Floor and ceiling insulation for a cabin/studio in British Columbia
I am going to be building a small (8×13) cabin (studio) to go in my backyard near Vancouver, BC. It is going to get used regularly in the summer, and occasionally during the winter. When not being used in winter, it will remain unheated, and when in use, we plan to use a radiant oil filled electric heater. It will be a post and beam structure, and I have a large supply of 1 1/2′ T&G cedar that was reclaimed from a panabode cabin that will be my walls.
For the floor, I am planning to do an ordinary floor with PT 2×6 joists (set on concrete blocks about 8″ above ground), 3/4 ply over the joists and then some hardwood finish flooring inside.
The ceiling will be a cathedral ceiling (with a sleeping loft) and the roof will be metal, pitch 10-12.
I would like to insulate the floor and the ceiling, so it holds a bit of heat when being used in winter (there is no point trying to insulate the walls) I originally considered using mineral wool between the floor joists, but after a bit of reading on this site, I am thinking XPS over the subfloor, with a second layer of ply on top, is a better way to go.
For the cathedral ceiling I am also considering XPS, but am not sure about moisture management. if the studio stays cold for weeks on end in winter, through rain and freeze thaw, where will moisture build up , and also over the cathedral ceiling.
I have read numerous articles here on issues about moisture management in cold/damp conditions for heated houses, but I haven’t seen anything about what happens when the interior spaces are left to get cold. Does it change the moisture management strategies, drying from inside out, or outside, in, or what materials would be best?
Many thanks,
Paul Nash,
Sechelt, BC
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Replies
Paul,
If you plan to occupy this space or use it for guests, you will need to keep it conditioned. If you don't, the interior furnishing will suffer, mold will grow, and no one will want to use the space. On the insulation, you can sometimes save quite a bit by using reclaimed rigid foam (installed on the structure's exterior). If you maximize the insulation for your climate and pay attention to air sealing, the energy load to keep the space comfortable will be quite reasonable.
Malcolm,
Good job working around the clematis. :0
Paul,
With the exception of the Panabode logs for walls, you are describing the she-shed I built for my wife this fall. The floor is a layer of 1/2" plywood with 2" EPS then of layer of 3/4" t&g plywood. The walls are insulted with 2" of exterior EPS and the roof is a sandwich similar to the floor.
Both the plywood and the rigid insulation were taped, making the structure quite air-tight. The disadvantage of the Panabode logs is they leak air and there is no good way to seal them. Then again with a building that size, who cares?
Being full of books, we keep the shed minimally heated with a portable oil heater in the winter, but even without heat I doubt you will have any moisture problems at all. The small amount of moisture you will generate while using it can be dissipated by opening the door or cracking a window. My similar uninsulated tool shed only shows a bit of discolouration on the ceiling plywood because the roof is uninsulated.
Thats a long way of saying I think your plan will work fine.
The she-shed before taping the insulation:
Once the weather improves and I get the rain-screen and siding on, I'm going to built an arbour in front of the door and hopefully in time the Clematis will cover it
Just for fun, here is the inside:
Malcolm,
Nice! Maybe I'll have to start building sheds to solve my "too many books" problem. Every now and then, I'll just build another shed.
Use EPS or polyiso rather than XPS. XPS is blown with very high global warming potential (greater than 1000x CO2 @ 100 years), whereas EPS and polyiso are blown with pentane (~7x CO2), most of which leaves the foam and is recaptured at the factory. As the HFCs leak out of the XPS over a few decades, it's performance drops to that of EPS of similar density (at full depletion.)
With the foam sandwiched between two layers of plywood, the compression rating becomes irrelevant, so really any density foam is fine.
If you can find a source of reclaimed foam (even used XPS) , that's a greener solution than any type of virgin-stock foam. There are many specialty recliamers and materials salvage companies trading in used foam board.
This surplus foam is sure priced right, and probably more than enough to wrap your whole cabin:
https://vancouver.craigslist.ca/rds/zip/d/free-scrap-rigid-foam/6423130938.html
https://vancouver.craigslist.ca/search/sss?query=rigid+insulation
Martin,
Getting 2000 odd books out of the house has been great, but the project has been a mixed blessing. My wife has found some of her friends now have shed-envy. Relationships have become strained.