Basement remodel questions.

[ynnek63]

Hello all,

I am planning on turning about 325 square feet of my basement into a "man cave". My house is about 90 years old and the basement has block walls. When I bought the house about 4 years ago, i had the basement water "proofed" by B-Dry Basement Water Proofing. And in the 4 years since, the basement has been very dry and odor free, with the help of a dehumidifier as well. I am pretty happy with the B-dry system. Oh, the basement had been finished when I bought the house, but I had to pull all the perimeter stud walls out so B-Dry could jack hammer and install the drainage system. There are still some partition walls remaining.

I am attaching a picture so you can see what I will be dealing with. It is pretty much representative of the whole area I will be renovating.


The ceiling height is only 84", but I am ok with losing a few inches, since this will be a "cave" after all.

So, my plans/questions are as follows:

For the floor I am thinking 6mil poly, 1" rigid XPS foam and 1/2" plywood and having this a floating floor. Good idea? I've read in other posts that another layer of plywood is good. This floor will only have padding and carpeting on it. Would I need to attach the floor to the slab if it feels solid? For the most part, the slab is level.

For the walls, the base plate will be at the edges of the plywood. As you can see in the photo, B-dry had to create a 5 to 6 wide by 1 to 1.5 inch high lip because of the way the foundation blocks were laid out or something like that. Anyway, the point is that the stud walls will be 5 to 6 inches away from the block wall (which itself is covered with 6mil reinforced poly). Will this gap present any issues other than losing floor space? How would I insulate that area?

I am also planning on spraying closed cell poly foam into the rim joist cavities.

Also, for some reason I can no longer remember, B-Dry uses a brittle cement over their drainage system and the only way to attach wood is with adhesive, which I why i would prefer to have the floating floor with the walls on it.

Because my house is "under water" so to speak (I owe more than it is worth) I want to do this correctly, but also as cheap as possible.

Pic below.

 

[Dana]

On the floor your basic approach is solid- use tongue & groove plywood over the XPS, and anchor it to the slab with TapCons to avoid "leaf curl" or "potato chipping" warpage lifting at the edges or big waves. If that's going to be an issue for your floor sealer, skip the poly, use specially formulated foam-board construction adhesive to bond the foam to the slab, and the plywood to the foam- it'll stay in place without the TapCons, and the inch of XPS is sufficiently vapor retardent to protect the plywood & rug underlayment from any ground moisture wicking. If you want to save a coupla bucks, use Type-II (1.5lb density) or Type-IX (2lb density) EPS instead of XPS- it'll all have the same R value after 50 years anyway- the XPS loses most of it's (envirnomentally damaging) HFC134a blowing agent that gives it the higher R/inch in that time frame and the extra R0.8 isn't going to be a significant performance improvement at Louisville's 56-58F subsoil temp. R4.2 (an inch of 1.5lb EPS) is plenty for dew point control. Stagger the seams of the plywood with those of the foam by at least 8" to avoid rockering-compression issues along the foam seams.

The wall plan needs revision though. Ideally you'd put at least 1-2" of EPS (cheaper than XPS, and with far lower environmental footprint than XPS) against the wall-poly, snug up the studs to the wall-foam, and insulate the studwall with UNFACED rock wool (preferred, on multiple grounds) or fiberglass (still OK) batts. That way the stud edges and foam/fiber interface stay above the wintertime dew point of the interior air to keep condensation under control, the wood can always dry toward the conditioned space interior, and there are no thermal bypass air-movement cavities between the insulation & foundation. Putting the bottom plate of the studwall on the projecting lip is fine- put an inch of foam under it as a thermal & capillary break against ground water wicking and you'll have no worries.

In a Louisville climate you can use open cell foam to insulate and air-seal the band joist & foundation sill to your layer of wall-foam and the top of the studwall. It not only air-seals better, it's a far better R/$ proposition than closed cell, and uses much less environmentally damaging blowing agent (water, instead of HFC245fa ) to boot.

 

[Sponsor]

 

[ynnek63]

First off, thank you so much for your response!


Since the cement on the lip is so brittle, it would be ok to use construction adhesive on the foam and base plate? Or, and this would be overkill I know, could I use 4 to 5" of the EPS foam to allow me to bring the wall off of the lip? My concern with using the lip is that in the corners, the lip curves and is deeper and the hard plastic they used near the bottom of the wall is also curved in the corners. And getting the sub floor high enough to meet the base plate would probably cost me another inch or so of headroom.

Also, either way I did it, how would you recommend I attach the foam to the poly on the wall since it hangs somewhat loose?

Again, thank you so much for your time and effort in answering my questions.

 

[Dana]

4-5" of foam is overkill but hidden wide-open cavities like are prone to becoming convective thermal bypasses. To avoid wintertime condensation and mold issues on the poly, the exterior ~R4 or more has to be air-impermeable insulation (foam).

You could just blow 3-4" of open cell foam (~R13-R15) on the wall-poly all the way from the floor to the band-joist and leave the studwall uninsulated, at a cost of about $1-1.40 per square foot. That way any air-gap remaining behind the studwall is completely inside the pressure & thermal boundary, and air movement in that gap won't translate into a heat loss. But if there's ANY chance of a flood you don't want to have open cell foam anywhere near the high-tide mark as it'll take on liquid water. As long as you insulate down to at least 2-3' below grade you'll have gotten most of the value out of insulating the foundation wall.

In general it's cheaper as a DIY to put R4 of EPS (at ~40 cents per square foot) against the foundation, and fatten it up with cheap R13-R15 fiber in the studwall (~R10-R11 after the thermal bridging of the framing is factored in) for an ~R15 wall, which is VERY comfortable year round in your climate zone, and probably comparable or better thermal performance than your above grade walls. (A 2x6 studwall with fiberglass cavity insulation typically comes in at about R13-15 after thermal bridging depending on the stud spacing and siding options.)