Will this rainscreen assembly work for this application?
TinyCobalt
| Posted in General Questions on
I am building a tiny house on wheels and I wanted to have a rain screen included. I would just like to use a mesh product for ease of installation and fastening options later.
I live in southeastern South Dakota. I will be using a 1″ standing seam metal roof with a 5/8:12 pitch.
I have included a rough sketch and was wondering if it would work to have a continuous and connected wall and roof rain/vent screen?
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Replies
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.)
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.
And I was thinking of the open cell option, but I'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?
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.
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...?