Insulating a 3 season cottage in Ontario Canada
We have recently purchased a 3 season lakeside cottage a few hours north of Toronto Ontario, climate zone 6. The building is only around 750sq ft, one story built on piers with a crawlspace circa 1960s. Construction is 2×4.
Current construction is as follows from outside cement board siding-> furring strips->old wood siding (used as sheathing I think)-> 1/2″ white styrofoam-> insulation bats (craft faced) -> wood paneling /drywall.
Ceiling has a poly vapor barrier, 1/4″ plywood and drywall / ceiling tiles.
Floor is completely uninsulated but has a poly vapor barrier stapled between the joists.
The place was in fairly rough shape cosmetically and we have pulled everything back to the studs. To my surprise the whole building is in great shape with no mold to be found.
We looking to insulate the walls / ceiling for now and install new drywall everywhere. Cottage will remain 3 seasons for now, but I want to do everything to the best of our abilities while everything is open so that in the future the transition will be easier.
I will be adding a minisplit with 3 heads to condition the space but it will be closed down for the most part all winter. I would like the ability to use it but winter usage wont be a primary concern.
I want to make sure that our insulation strategy does not create the potential for moud growth during the summer when air conditioned, and also when it is shut down over the winter.
Right now we are looking to use Roxul in the stud bays with a poly vapour barrier or membrain. Another option is spray foam. Additionally we could add a layer of comfortboard to the exterior and replace the siding with a rain screen. Likely blow in insulation for the attic. We will not be insulating the floor for now.
I also have concern with leaving the styrofoam in place and having two moisture barriers.
Are there any issues with this strategy?
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Carpenter ants like spray foam. I had a major problem with them in both my bunky and my cottage in Parry Sound. Stay away from it.
The big concern when insulating is trapping moisture in the wall. When the two sides of a wall are at different temperatures you get what's called "vapor drive," where the temperature differential tends to drive moisture from warm to cool. So the simplest approach is to block moisture from entering the wall on the warm side, and construct the cool side so that moisture can exit freely. So in a place like Ontario you put the vapor barrier on the interior and make the exterior vapor open, and in a place like Miami you put the vapor barrier on the exterior and allow the wall to dry to the interior (and have the air conditioning remove the moisture that enters).
In a place that has cold winters and hot summers the warm side of the wall changes with the seasons. In a really cold place you just assume that the vapor drive will be so much greater in the winter that it will drive out all the moisture that accumulates over the summer. In moderate climates you do a couple of things. On the interior you use a vapor retarder instead of a vapor barrier, and you might even use a smart vapor retarder that is designed to block vapor in the winter and allow it to pass in the summer.
If your wall construction doesn't allow drying to the cold side you have what's called a "wrong side vapor barrier" -- WSVB. Your wall isn't going to be able to dry to the cold side. What you have to do in that case is use an impermeable insulation -- closed cell foam -- and have enough of it on the cold side so that moisture won't condense and accumulate. You'll still get vapor drive, but it will build up and eventually reach equilibrium with the warm side and the drive will stop.
So that's the theory. The question is, which scenario are you? Which side of your wall is the predominantly warm side, and what is the ration of heating to cooling? If you were living there year-round the answer would be easy, but if you're only there three seasons it gets trickier.
The safest thing would be to treat it as if it's a WSVB. Famed building scientist Joe Lstiburek calls that "the wall that works in any climate." Interestingly, from your description -- foam board behind fiberglass batt -- it sounds like that's the way the wall was previously done.
The issue is that it’s only 1/2 of eps. I’m not sure if it will prevent the condensation although it does allow drying to the outside, just not too much.
I’ve attached some pics for the existing construction.
Seems to me you need to dig a little deeper and understand if you are building on a proper foundation.
All to often lake houses start out quite differently than they end up. The foundation maybe nothing but a few rocks setting on the dirt under the floor you are standing on.
It sounds like your goal is a modern tight well insulated building but that is a whole system you need to start at the foundation and water barrier under the siding proper window flashing that will keep the rain out of the insulation in the walls a continuous air barrier to keep the wind from blowing thru the insulation if you start skipping steps things can get very ugly.
This place may well have been doing just fine with piss poor flashing around the windows because who care if some water gets in without insulation there is lots of heat energy to evaporate the water and without an air barrier there is lots of air movement to carry away the moisture before anything can rot.
Walta
Since what you had before worked, I would not go too far from it. The closest assembly I can quickly find is this one:
http://effectiver.ca/calculator/wall.php?id=7063
This has 1" of EPS though. As long as the 1/2 EPS is unfaced, it is relatively permeable and should allow for some drying.
I think membrane (this is silly priced up here in the great white north) or faced batts are a better option for this. Whenever you re-do the siding, you can look at adding a bit more exterior insulation to improve it if you want to use it full time.
3 head multi split feels a bit overkill for 750sqft cottage. I have a bit smaller 550sqft one up near Algonquin park and heat and cool the whole thing with a single wall mount mini split and some baseboards (baseboards are really only needed to bring the place up to temperature quickly). With the cost of hydro in remote areas, if you want cheap heat in the winter, a wood stove is the way to go.