Flooring options for basement

I forgot to mention that some of the Dow board does have a poly facing on it. See if yours does. If so, go ahead and peal it off.

Good reminder about the poly facing. There didn't seem to be any facing on these Dow XPS boards.
In case anyone's tracking prices, I paid $20/board for a big stack of 1.5"x4'x8' boards.

One more materials question: I'm not confident that any of seam tapes I've read about are long-lasting or reliable. What kind of tape should last the longest? Or, is caulk/foam a better option for seams?

Hmm, carpet tiles. That could be a good option on top of a couple layers of subfloor material. But probably not so comfy when installed directly onto the slab.

mrcoco said:

Thanks for the response, and especially the temperatre analysis.

I do have extensive downspout revisions, and an interior footing drain with sump. So although there's no such thing as a guarantee, all the usual sources of liquid moisture are cured. The efflorence comment just goes to how damp the ground is generally.

Ok, so your reply answers the second part of the question about whether 1" XPS woud put my studs at risk, and it sounds like not as long as I don't put too much glass in front. So that's good. But I still have the primary question: I already have the 1.5" XPS in hand, paid for and ready to install, but I wonder if I actually should switch to 1" for drying reasons to protect my sill plates.

Click to expand...

What nukeman said. In Seattle you don't have nearly the wintertime condensation issues that you have in cooler climes, and the fraction of the total R that needs to be foam is quite small. If you watnted (for some unknown reason) to put in unfaced R30 with that R7.5 XPS you'd still be fine from a condensation point of view. Unfaced R13 is pretty cheap stuff, and in combination with the R7.5XPS would be more than adequate from an energy-use point of view. R15 batts would lose less R value when it's really cold out though (density is an important factor when using fiberglass batts.)

Unfaced 1.5" XPS would be fine, just make sure you don't make it any more vapor-retardent than that. Seal every seam between panels, and pay attention to sealing it at the top. Insulate & seal the foundation sill & band joist with it too, sealing every joint. Duct mastic painted or troweled on 1/8-1/4" thick (when wet) works fine as seam-sealer, but for rougher sections with big gaps one-part spray foam might be called for. Most caulk won't form a sufficient bond to XPS, but foam-board construction adhesive could be used. (Use only foam-board adhesive- other stuff often has solvents that would damage the foam.) The difference drying capacity between 1" and 1.5" isn't enough to be worth switching, as long as you've sealed the foundation with masonry sealer (including the exterior portion wherever accessible.)

If you have a lot of interior efflorescence issues you may need to do more than simply sealing it with masonry sealer. If there isn't already, some amount of near-surface drainage many need to be put in (French drains, or splash-pads sloped to direct bulk water at least 2-3' away from the foundation etc.

But sealing it is still a must to protect the studwall. Most masonry sealers are vapor-permeable (check the specs- a few are highly vapor retardent, but most of those are sold as radon-sealers)- the foundation will still dry. The primary functoin of masonry sealer is to reduce the amount of capillary draw- the wicking of water through the material. Stopping the wicking on the surface maintains the integrity of the cement, which would otherwise be leaching mineral content to the surface and losing structural strength over time. In rainy-foggy Seattle, sealing the EXTERIOR above grade is critical, since it reduces the amount of dew/rain that gets sucked in, without reducing the drying rate very much. If you want to maximize the drying rate to protect the foundation sill you can seal only the below-grade portion of the interior concrete (but I'd personally take the slight hit in drying rate and seal the whole shebang, lest the above grade portion become a major efflorescence-point.)

Don't put any form of carpet down on an uninsulated slab or it'll mold, even in the relatively low summertime dew-point air of the Pacific Northwest. During the summer the subsoil temps are below the outdoor air dew points, and your indoor air will be even more humid much of the time. Carpet insulates the slab from the room air, lowering the temperature of the slab to something closer to the subsoil temp, so the room air that reaches the bottom of the carpet will exceed 70% or even 90% RH all summer (even if it doesn't condense.) It doesn't take a lot of insulation to "fix" this- 3/8" fan-fold XPS is about R2, which would raise the temp of the bottom of most carpets sufficiently to stop the mold conditions in Seattle (but not for the upper midwest or New England.) From a barefoot-comfort point of view, putting down R5 (1" XPS, or 1.25" EPS) is better than R2 with your subsoil temps, and unlike the wall situation, under slab going with something highly vapor-retardent like poly faced EPS is fine. The slab-sealer can be as high-perm as you like as well. The walls only need to be vapor-breathable to limit the amount of moisture reaching the foundation sill, whereas on the slab that's not an issue.

DO put in a capillary break between bottom plate of the studwall and the concrete though, even if you've used a sealer. Foam sill gasket are far better than nothing, foam gasket + 6mil poly is better.

awesome info here in regards im having similar issues -

I wanted to give you guys an update. I was able to use your information to persuade the inspector to allow me to add unfaced FG insulation in the studwall cavities, bringing my walls up to R21. He was quite cordial about it. Thanks again for arming me for the discussion.

On the floor, I ended up following Dana's guidance and putting down 1" XPS and 3/4" OSB with lots of Tapcons. Not a fun project, but that floor is going to be so sweet, and the room will be the most comfortable room in the house.

Thanks for everything guys!!!

One more update. I now have the room all closed in, and I can't believe how comfortable it is. It is hovering around zero degrees F in MN this week, and the new room is on average about 8 degrees warmer than other rooms in the house. The room is roughly 300 sq. ft. and has one heat register. I even tried a test and closed the register overnight, and still the room was 4 degrees warmer in the morning than the rest of the house. This room was super cold previously, and you needed a jacket on down there in the winter, even with the register wide open. The whole family will live down there in the winter...

Thanks again to everyone who provided guidance on this project. Great team effort!