Exterior Insulation Detail and Moisture Risk
Currently designing a 2×6 wall assembly for a home in Minnesota – Climate zone 7.
The owner wants to use R-9 zip-r sheathing with fiberglass or rockwool batts for the 2×6 cavity. The reasoning for using R-9 is to get the advantage of exterior insulation but still thin enough to not warrant buying specialty tools or change the building process too much.
Here is his desired assembly:
-LP smartside lap siding
-3/8 plywood battens
-R-9 zip-R
– 2×6 wall with R21 fiberglass or mineral wool
– Smart Membrain vapor retarder
– 1/2″ drywall
Although this assembly may be bullet-proof in other climate zones – I’m worried this could be a risky assembly in climate zone 7.
From my understanding – in climate zone 7 you would need R-15 exterior insulation to eliminate the risk of condensation in the 2×6 wall cavity. Is this still a large concern using zip-r where your sheathing will be nowhere near the condensation point in the wall? what if you were to use a hydrophobic cavity insulation such as rockwool? Would this still be a worrisome assembly if the wall could dry to the inside?
What if we were to step up to the R-12 zip sheathing?
We love the idea and simplicity of zip-r but making it work in climate zone 7 is a challenge.
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Replies
What is especially confusing is that the IECC only calls for R20 cavity + r5 exterior insulation. That is even less than r9. I’m curious to hear what other think of zip r9 in climate zone 7.
I agree the code is misleading. Martin Holladay addressed this in this great article.
//m.etiketa4.com/article/the-2012-code-encourages-risky-wall-strategies
Im less concerned about building code and more concerned about the performance and durability of the assembly. Mostly - will i get condensation in the wall cavity - and if I do - is it enough to be concerned about?
The R15 is for a wall with painted drywall as the vapor retarder. By adding the smart membrane, you can use less exterior rigid. You can read more about it bellow:
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed