Insulating an A-Frame roof.
We are planning on building an a-frame cabin up in the U.P. of Michigan that will not be a primary residence. My understanding from the local inspector is that I am not required to meet the energy code due to the sporadic and non-primary use of the building.
I was considering saving some money by using T&G pine as the roof decking/interior ceiling, then waterproofing, then several inches of rigid foam, then some purlins/metal roof. I have seen a few videos on youtube using this approach, but I am a little skeptical.
I was curious if this could work out, assuming I can size the wood stove to account for the lower R-value in the roof. We only use it a few times in the winter, and I am fine with just going through more wood.
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Replies
It can work. You will need to tie the air barrier of the roof/ceiling into the air barrier of the walls (gables for an A-frame) to keep the layer continuous, have a plan before you start.
I have an off grid cabin and you just cant think of it in the same way you would a modern on grid house. Build what works for your situation. You can size your rafters for structure and not insulation levels.
I am all in on burning an extra log or 2 when needed to make up for reduced insulation on a seldom used structure, it just makes sense.
Have fun!
huckarcher,
The two things I would look out for are:
- As plumb bob said, the continuous of the air barrier, and how it transitions from the top side of the roof sheathing to the walls. You can lose a lot of air through the t & g ceiling where it goes over the top-plates.
- What you use as the layer you identify as "waterproofing". It's too far down the assembly to be useful as a roof underlayment, or part of the roof waterproofing (except during construction). It's primarily an air-barrier, but you may want to use either a self-adhered WRB, or peel & stick membrane anyway.
You may want to think about the future use of the building over its lifespan. If you go with a roof assembly that is designed for very occasional winter use and that changes, you are stuck with either living with an inadequately insulated (but recently built) cabin, or doing costly renovations.
Also, if you are building on weekends and spare time, critters can be a major issue. I had the vaulted roof insulated and poly installed, but the soffits were open, and we got invaded by packrats (bushy tailed wood rat). I ended up having to tear out and re-insulate the ceiling TWICE to clean out the nesting materials and rat shit. No good
plumb_bob,
I have a friend with a hunting cabin up near Prince George. He is constantly besieged by pack rats - especially invading his skirted crawlspace. Seems like they are a real problem up your way.