Replies

1. Expert Member
   BILL WICHERS | Jan 05, 2019 03:11pm | #1

   Sounds dangerous to me. If any water gets into the middle layer with ice and water shield on both sides you’re screwed, it will rot with no possibility for drying. Granted, it’s not uncommon to have a thick layer of closed cell spray foam applied directly to the underside of the sheathing, and then to have a layer of ice and water shield on top (basically sealing the sheathing on both sides). As I see it, with only the sheathing sealed up at least you don’t have any voids so a little bit of moisture that might get in can’t easily move around which limits the possibility of large problems forming. With an open cavity (essentially, with the rockwool layer), there will be voids that can channel water and spread the damage.

   I think it was someone at the building science Corp that basically said it’s fine to seal up both sides of things if you know water can’t get in. But roofs can’t leak either, they say, so you have to assume that water can always get in and make sure there is a way for it to dry out if it does.

   Bill

2. Expert Member
   Dana Dorsett | Jan 05, 2019 04:28pm | #2

   The dual layers of Ice & Water creates something of a moisture trap, and aren't necessary. The exterior Ice & Water only would be preferable.

   The Ice & Water on the interior layer of Advantech is OK, but unnecessary in an RI climate. The 2" closed cell foam + Advantech puts it under 0.5 perms, and protects the exterior sheathing from interior moisture drives just fine. The 5" rock wool (about R21) and 2" of closed cell ( R12) adds up to about R33 outside the of the 7" open cell foam ( R26) for about R61 total, whith R32/R61= 52% of the total R outside of the open cell foam. In zone 5A (RI) the code prescriptive is a minimum of 41%, outside the first low permeance layer (the inner face of the closed cell foam). That's huge dew point margin.

   Skipping the closed cell and doing 9" of open cell (R33) instead of 2" + 7" wouldn't be a miniscule thermal performance hit, but would improve the drying rate considerably. The total R would be about R54, and R21/R54= 39%, just shy of the prescriptive, but RI is at the warm edge of zone 5A, and the additional R of the exterior Advantech layers and drainage mat would bring it to about 41%- not a big risk.

   But they'd get less applause from the sponsors that way, eh? :-)

3. 
   brianvarick | Jan 05, 2019 11:58pm | #3

   I thought it looked a little ridiculous when I saw them putting it together. It’s like two belts and two suspenders which is worse than just a belt and suspenders.

4. 
   graham78 | Jan 06, 2019 01:07pm | #4

   i wonder if they needed a temporary weatherproof roof until they got back to the rest of the insulation and final roofing?