More Q&A Spotlight
Tom K has plans to build an outbuilding this summer in Alaska, and he will include a continuous layer of exterior insulation—assuming he can get his hands on the materials he needs.
“It is currently not possible to get any kind of rigid mineral (vapor-open) insulation,” he writes in this recent Q&A post. “It does not seem to even exist in the Pacific Northwest and my attempts to find a place to freight it have failed.”
Tom needs insulation that is both fire resistant and vapor-open. He’d prefer to stay away from rigid foam, and wonders whether it’s feasible to use mineral wool batts instead. Another option would be to build some type of exterior frame, such as a Larsen truss, and fill the exterior wall cavities with blown-in mineral wool or cellulose.
That’s where we start this Q&A Spotlight.
Using batts may be problematic
In regards to the apparent unavailability of Rockwool Comfortboard, Jberks suggests that Tom call Rockwool directly. If there are any distributors in Alaska, the company could point him in the right direction.
But about those batts—that’s probably not going to work, Jberks says.
“If we’re talking about the same batts, my personal opinion is it wouldn’t work, or would be a horribly cobbled attempt,” he says. “The batts compress quite a bit. How are you doing to affix them while maintaining the depth? Also, I wouldn’t know how you could do an air gap and siding, unless you built an exterior frame, which is essentially a double-stud wall and what you suggested with the Larsen truss.”
The point of adding a continuous later of exterior insulation is to reduce thermal bridging through the wood frame of the building. And in one assembly…
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9 Comments
I am surprised a wrap and strap wall assembly was not considered. It accommodates the builders desire to not have foam and is pretty straightforward in construction. And you could use rockwool batts in that wall.
it might be easiest to work with a distributor in seattle that can deliver to Alaska Marine Lines terminal for barge service to most of alaska. Its slow, but they take everything from one parcel to a container to a mobile home.
StuSid,
Would a Mooney Wall give you enough additional insulation for Alaska?
Best they can do is about $12 a wall square foot for the comfortboard 80 at 4 inches, delivered to the nearest shipping hub but not to the site. That would be about twelve thousand dollars for me. The comfortbatt insulation from a box store plus extra framing plus structural screws is around a dollar a wall square foot.
Tom,
Kudos to you for attempting to eliminate foam in your build! $12/sqft is a lot! I can get it for about $5.60/sqft here in Maine. You have been given a lot of good info so far but look into this: https://maineindoorair.org/a-practical-easily-built-wall-system-r-30-and-beyond/
I have no doubt that you can acquire all the necessary materials for this assembly its just depending whether its enough insulation for your climate zone. AK has a wide range of climates from SE AK all the way to the arctic.
Lastly, you are probably aware of this resource in Fairbanks: http://cchrc.org/
Good luck!
Thanks, Stu. I actually found that first link when I was looking up what wrap and strap meant. There's actually a link to it on this site in another article, so you're not alone:
https://www.greenbuildingadvisor.com/question/thoughts-on-wrap-and-strap-wall-for-zone-5-house
If anyone has access to the older article in Fine Homebuilding (A Superinsulated Saltbox), I'd enjoy reading that. Might check the local library. I do agree with Michael that the installation details on a more conventional double wall are easier.
But if I can get the Rockwool board at retail price, that's easier still, and my personal preference. I can also add the exterior insulation after I get the roof on, so if it takes all summer to get it sorted out then so be it. Worst case is just adding Swinburne trusses in the fall if nothing else works out, and until then using Tyvek as siding like most of my neighbors.
Stu, there are many other wall options but the OP had specified "exterior insulation" and wrap-and-strap (or Mooney wall) is usually an interior approach. Having used that system myself, I prefer double-stud walls: similar effort and material use but more conventional fastening for drywall, electrical boxes and interior trim.
Last week I talked with Dan Edelmen, the national sales manager for Rockwool, who said that as a result of this Q+A they now have now established a sales channel for Alaska.
We do now have distribution. It can be purchased from a local lumberyard or even Home Depot or Lowe’s and that comes from our distributor PRIMESOURCE. We also now have another option as we have heard that even these channels say they cannot get it. Reach out to me directly at [email protected] and I can connect you.
Thanks for the response Michael. I had overlooked that detail in the OP so I understand why you went in that direction.
Got an updated quote for Comfortboard 80 through the local distributor, it's down from $12/wsf to $9.67/wsf, compared with about $2/wsf current market price for the Swinburne truss design.
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