Drainage plane behind exterior iso insulation
I have a home built in 1870 with all original siding, framing, etc. There is no insulation or exterior sheathing- just the original cypress boards. I am going remove the siding, sheath with either Zip system or GP ForceField, and then I want to add exterior insulation over that. Rockwool Comfortboard is hard to come by here, so looks like I will be using polyiso foam board. Question: I intend to seal my sheathing with tape, liquid flash, stretch tape in windows, etc. and wondered if using Tyvek Stuccowrap over my sheathing would allow any water that gets through the 2 layers of 1″ polyiso to drain, ventilate, and dry? Or is this a really bad idea? Comfortboard would solve the “drain and drying out” issue, but I also like the radiant barrier. I am putting a rain screen behind the new siding so it shouldn’t ever get wet that far back (all seams staggered and taped), but I want to make sure I’m not setting myself up for disaster. I can add a pressure treated bug plate under the insulation with the stuccopwrap going behind it (with small gap and all covered by screen) or just do the screen and no plate under the insulation.
Any help would be great. Thanks!
Replies
Question. One, why are you using Zip and polyiso separately when Zip makes a combo panel with 1"- 1 1/2"- 2", and 2 1/2" polyiso options already on the Zip? Two, have you read any of the studies about how much less effective polyiso is the colder the temperature gets? Interesting findings. Three, if you're set on using separate polyiso, why are you using 2 layers of 1" instead of a single layer of 2"? It's more work and more expensive.
1) Zip-R is a neat product but does not have the radiant barrier I want. 2) I'm in the deep south I am more concerned with keeping cool than warm :D 3) Polyiso- unlike comfortboard- shrinks slightly over time. You want to use 2 layers and stagger them to compensate for the shrinkage, and tape the outermost seams.
What climate zone are you in?
2. Want to keep the heat out! I rarely turn the heater on in winter.
My opinion is not that it’s a bad idea, I think it would work, but totally unnecessary. I can’t see how water would ever get to it. In a typical rainscreen system there’s a WRB over sheathing, behind the siding. You’re essentially putting 2 WRBs in. You’re taping the polyiso seams. If you tape both layers you’ve got double the protection. Just my 2 cents.
Thinking about this again, and I’ve changed my opinion, for what it’s worth. By putting a drainage plane between insulation and sheathing you’re introducing air movement, in your case hot air, behind the insulation. You’re also, depending on whether you’re in a warm humid or dry climate, introducing moisture behind your insulation. So my amended humble opinion is it isn’t a good idea.