Finishing My Basement - Insulation Questions

Hello everyone. I stumbled across this site because I’m doing some research before starting to finish my basement. I’m glad I did there is a lot of good info here.

Anyway, I wanted to get opinions on my basement finishing plans – at this point mostly the insulation plan… Dana please chime in :)

First, some background. I live in Columbus, Ohio. My house was built in 2010. I have a cinder block foundation/basement walls. I have a Tremco barrier solution on the exterior and (I think) 1" of the Tremco WARM-N-DRI insulation on the exterior (below ground only). The builder claims I have R5 on the exterior.

I've read the Building Science stuff like 100 times but I just want to get opinions. So here is my plan:

Floors: I would like to keep the subfloor height to around 1.5” total so I can either use 3/4” XPS and 3/4” OSB or 1” XPS and 5/8” OSB. Are either a viable option? I was planning on taping the XPS seams on the floor. I was not planning on using a sheet of poly on the concrete floor. Thoughts? I will probably be using carpet as the finish floor in most of the area, but may use some cork flooring also.

Walls: I keep going back and forth on this option. I originally was thinking 1.5” XPS because it’s a bit less expensive than 2”. But now I’m wondering if I can get away with 1” since I have R5 on the exterior. Whatever I choose the seams will be foam sealed and taped (or is there a better, less expensive way to seal joints?) Up against the XPS I was going to frame out the wall and fill the stud cavities with R13 unfaced fiberglass insulation. Thoughts on 1.5” XPS vs 1” XPS?

Rim joists: I was planning on using pieces of 2” XPS and foam sealing the edges.

I think that’s all my questions for now. Please let me know what you think. Thanks in advance for any feedback. It will be greatly appreciated as I've been driving myself crazy with this stuff.

The Warm-N-Dri spec calls out R5 for the 1-3/16" product, so that's about right.

With the slab supporting the full span from below you don't need 3/4" OSB to handle the floor loading- 5/8" is fine (even 7/16 sheathing can work.) The heavier OSB subflooring is only necessary for supporting spans between joists. Columbus is in US zone 5, and at your subsoil temps the additional R of going to 1" (or even 1.5") is cost effective in the longer term even assuming heat-pump or natural gas heating costs. Even 1" of XPS is more than enough capillary break and vapor retarder against ground moisture, and will keep the subfloor above the dew point of the summertime air as long as you keep the basement under 60% relative humidity (which will take either an air-conditioned house or a basement dehumidifier in a Columbus OH climate), even with the insulative effects of carpeting or cork flooring.

On the walls, yes, the exterior R counts, but so does the capacity of a CMU wall to wick ground moisture up from the footing. If there is a membrane or metal capillary break between the foundation & foundation-sill you can put foam of any permeance on the wall and it will be fine, but if it's a thin foamy-thing or none at all it's better to use a higher-perm foam than 1" XPS. From the point of view of keeping the stud edges dry in winter you'd have plenty of foam-R with an inch of XPS, but 1.5" of unfaced EPS (R6) would have similar R, but provide 3-5x the drying capacity for the CMU to better protect the foundation sill. (It may even be cheaper.) Tapes don't seal EPS very well, but low expansion foam does, as does fiber-reinforced duct-mastic applied with a putty knife 1/8-1/4" (initial thickness), ~1" either side of the seams.

Run the floor XPS all the way the CMU, put the edge of the wall-foam on top of that, and run the subfloor all the way to the wall foam, and the bottom plate of the studwall atop the subfloor. That way both the studwall plate and subfloor edge has full capillary & thermal breaks from the CMU & slab.

Yes for 2" XPS foam-sealed on the band joist and sill, sealing it to the top of the wall-foam.

Thanks so much for the reply!

I do have a foam sill gasket. So you are recommending that I put 1" XPS on the floor with no poly?

And then 1.5" EPS on the walls instead of 1.5 XPS because the eps allows for better drying of the block wall?

Can I use caulk to seal the edges of XPS in my rim joist? Most of my XPS is snug enough that I probably don't need to use expanding foam. So I was wondering if there is a certain type of caulk I could use that is safe on XPS. Thanks!

I'm finally starting to make some headway on my basement. I have 1.5" EPS to go on the walls and 1" XPS to go on the floor. Should I be putting 6 mil poly on the floor under the XPS or am I OK to just use the XPS on the floor? Thanks!

You can put poly down if you like- it doesn't hurt, only helps (and is pretty cheap), but if you'd rather not bother that's OK too. You don't absolutely need the poly. The wicking potential of XPS is very low, and at 1" it is moderately vapor-retardent too, so it's not going draw ground moisture into the subfloor at a rate faster than the subfloor can dry to the interior.


Standard grades of caulk is not a good sealant for polystyrene foams (XPS or EPS), since it has fairly poor adhesion to those materials and is guaranteed to fail over time. Acoustic-sealant type caulks would work better due to it's long-term high flexilbity. Using foam board construction adhesive as a caulk works too, but it's moderately expensive. Duct mastic can air-seal most materials pretty well and is cheap & easy to apply, but expanding foam is a slightly better approach for this application.

Thanks so much for the response, Dana!

I used an adhesive caulk that said it could be used on foam board insulation. seems to be working ok so far. Most of my XPS on the rim joist were tight enough that I had to hammer them into place and the big gaps I used spray foam on so hopefully I should be ok.

Since my builder put fiberglass batts in the rim joist is it ok to put that back in after I put the XPS in, or should I throw it out?

Sure, you can put it back after the XPS is installed and sealed. Unless it has an air-barrier on the interior side it won't perform anywhere near it's labeled rating (due to convection losses at the temperature extremes), but it'll still be doing something.

Thanks again Dana!

One more question. One side of the stairs that lead down to my basement the stringer butts right up against the cinder block wall. So I can insulate the block wall above and below that stinger but not between the stinger and the bock wall. I only have 1.5 inches, or so, of stinger thickness to work with to get insulation and drywall on. Any suggestions on how to handle that?