Region 5 below grade basement insulation techniques with intention to finish
I’ve probably read most or all of the articles on the subject 5-6 times over, but am still left with many questions as the advice can quickly change as the circumstances do, and it’s difficult to keep track of all these factors.
My goal is to finish my below grade basement in 18301 Region 5. Cape cod style home built in 1968. 954 sq ft basement.
Standard 16×8 block wall, ranging from 81.5″ high to 84.5″ high on the side with the sump pit. Slab appears to be about 1.5″ thick. Very minimal gravel under the slab. The outside of the block wall appears to have been painted with a black waterproofing paint. I don’t know if any other water management measures were taken on the exterior, but I don’t believe so.
Block wall does not appear to be topped with capstone block, just standard block with holes at the top. I only know this because every 176″ on center, the builder doubled up the block towards the interior to provide stability, and the part of this top layer that is inside the basement has the holes going through.
Then there was a metal termite barrier added on top of the block wall which extends out an inch or 2 into the interior. Then a very thin layer of fiberglass and then 2×8″ sill plate, and then 2×10″ band joist and floor joists. Presently there is fiberglass insulation in the floor joists (and craft faced on both sides oddly), though I intend to remove this mainly as it’s full of mouse droppings. I’m actually considering adding fiberglass back in the joists here but only for a measure of sound absorption between the floors, and I would just use unfaced fiberglass, somewhere between R-19 and R-30.
I have gutters along the exterior which underground pipes carrying the water away. Grading is fairly well done. There does not appear to be any water that comes in through the walls, and the floors also pass a moisture test (taping plastic to the floor does not result in any condensation bubbles).
There are also 5 windows along 2 of the walls, the openings measure 32″ wide by 14.5″ high.
I do have a mildly high radon level, and so I intend to have a contractor install piping and a fan (likely requiring an exterior penetration, unless we find a way through the interior walls and put the fan in the vented atttic and through the roof.
I would like to finish the entire basement, and am trying to decide on the best method to insulate/air seal as well as choose on flooring. I am also trying to take into account electrical receptacle box locations.
I had initially considered method (A), framing 2×4 studs 1″ off the block, and spraying with Lapolla 4G closed cell, 3″ – so 1″ continuous layer and then 2″ into the stud wall. Providing around R-19.5. However, I do not love the “permanence” of the installation. In case I ever needed to inspect or address a problem with the wall.
I then considered (B) a 1″ continuous layer of XPS, caulked at the gaps, and then taped, and framing over that, and then either cutting in additional XPS to the stud bays, or installing Roxul mineral wool.
Though I am also considering (C) a 3″ continuous layer of XPS, with 1″ furring strips and drywall screwed into the furring strips. However, I believe this method would require surface mounted electrical boxes which I’d rather avoid. OR, I could cut into the rigid foam board just where the electrical boxes – I think I’d still retain enough of a layer to keep my air sealing intact.
I believe regardless of the method chosen I will spray 3″ of closed cell on the band joist, wrapping down to connect with the XPS. I may end up using a DIY kit for this rather than hire a contractor. I was just going to install a drop-ceiling in the basement so that I have access for running wires in the future. I know the drywall is a sufficient fire barrier to cover the XPS on the wall, but since this would end at the bottom of the floor joists, I am not sure if I would be required to do anything more to protect against the top of the XPS and the closed cell on the band joist, if I am just installing drop tiles and not drywall on the ceiling.
I am seeking opinions on my stated options for insulation taking into consideration ease of install, costs, ability to maintain and access as much of the house as possible, and overall effectiveness. I like XPS over PolyISO as PolyISO loses R-value in colder temps. And I like XPS over EPS as it has a higher R-value per inch. My understanding is that I would not have a vapor barrier at all. Would this be OK, or would I need to add one? Much of what I’ve read seems to suggest it doesn’t really matter as half the year moisture will want to dry to the exterior and half the year to the interior. But I still want to make the best choice here. Also take into consideration that I already have foundation coating painted on the exterior of my block. Exterior insulation isn’t an option in my situation. I’d like to maximize my interior living space, ideally have access to run wires and receptacles in the walls, and optimize my energy efficiency and air sealing.
For flooring, I have debated between just painting an industrial grade epoxy on the entire slab, or a combination of epoxy, and ceramic tile or vinyl applied directly to the concrete. I’m leaning towards the comnbination, mainly to avoid the excessive prep work required for epoxy, and the need to do the entire thing in one shot to get a consistent finish.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part