPS with wood fiber or Roxul (both are vapor open) but then you need an air gap. If your roof is flat, then venting the air gap is difficult.
I would put 6 inches of polyiso on the exterior and one layer of Roxul Batts on the interior. You'll have to get clever to prevent them from sagging.
Manny, I re read your post. You said 2 layers of 2" EPS. I read it as 2" total.
Probably still a little light. Adding another 2 inches might help but that is easier said than done. Switching to polyiso would help some. I would want the exterior insulation to comprise at least 30-40% of total roof r value in your climate zone. You might need to add another layer on top and remove a layer on interior.
Manny - I agree with Rick. I am in Northern NJ too and I used 2 layers of 2" polyiso to get me to my Zone 4 minimum ratio of exterior to total of 31%. Roofing contractors who do flat roofs use polyiso all the time, plus you can get it tapered to create a drainage slope. Allied or ABC sell it and also design the tapered roof for you. You may also want to consider conditioning that unvented attic. I have read stories of moisture issues in unvented attics that are cured with a little conditioning.
I am not a big fan of 3/4" furring with ComfortBoard - tough to get everything coplanar. Hardie required I use 2x4's given the weight of the siding, and I am glad I did. You also want to pre-drill the furring material so the HeadLok fasteners sit flush.
People told me I was crazy too - and while there is quite a bit of truth in that, everyone loves the house now.
So I did forget to mention that the attic will be finished and will also be conditioned.
Hi Manny,
Your wall assembly seems fine, but as Rick and Jonathan have pointed out, your roof assembly may require a bit more research. Here are two articles that should be helpful:
https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation
I'd also investigate not using wood sheathing just below the roof membrane. And when it comes to meeting code (eg, external foam R value), be conservative.