How to Insulate basement below and above grade concrete walls, already framed with plumbing and electrical
Hello, I am renovating my slab on grade house, I live in Ontario Canada, cold winters, hot summers…
My contractor wants to insulate the lower level by putting the pink batt insulation and then poly, however the walls are concrete , some are partially below grade, and two are above grade … 2×4 frames are already up and plumbing and electrical are done as well.
I started researching and found that putting the pink batts up next to concrete is not a good idea nor is the poly vapour barrier in this situation.
I now plan on using the Certainteed Membrain smart vapor retarder instead of the poly but dont know how to best insulate, I cant ask them to undo the framing at this point. Thought of adding Tyvek behind the studs against the concrete so the pink batt insulation doesnt touch the concrete…or…. cutting very thin rigid foam board and sliding it behind the framing and gluing it and sealing all the edges somehow… or….. spray foam?
Contractor is worried about the spray foam option in case there was ever a leak or issue with the electrical in the future..
You can see in the attached picture there is currently alot going on in the walls framing in terms of plumbing etc..
Any advice would be greatly appreciated!
Thank you,
Jewell
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Replies
I always recommend using some sort of foam against the concrete when insulating basements in cold climates, but if that's really not an option, I would cover the concrete with poly sheeting, insulate the bays with mineral wool or dense-packed cellulose, and very, very carefully install and air-seal a variable permeance vapor retarder on the interior. Membrain works but Intello and Majrex are far tougher and what I would use in this situation.
Thank you for the replay, may I ask why using tvyek or typar wont work as a buffer between the concrete and insulation? I had seen this in a video as a way to prevent insulation touching concrete and causing condensation
And when you say foam then rigid foam board would be what you suggested as well?
Should it be glued?
Polyethylene is a vapor barrier, and you need that here to keep moisture from the concrete from getting out into the insulation where it can cause problems. Tyvek and the like are WRBs, Water Resistant Barriers, but they are intentionally vapor open, i.e. they are NOT vapor barriers and will NOT block moisture migration.
Rigid foam typically works as a vapor barrier when sufficiently thick, or if faced with a impermeable material such as polyethylene or metal foil that hasn't been perforated.
Bill
Thanks for the reply Bill, does anything change in the setup if I am installing a fridge and stove on a partially below grade concrete wall?
And does anything change on the fully above grade concrete walls?
It seems that I may need to do spray foam on the above grade wall with all the plumbing in it , do I need to go thicker behind/around the plumbing?
I am thinking of doing rigid as suggested on the wall that is partially below grade as it has space behind the studs but this is the wall which will have stove /fridge and the wall beside it has the plumbing and much less space to add rigid, so can I mix insulation in the same space as long as it is properly sealed at the area where walls meet?
Also we have laid drycore down on the concrete slab as a subfloor as it is a slab on grade home, and framing is sitting on the drycore, how do I finish the foam board and spray foam down where it meets the drycore subfloor? Do I need to seal this area? Especially if I am not using a vapour retarder behind the drywall, or should I use membrain?The contractor filled most of my cove joints with hydraulic cement , there a few open areas still , which I think is a good thing?
Thanks so much for the help, I think these are my last questions lol...
Jewel33,
Just to add to what Bill said.
With basement insulation you are trying to do two things: Stop the warm, moist inside air getting to the cold concrete where it will condense, and stopping moisture from the concrete moving into the basement adding humidity there.
Using an air/vapour-barrier like poly against the concrete does the latter, but doesn’t help with the former, as by the time the interior air get to it the poly is below the dew point - so while it stops that air condensing on the concrete, it instead condenses on the inside face of the poly. That’s why foam is recommended. It is an air/vapour retarder like poly, but its inside face stays warm enough to stop the condensation.
I would use rigid foam on the entire inside surface of the concrete wall to be safe. Concrete can wick moisture, which means it can carry moisture up higher than the level of the soil on the exterior. Rigid foam is the safest way to go here. You can use batts on the interior side of the rigid foam if you want, since the rigid foam in that case is providing the moisture control for you. I prefer to use ONLY rigid foam myself when doing this sort of thing, I do not like to use batts at all in underground spaces like this.
Note that you can use rigid foam on the sides of the pipes and then fill in around the pipes with canned foam. It's not necasary to use spray foam over the entire wall surface if you're trying to avoid using spray foam.
Bill
I'm not an expert but I've been reading up a lot.
It appears you might have the perfect opportunity for exterior insulation. If you install enough of that, you don't even need any interior insulation at all.
I'm from Ontario too. Check out this guide: https://natural-resources.canada.ca/energy-efficiency/home-energy-efficiency/keeping-heat. Lots of good info in there.
Thank you for the reply, unfortunately I cant do the exterior at this time.
In that case I would go with Akos' suggestion to move the walls out enough to insert rigid foam sheets. Check with your builder and building department but based on what I'm reading on the nrcan site you will want R-10 foam sheets which I believe are 2" thick. See section 6.2.4 here: https://natural-resources.canada.ca/energy-efficiency/home-energy-efficiency/keeping-heat-section-6-basement-insulation-floors-walls-crawl-spaces#a6-2-4
thank you for your responses and help :)
Bellow grade, there are only a few options that guaranteed to work. The pink+poly is not one of them, it is prone to turning into a moldy mess.
I think the simplest is to have the contractor release* all the studs and move it way from the foundation 1.5". Nail a 2x2 to the sides of the top and bottom plate to extend them to the new stud edge. You can take out a couple of studs that don't have wires through them and slide sheets of rigid through this gap in behind the rest of studs. Tape the seams of the rigid and seal it with canned foam at the top and bottom. Once the rigid is in, insulate the stud bays with unfaced fluffy and install interior drywall (NO POLY behind drywall). Simple, uses box store materials and guaranteed to work.
If the rigid install is too much for the contractor you can spray foam the foundation with 1.5" of closed cell and insulate the same as above. Again, very effective worry free wall except for the usual issues with spray foam (cost and rare bad install that is near impossible to remediate).
You can also spray foam as is but you have to get the spray foam in behind all the studs. You essentially need to create a solid air barrier between the inside of the basement and the concrete. The spray foam would now reduce the space inside the studs but you can insulate with either R8 rolls or squished batts.
You want a thin layer of spray foam (the 1.5" above) so won't encapsulate any of the electrical and you can wrap the plumbing pipe with poly beforehand to keep it from sticking to the spray foam.
*releasing the studs is easy. Cut the top and bottom with a demo blade on a sawzall, you can move that whole in under an hour.
Thank you for the response, how thick does the rigid foam board need to be and do I need to glue it?
Depends on what your building department requires. If it is R20CI than you need R12 batts+r10 rigid. If it is R15CI than R12+R5 rigid. Since a lot of the wall is above grade, I would go for the most you can install. R14 batts with 1.5" rigid is a pretty solid assembly.
The rigid needs to be tight to the concrete, glue is the easiest. You also have to air seal it around the perimeter to prevent air from getting behind the rigid, a bead of acoustic caulk along the bottom and top of the wall is the easiest. You can put the caulk on the wall and slide the rigid over it and push it into the adhesive. Tape the seams between rigid panels.
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