Feedback on a Pretty Good wall for Zone 6
Feedback on a Pretty Good wall for Zone 6
I am designing a small Pretty Good house in coastal zone 6 and would appreciate input on my assembly. I’m a carpenter and would be building the house myself with my partner, so I can take the time to do the details right. Besides energy efficiency, my priorities are: healthy building materials, a resilient & long lasting wall, and simplicity.
from interior to exterior:
– blueboard with skim plaster
– 2×6 studs with 5.5” Timber Hp batts
– diagonal board sheathing locally milled
– Henry blue skin or similar high perm self-adhered WRB/air barrier
– 3” Timber Hp exterior insulation board
– vertical strapping/rips for a vented rainscreen with additional horizontal strapping for siding nailbase
– cedar shingles
My main concerns all relate to the added complexity caused by the exterior insulation. I don’t mind innie windows or dealing with the extra strapping for shingles, but I do worry about the timber hp board being unprotected. However, with board sheathing I need a self-adhered WRB/air barrier and I’ve learned that they don’t stick to the wood insulation board products.
My options seem to be:
1) switch to Rockwool for exterior insulation and leave it exposed behind the siding/rainscreen
2) install a layer of 15# felt paper over the Timber board as a secondary WRB.
3) risk it with the unprotected Timber Board
4) switch to a double stud wall and forget the exterior insulation
What would you do? And what else should I be thinking about? Thank you!!
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Replies
Your interior/exterior insulation ratios are all good for CZ6, so that works fine.
I've never worked with Timber HP, and I don't know of it being available in my area (southwest Montana). First glance at their website indicates it can get wet and dry itself out, and that "Our installation information routinely indicates the WRB location against the wall sheathing. This location allows for straightforward flashing at penetrations and overall WRB continuity."
Not sure how wetting impacts durability. I would want to talk to a rep, but it sounds like as long as you have good cladding over it (and with a 1.5" rainscreen gap + cedar shingles, I think you're covered for "good cladding") you'll be fine long term with your SA WRB at the board sheathing, then timber HP outboard of that.
If your question is more about prolonged exposure + wetting while you self-perform the work, that's where I would definitely want a rep's opinion.
I'm doing 3" of Rockwool on a small reno at my own house, and I'm glad Rockwool has no issues with water because the exterior work is taking me forever and it's been getting wet, dry, rewet since September. I bucked out my windows; just seemed easier to manage the waterproofing.
Why board sheathing? Just because you can get it locally milled? I'd be mighty tempted to use Zip or something, which for an owner-builder gives you a big jump in speed of install & dry-in.
Big props for going for cedar shingles + rainscreen (battens, not mesh). I love working with cedar shingles, and really love the look, but have never wanted to give myself the brain damage of putting them on a double-batten rainscreen. I commend the effort to do it right!
Thanks for sharing all these thoughts. I’m in Maine so Timber Hp is local, hence my insistence on it. My understanding is that some builders and architects are concerned about water infiltrating any cut edges of the Timber Board that wouldn’t be protected by the wax coating, as well as it’s susceptibility to insects and moisture over time. It’s been used outboard of the WRB (or as the WRB) in Europe for about 20 years but the consensus seems to be that it’s unclear as of yet how it’ll hold up long term exposed behind cladding. But I tend to agree that a generous rainscreen with an insect barrier would go a long way. Since my cladding choice has so much longevity it’s important to me that I don’t need to replace the exterior insulation before the siding.
Rockwool definitely has the advantage on water resistance. Are you finding it easy to install? I haven’t worked with it but I know some people find it compresses too much.
As far as the board sheathing, you make a fair point. But, reasons…I want a vapor open assembly and I think the boards will do best at delivering that. It cuts down on waste. It’s a local natural product. And finally, I work with Zip panels a lot and I’d like a break from that toxic dust on my own build!
We’ll see how I fare after that brain damage shingle install! I’d love an alternative but I can’t seem to figure out any other way.
Good luck with the rest of your reno and thanks again!