We have a Sears kit cottage built around 1964. It is built out of solid red cedar, 2”x6” tongue and groove walls run vertically. There are no studs, the boards are the interior, exterior and structural pieces all as one. A small wood stove takes the chill out in cooler weather but no insulation and single pane windows and being up on posts does not make for extended season enjoyment. I have been considering some insulation. There are web rafters so I could blow in some roof insulation. The crawl space, from 1 to 3 feet, I can see 2” rigid insulation with a exterior finish al la mobile homes skirting. The walls could then take 1” rigid covered with maybe board and batten siding. Then start replacing windows.
My concern is that will I be risking to rot it from the inside out once I seal the exterior walls? Is it’s longevity due to its beathability? I see no sense in tyvek with the impermeable rigid outside of that. I suspect digging in the 2” insulation a foot should take care of the plumbing and I will need to ventilate the under space. It is on a swamp so humid and north shore Lake Erie so dead of winter not my aim.
Replies
It is good to think about this before proceeding.
A single vapor control layer is a good idea. sounds like your primary worry will be the heating season, so a vapor barrier on the inside would seem called for.
But if you plan for occasional use, putting the vapor control layer on the outside of the current vertical boards would likely be ok as well. R value of cedar is around 1.4 per inch, so if the boards are 1.5 inches the existing walls would be a bit over R2. Two inches of foam at R20 would put the interface 2/22 or 1/11th of the temperature gradiant from inside to outside.
In other words, as soon as you heat up the wood walls the inside surface of your exterior foam would likely be over the dew point temperature. Between occupations, it would be good to provide for ventilation.
The bottom would be better, it seems to me, to seal and insulate under the floor. Foam board also is possible, but as with the walls, you need an exterior covering to protect the foam from damage from elements and critters.
the swampy location means there will be plenty of frost heave.
Before adding insulation to the attic, make sure it is properly vented, and find a way to air seal/vapor control the ceiling.