Air sealing ceilings in a log home
cwadeson
| Posted in Energy Efficiency and Durability on
Hello all,
My wife and i will be building a log home this year and i am working on the details for the roof and framed wall assemblies. The walls will be true wall logs 8″ thick with each course sealed with gasket and sealant. The gable ends and roof will be framed/trussed with sheathing and cavity insulation. My original thought is to use rockwool cavity insulation for the framed portions of this project with a siga majrex membrane. We are building in climate zone 6A. The interior framed walls and roof will be covered in tongue and grove. I’m wondering if i am on the right track here. Thanks in advance!
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Replies
cwadeson,
You can insulate the framed walls the same way as they are done in conventionally framed houses. Adding more insualtion than fits in a 2"x6" wall probably doesn't make sense, as your log walls are proportionally going to be much less efficient.
For the roof, make sure you use framing members with sufficient depth to hold the insulation you need. Otherwise you end up with very limited and expensive options. https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
The biggest challenge is going to be air-sealing. The framed gable and interior walls will have very different rates of shrinkage than the log ones. So you need to come up with a detail for how you deal with that - and also how you deal with the same problem at door and window openings in the log portion of the walls.
Thanks Malcolm.
1) framed walls will have R23 Rockwool
2) Roof will be trussed with flat trusses on two wings and parallel cord trusses through the center cathedral section so depth of insulation wont be an issue there.
My main concern, since I've never built with Majrex before, is if that combined with the rockwool and T&G is an adequate assembly for air sealing at ceilings/roof.
The T&G doesn't provide any air sealing. The Majrex is the only thing providing that.
I conduct about half dozen energy audits and building investigations on log homes a year. A problem that is consistent with nearly of them is how the roof air sealing is connected (and made continuous) to the log walls. This thermal image is from a log home I was at last week, a 2000 build at 10 ACH50, about double what I typically see from stick-built homes of the same era. A good amount of the total leakage was the wall to roof interface. Very difficult to fix after the fact.
Randy,
That’s the worry I was alluding to when I brought up the differential settlement of the gable walls and the log ones. The air-sealing connection between the roof and walls also has to take into account that the log walls will shrink over time, while the stick-framed ones won’t.