What is the best insulation for basement walls?
I want to insulate the basement of my ~850 sq. ft. 1924 bungalow and would like to see what kinds of suggestions might be out there as to how best to go about this. I am in climate zone 7a. The foundation is poured concrete and is in pretty good shape though I do get some seepage when the ground is saturated so that needs to be a consideration.
I have a target of R24 and would like to keep things as simple as possible. I am not thinking of doing the work myself so my choices will depend on what local contractors are familiar with and possibly what is permitted by building code, which would include a vapour barrier on the warm side of the insulation.
Insulating the basement without a frame wall interests me but I’m not sure if it would be possible to achieve R24 without one. Could 2” of EPS with another 4” of rock wool be installed without a frame wall? Even if not required I imagine it may be advisable to protect the insulation with some kind of covering after.
I have also wondered whether my poured concrete foundation wall is flat enough to use an EPS layer as vapour barrier. The poured concrete foundation appears to have been formed with square sheets of plywood maybe a couple of feet to a side and bulges a bit in the middle of each square. Also, I need to ensure that any seepage in the wall can escape so I wouldn’t be able to seal the EPS at the floor. If the EPS as vapour barrier is not advisable then maybe the 6 mil poly with something less rigid as the first layer to press it against the foundation wall would be better. But would that work without a frame wall to hold it in place?
thanks,
Paul
Insulating a basement properly to avoid mold and rot is not hard to do, but as you mention, Building Code and how your inspector interpretates it can be one of the main hurdles. And a vapor barrier on the warm side of the insulation is exactly the problem. That's the worst thing you can do, but inspectors often don't know that, so they force you into a design that is doomed to fail. First I think you should read a few of these pages -
Those pages should help a lot, but a few things to mention - I see your point about how the uneven walls will be tricky if you count on rigid foam boards against the concrete to stop the inward drive of moisture. Myself I would still put rigid EPS insulation against the wall, but I would put include a poly membrane as well.
The problem with basement vapor barriers is just that for decades they were put in the wrong place. As you seem to know, you want to keep a framed wall INSIDE of the vapor barrier. If you put a poly membrane against the concrete you will stop the moisture, then the EPS.
If you are looking to do this without framing a wall you will still need to cover your rigid insulation with drywall for fire protection. A method that will work (providing the inspector will sign off on it) would be poly and EPS held to the wall with furring strips. You could do that with long Tapcon concrete screws, though the thicker the insulation the longer the screws need to be and that will get expensive. but that allows you to cover it with drywall without a framed wall.
One product that may help would be these Thermal Wall panels, they have a metal rail embedded in the middle that you use to fasten the panel, then you replace the foam track and attach strapping to the same rail. so you can pull this off with shorter and much cheaper screws.
And there is no reason you couln't cover the EPS with a poly vapor barrier that will help pass inspection as the EPS foam can get wet, so there is nothing behind the poly that will lead to rot. Hopefully that helps, feel free to post a follow up question if not.
As for moisture seepage through walls - it would be wise to keep the drywall up a few inches off the floor in case of flooding, you can cover that with a trim board, so if there is any flooding the trim may dry out fine and you'd have no drywall to replace. And be sure to install a sump pump to avoid flooding.