Does my wall assembly seem adequate for my area?
Hi everyone! I have a question about a wall assembly I want to use at my off-grid mountain cabin. It’s a 1986 build cabin but the weather and environment with a lack of previous owner’s attention have made the project list somewhat large. We’re overhauling our solar system so I plan to practice these wall assembly details on the powerhouse shed before attempting the main cabin.
Here’s what I have planned:
1) interior insulation currently fiberglass, want to install closed cell spray foam in some areas and leave fiberglass in other areas
2) 1/2″ CDX plywood
3) Delta vent SA WRB
4) 1 layer of Roxul comfortboard 80
5) 1×4″ furring strips
6) hardie siding with metal skirt near bottom for snow (may leave oak batten board if viable)
Also to note, the cabin is technically zone 5 in Eastern Washington but sits in the mountains at higher elevation so winters are cold (below zero isn’t uncommon) with a good snow pack and warmer summers.
Any help confirming my wall assembly or suggestions for improvement or trade out of WRB is appreciated.
The delta vent SA seems hard to find and I’m not sure if I need two layers of the Roxul insulation.
Lastly, what fasteners are people using to attach the comfortboard to the wall assembly?
Much thanks!
Zach
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Replies
To even begin to assess this we'd need to know:
The depth or R-value of the fiberglass insulation.
The thickness of the Comfortboard 80.
Mountain,
A few things are unusual about your choices. You've chosen an expensive WRB, for instance.
Installing a continuous layer of exterior mineral wool can be a good detail, but it's also fussy to install. (The squishiness of the mineral wool can make it hard to get the furring strips to be coplanar.) Almost all builders find that installing exterior rigid foam is easier than installing exterior mineral wool.
Dana - main cabin is 2x6, shed assembly is 2x4. Thickness of comfortboard 80 is 1.5", R-6 I believe.
Martin - Due to being off-grid, having a tight home is important to save on heating the home. I'm open to other wall assemblies. I'm in a high fire area so I like the fire resistant properties of mineral wool vs. foam which has been shown to help ignite fires. From what I've read, it sounded like the comfortboard 80 is denser and a bit easier to handle with install, but maybe this isn't truly the case.
Mountain,
I agree that the exterior layer of mineral wool is a better choice than foam for many reasons. I have used mineral wool on a number of projects (both residential and commercial) with builders that have never used it, and particularly at 1 1/2" it really isn't that difficult to install properly. It is usually held in place "temporarily" with a few cap fasteners and then the furring strips are fastened through it to do the work.
I would suggest sealing the seams of the plywood using a good quality air sealing tape (like Siga Wigluv) if you are working in dry weather or Prosoco Airdam if it's damp to create the air barrier, and then using an inexpensive permeable WRB on top prior to installing the mineral wool.
In zone 5 you'll need more than 1.5" ComfortBoard on the 2x6 section for dew point control, but 2" would be enough. For the 2x4 section 1.5" is enough.
If you're at a sufficiently high elevation that it's really zone 6 from (on a very local heating degree-day basis) it'll need to be 2" on the 2x4 part, 3" on the 2x6 part.
Mountain,
I disagree with Dana. There is no need to worry about dew-point control when using exterior mineral wool. Since mineral wool is vapor-permeable, the existing sheathing will simply dry to the exterior (as it does on almost any wood-framed house in the U.S. and Canada).
The mineral wool is semi permeable, but the half inch CDX is not. Even with rainscreen and permeable exterior insulation the vapor permeance of the CDX is low enough to matter- it can still accumulate moisture faster than it purges it in winter unless there is an interior side vapor retarder.
Yes the CDX will dry more quickly if it can dry in both directions, but it's still better to have sufficient exterior R to not load up with moisture over the winter from interior moisture drives in the first place.
In zone 5 the exterior drying capacity of the back vented siding alone would be enough to pass muster with the IRC from a moisture point of view (with or without the exterior R) but it's suspect in zone 6. Labeling a county in eastern WA as zone 5 applies to the populated lower elevations where most of the building is happening, not the remote mountain locations. The elevation differences in many of those counties are significant enough that I'm assuming it could easily really be zone 6 (or even zone 7 nearer the mountaintops) by the HDD/CDD definitions. eg:
Kittitas County WA is officially zone 5, and most towns are lower than 2000'. Even the town of Cle Elum just shy of 2000' in elevation runs 7000 HDD65 (source: https://wrcc.dri.edu/cgi-bin/cliMAIN.pl?wa1504 )making it the cold edge of zone 5. The highest town of any size is Liberty at about 2600' (couldn't find weather data, but I'm betting it's zone 6), but there are all sorts of locations accessible by fire-road in that county above 6000' (been there, skied that) that could easily be the cold edge of zone 6 or even warm edge of zone 7. (The mountaintops in that county are above 8000', some pushing 9k'.) This sort of thing is true for several eastern WA counties.
But perhaps MOUNTAINCABIN can give us higher precision on the actual elevation of this house (?).
From a thermal adequacy point of view 2x4/R13 + R6 ComfortBoard is barely better than the IRC code-minimum R13+ R5 c.i. for zone 5, and below the code-min 2x4/ R13+ R10 c.i. for zone 6.
A 2x6/ R20 + R8 ComfortBoard wall would be decent enough in zone 5, but barely ahead of IRC code-min for zone 6. Bumping that to 2x6/ R20 + R12c.i. removes all question on the dew point control issue and delivers a more appropriate whole-wall-R for zone 6 where it's considered "...important to save on heating the home...".
> suggestions for improvement or trade out of WRB
Consider MemBrain on the interior side, cellulose (vs fiberglass) and Typar (more optimal perms).
It might be an option to not use sheathing and use diagonal steel bracing.
I agree with both Dana and Jon. More exterior rockwool would be better / safer as would adding Membrain at the interior if you can. I'd stick with the plywood as exterior air barrier and would ditch the spray foam. All this talk about walls (the easy part) but what about floor and roof and the transitions?