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How to insulate a sort-of year round cottage

Glen Griffiths | Posted in Energy Efficiency and Durability on

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.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Glen,
    Q. "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?"

    A. I don't foresee any problems. I'm not sure what you are worried about. If you show up and the interior of your house is 20 degrees F, the interior air will be quite dry. (Cold air doesn't hold much moisture.) When you fire up your heating appliances, the air will be even dryer. I don't know what moisture you are worried about.

    A foam-sheathed wall is designed to dry to the interior. As long as you don't install any interior polyethylene, your walls will stay dry.

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