I am adding on to an existing A-frame cottage. The new addition will have a basement with ICF walls and rigid foam under basement slab (we are in Ontario – Zone 6). The walls will be 2 x 6, 24″ O.C with mineral wool R-22 insulation and rigid foam on the exterior as per thickness table.(https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing). The main floor ceiling will be insulated with blown-in cellulose to R-40.
The addition will be about 600 ft2. Existing cottage is about 800 ft2. High eff. propane furnace in existing basement (venting to basement and main floor – when opened) and will add a direct vent propane heater to new basement – to be a workshop, so isolated.
Ceilings in basement are insulated. In the winter we leave the heat on in the basement around 45 – 50 F to keep frost out of the slab. The main floor plumbing is drained and antifreeze added to toilet / sinks.
New addition will have a masonry heater (http://www.tempcast.com/) and we will keep the existing efficient wood stove.
My worry is when we show up on a weekend in the winter, the main floor is below freezing or thereabouts, and we fire up the heat – furnace and stove/fireplace. Do I have to worry about excessive moisture getting trapped in the walls? Should I forgo the rigid insulation on the exterior to allow for easier drying.
I want to insulate and seal the addition as well as retrofit the existing building properly (only 2 x 4 studs) as we will use it more in the winters as we get older and retire. The retrofit poses problems I’ll leave for another question at another time.
I would prefer this not get into a discussion about masonry heaters – it’s a luxury I’m willing to pay for. My main worry is vapor and moisture problems.
Thanks for any advice.