Replies

1. Expert Member
   Dana Dorsett|Jul 21, 2016 04:06pm|#1

   Yes, a 1/4 mesh type rainscreen will work. Ideally you'd have some amount of venting at the eaves and more than 1/4" for the above the roof deck venting.

   That's a heluva lot of closed cell foam that's buying you next to nothing in thermal performance over half-pound open cell foam. You'd get about the same roof performance using only 3-3.5" of 3lb foam on the roof, and making the foam the finish roof (no standing seam). If you used half-pound foam in the rafter & stud bays with MemBrain under the gypsum you'd have better drying capacity, and nearly the same thermal performance.

   To bring the thermal performance fully up to what it would be with closed cell foam, a continuous layer of 3/8" fan-fold siding underlayment between the rainscreen mesh and the roof/wall sheathing gets you there. The cost of MemBrain + 3/8" XPS is small compared to the price delta between closed cell and open cell cavity fill, and the environmental hit MUCH MUCH smaller (it's bit more than 25% of the total polymer, and less than 10% the quantity of HFC blowing agent.)

2. 
   Damon Beck|Jul 21, 2016 04:33pm|#2

   Ideally I wanted to have all of my insulation to the outside of the sheathing, but due to size limitations with tiny houses, we would have lost a lot of interior space and crucial loft headroom.

   我想打开细胞的选择,但我ve heard that closed cell can make a structure seem more rigid, which is a plus in my tiny house construction. Is that true about closed cell foam's rigidity factor?

3. Expert Member
   Dana Dorsett|Jul 21, 2016 04:52pm|#3

   At 3" or more closed cell foam is fairly structural (you could almost skip the framing if you didn't need it for nail retention), but let-in bracing and metal framing ties are a lot cheaper than a whole bunch of buck-a-board foot foam.

4. Expert Member
   Malcolm Taylor|Jul 21, 2016 08:59pm|#4

   Damon,
   Tiny houses fall outside of building codes so you can do what ever you'd like, but some building codes, such as ours here in Canada preclude connecting the rain screen with the roof structure because of fire concerns. The idea being you create a concealed space connecting the two where fire can spread. How much of a threat a 1/4" gap represents is...?