Replies

1. 
   STEPHEN SHEEHY | Aug 06, 2017 11:42am | #1

   I'm no expert, but you still need a rain screen with most siding. And closed cell foam is very expensive and does nothing about thermal bridging through the studs. If I were building another house, I'd look at Zip R sheathing and dense pack cellulose.

2. Expert Member
   Dana Dorsett | Aug 06, 2017 12:54pm | #2

   Closed cell foam between studs is a waste of (environmentally & financially) expensive foam. In a 2x6 assembly it adds less than R2 to the "whole-wall R" performance, after thermal bridging is factored in. It doesn't keep the sheathing any warmer in winter, and at ~0.2 perms it all but prevents drying toward the interior. A rainscreen (or vinyl siding) would be prudent to give the sheathing some drying ability, but with 5" + of 2lb foam the sheathing could rot away and the house still wouldn't fall down, due to the structural rigidity of the foam, not that it's a desirable outcome.

   IMHO i's highly unlikely that 5" of closed cell foam between 2 x 6 will become the standard. From a lifecycle verditude point of view it's more than 2x the polymer per R of open cell foam, and even open cell foam has a higher environmental footprint than cellulose, rock wool or fiberglass.

   See:

   https://www.greenbuildingadvisor.com/articles/dept/musings/installing-closed-cell-spray-foam-between-studs-waste

   https://media.licdn.com/mpr/mpr/AAEAAQAAAAAAAAv3AAAAJGVlYWRjNTk5LWEyMTQtNDk3YS04MjFjLTZlYjdjMWEzNjRkZQ.jpg

3. GBA Editor
   Martin Holladay | Aug 08, 2017 10:39am | #3

   William,
   These two articles address your question:

   Installing Closed-Cell Spray Foam Between Studs is a Waste

   How to Design a Wall