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My new (to me) house has a 60-year-old block foundation. The previous owners did little to no insulating in the basement. However, they started to finish a bedroom down there, and got as far as framing the walls and wiring in the outlets and lights. I'm interested in finishing the job.

The framing was done using pressure-treated 2x4s for the sill plates and the studs that border the foundation, and ordinary 2x4s everywhere else. The distance between the studs and the foundation walls varies, but is never more than an inch, and in some places it is barely 1/8 of an inch.

My research tells me that the "right" way to do this would have been to cover the foundation walls with sheets of rigid-foam insulation first, then build the walls in front. Given that it's too late for that, what's the second-best option? Spray foam (expensive!)? Cut rigid foam to fit the spaces between the studs and caulk the seams (time consuming!)? Paint Drylok between and behind the studs, then fill with fiberglass batts? Just use fiberglass and hope for the best (I've been told fiberglass will eventually grow mold if you install it in the humid space adjacent to a foundation wall)?

I live in an area where it stays below freezing for most of the winter. The soil around the house is sandy and well-drained.

  • With a block wall I would be very cautious. Block walls are the most prone to leaks. I would want to make sure that the walls don't weep or leak before installing any insulation or sheet rock. You mentioned that it was sandy and well drained but is there any evidence of leakage water stains on the walls? It may be fine for a few years, then with a bad winter the walls start leaking and all your hard work and $ go down the drain. This happened to me when I lived in Ohio. – Ed Beal Apr 15 '16 at 21:29
  • @EdBeal That is something I worry about. There is some evidence of water leakage in a different part of the basement (that will remain unfinished), but this part looks ok. But just to be safe, I have separately dug out 4-5' deep outside around the perimeter, sealed up anything that looked like a crack with hydraulic cement, and painted the whole thing with sealant. And I'll be watching it like a hawk this spring. – dlf Apr 21 '16 at 13:05
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here is how we do block wall basement finishing - in almost 30 years i have never had a callback or any complaints about leaks after the fact. you do have to make sure the foundation is in good shape with no failed blocks or footings.

1) hire a foundation waterproofing company to come in and shoot urethane foundation sealer (blueseal is what we use. like drylok, but cheaper and better - thicker coating in non retail 5 gallon pails). you can also do it yourself with a drywall texture sprayer set to the smallest orifice. you can use anything you want, but it needs to be silicone or urethane.

2) before it dries (just follow the installer around), throw big handfuls of chopped fiberglass (check in your area for a fiberglass supplier - its cheap and abundant). make sure you coat everywhere as much as you can.

3) once dry, spray again. dont worry if you miss a reapply window as the chopped glass acts as a mechanical binder.

4) insulate with mineral wool insulation. it can get wet and it wont grow mold. it also self-drains if it does get wet.

5) install an air barrier, not a vapour barrier. its important you do this step correctly. the urethane sealant has sealed the wall cavity from the outside, but the blocks still act as a partial insulation void. you may get some condensation inside the wall, but it wont freeze. install the air barrier so that the printing faces the studs - this way the inside of the wall can breathe if it needs to. its the opposite way its normally installed, but its not designed for this purpose.

this approach works surprisingly well because the urethane/glass composite acts as a strong but flexible sealed barrier that resists fracturing in the foundation, and even if cracks form, the urethane can stretch to make sure the water doesn't get in. the air barrier trick is an old one, but it works really well in walls that may have occasional moisture.

good luck

  • This sounds like a good plan for a new basement, but you haven't really explained how this will work with a pre-framed basement. – isherwood Apr 21 '16 at 13:19
  • @isherwood - not sure what the confusion is - this is the protocol for a pre-framed basement just as the OP asked. which part do you want clarified? – personal privacy advocate Apr 21 '16 at 13:46
  • So you spray through the framing? – isherwood Apr 21 '16 at 14:03
  • yes - you have to waterproof as best as possible. its not ideal, but oftentimes these jobs arent. in an ideal world, you waterproof, then stud - but thats not the case here. at least the interior cavity is protected from moisture, the studs are somewhat at risk due to the fact they are on the wet side of the membrane, but its the best compromise. theres only risk if there is a leak, but then the stud is the only thing that can rot. if you dont have the membrane, then the stud and everything else is at risk because the water can infiltrate everywhere. – personal privacy advocate Apr 21 '16 at 14:16
  • @personalprivacyadvocate What about the fact that the electrical is already installed? Is is possible to spray around that? – dlf Apr 21 '16 at 15:08
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Unfaced friction-fit fiberglass in R-11 or R-13 will do just fine. If you're concerned about them sagging, staple each batt to the top plate. They won't go anywhere.

Depending on your climate, do not apply a vapor retarder over the wall. Let it breathe to avoid mold and mildew.

  • No concerns about mold growing on the fiberglass? – dlf Apr 15 '16 at 21:23
  • I attempted to address that concern in my answer. :) What I describe is standard practice where I am, and we have active sumps and humid summers. – isherwood Apr 17 '16 at 15:00
  • Downvoter, care to comment? This is a simple and proven technique. If you disagree, you should explain why. – isherwood Apr 21 '16 at 13:19

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