Temporary basement insulation questions
Climate Zone 6, almost 7 in Ontario
I’m in the process of completely renovating our 700 ft^2 house. I have R60 in the attic, and my goal for the main floor walls is a minimum of R34. It’s currently R12. I’d like to match the R34 in the basement above grade and taper down to R20 near the footing (I think). I want to insulate the exterior of the basement and carry the insulation about 4 inches above the rim joist. I was planning on using the exterior insulation as a vapor barrier and tying it in with the interior vapor barrier on the main floor. The main floor walls are double 2×4 with 2-3 inches between them. I planned on using closed cell spray foam on the bottom plates to connect the interior and exterior vapor barrier.
For the basement, initially I was planning on using reclaimed XPS but I am now considering a Rockwool product because I need insulation now. I have a 50k BTU/hr wood stove in the basement and it can not heat the basement above 15*c (59*f) with outdoor temps at -10*c (14*f). I’m expecting a heat loss of 15k BTU/hr once I’m done all my renos but right now I’m likely over 60k btu/hr. We are using electric space heaters to keep the house livable but that’s costing near $300/month.
My basement is cinder block wall and I’ve built a 2×4 wall in front of it. The cinder block is poorly done. It wonders off course and is not plumb. It also has a weird 10″ horizontal section that looks like it is a poured slab about 5′ up the wall. All that to say, it will not be easy to put rigid insulation on it.
Since I ultimately want my insulation on the exterior but I need insulation for this winter, I’m wondering what I should do. The 2 thoughts I’ve had are to hang XPS on top of the studs and then temporarily put 1/2 drywall on top of it for a fire barrier, or to just hang some Rockwool panels on top of the studs. I’ll then transfer this insulation to the exterior when the time comes.
The basement does leak, fairly bad although I’ve improved it a fair bit with some exterior grading. I plan on water proofing the foundation this summer, which is when I plan to do the insulating. RH in the basement is 50-40% although since running the wood stove (only installed last week) it’s staying around 40%. Later down the road I plan to put vapor barrier on top of the slab, add R15-R20 XPS and then pour 2-3″ of cement with embedded PEX.
Presently the exterior is stone clad from slightly below grade right to the bottom of the roof deck. I have pretty bad ice dams already and am in the process of removing the top course of stone. I plan on completely removing the stone.
Sorry for the long post, I just wanted to try and cover all the details. Thanks for any comments.
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Replies
Make sure you’ve air sealed your rim joist. Air leaks can suck out much more heat than a poorly insulated wall.
I’d use some of whatever insulation you plan to use outside as your temporary indoor insulation so that you can reuse it when you move it. If you can’t do that, I’d look for cheap reclaimed insulation, maybe roofing polyiso but whatever you can get cheaply. I wouldn’t worry about a fire barrier if it’s just temporary for one winter, “during construction”, essentially.
I’m not sure using exterior below-grade foam is going to work well as a vapor barrier here either. In a new build it could work, but in a retrofit how will you tie it together with your slab vapor barrier that you said will be inside?
Bill
Thank you for your comments Bill.
Since I am living in the house with my wife and 3 young children, fire safety is a concern, even temporarily. Maybe if I was using my electric boiler instead of the wood stove I'd give it a go.
I have not air sealed the rim joist yet. I didn't want to seal it on the inside due to moisture concerns. I didn't want to limit it's ability to dry. It does have some rotten areas already that I will be addressing when I replace the exterior walls, but this may have been caused from roof leaks.
You are correct, I will not be able to tie it into the slab vapor barrier using exterior foam. The house is basically built on sand and the water table is at least 10 feet below the slab so I'm hoping with good drainage that this will be enough. Especially since I'm not planning on having interior insulation.
Since I really wanted exterior insulation, the only way I could think to tie the vapor barriers together would be to run a sheet of poly against the interior cement wall. This would then result in the concrete having a vapor barrier on both sides which would dump excess moisture in the rim joist area.
However, I did not realize Rockwool could be used below grade which would allow air drying of the concrete correct?
Would it be worth switching directions slightly in regard to my moisture barrier/insulation plan?
As an aside, the wood stove is able to provide enough heat so I don't think I need to rush with the insulation. I've been running it steady the last few days and I guess it just took a while to bring the thermal mass up to temperature. It's now 20*c in the basement, which is right around where we like it.
If you have free wood available for your wood stove, and it’s able to keep you warm enough, then the insulation becomes less of a concern. You could still do the *air sealing* work on the rim joist, even if you don’t insulate it. You’d be sealing the gaps in this case, probably using a mix of canned foam and caulk depending on the size of the gaps.
You can install a capillary break on top of your foundation wall to protect the rim joist. I’ve used thin pieces of HDPE sheet for this in the past. Dana here likes to use EPDM. both work. HDPE has the advantage of being stiff, so you can slide it under the rim joist more easily. EPDM is a common roofing material, so it’s easier to find in building supply places. Either way, you have to jack up your rim joist a bit to get the material into the gap. If you’ve already had issues with rot on your rim joist, adding a capillary break is probably a good idea.
Bill