XPS, is 1" enough?

Greetings all, my brother recently bought a house and the basement has basically 2x2 walls everywhere. When we removed the fake wood paneling, there is nothing under it except a 6mil vapor barrier tacked to the 2x2 "studs". So I've gotten advice on my basement remodel before I did that but my space was not as big as his so I've got a question about the XPS insulation (we're going to put XPs on the wall and then tape and foam it).

He went through the winter without much issue temp-wise down there so it doesn't appear to get that freezing down there (maybe the soil is regulating it to some extent?) so we're trying to decide what thickness of XPS to put against the walls. Around here (Michigan) you can get 1" 4x8 XPS for about $18 a sheet (SSE or T&G). Or you can get 1.5" for TWICE that (~$36) or 2" for again, twice that (~$36). Since we need a lot of this stuff, we're interested in knowing if there is some law of physics or chemistry or astrophysics that says 1" is not going to work for us?

For example, I know that some people put in the XPS and seal it, then do some fiberglass between the studs, with the XPS as a vapor barrier. I think we're undecided on if we're even going to put fiberglass in between the studs but if we do, and there's some law that states that 1" XPS is not thick enough to "count" as a vapor barrier, we obviously don't want to have potential mold/moisture problems with that fiberglass!

I think he'd like to go 1" with no fiberglass (since, with nothing, the space seemed decent in the winter) since that would maximize the remaining finished space, and be the least costly.

But I thought I would kick it to the pros for your opinions!

I think you should shop around a bit more, because I can get 2" XPS for 21-23$ any day of the week. It is up from 18-20$ 3 years ago.

Short answer is "yes, but"

If you only use 1" xps you will have a vapor/moisture barrier but not have enough r-value (mainly to prevent condensation when the heated air hits cold wall). So, what you can do is use unfaced fiblerglass in the 2x4 stud space in addition to the 1" xps. This should give you a r-18, plus the moisture barrier of the XPS. You do not use faced fiberglass because you will then have 2 vapor barriers, which will cause problems. In the basement, you want the fiberglass to be able to breathe.

If you only used 1" xps, you would lose the heat you put into the space because it would transfer too rapidly into the wall/earth.

It has been a while but I believe this is the best practice. I believe it is even better than just using thicker xps because, to make a long story short, the thinner xps will have less of a tendancy to trap water if it has migrated from outside (the foundation wall) to inside.

In a basement you don't need or particularly want a vapor barrier. If you DO have a vapor retarder, it wants to be between the studs and concrete, to protect the wood against ground moisture.

If you were planning to cut up the sheet foam and put it between the 2x2s, you will be wasting good foam due to the thermal bridging of the foam, and you will be putting the stud edges at risk. If you are putting it on the interior side of the studs under the gypsum that would put the studs on the cold side of the assembly and they would retain moisture in winter- bad idea. If you're putting the 1" foam between the studs and foundation (preferred), it's more than enough for 2x2 studs, but could be marginal for 2x4 studs depending on exactly which climate zone you are in.

The right stackup for basement insulation is to put the rigid foam against the foundation, trapping it with a studwall insulated with unfaced batts (rock wool preferred, unfaced fiberglass OK.) From a dew-point control perspective inside the studwall, 1" EPS or XPS is PLENTY if you're only adding 1.5" of fiber. If you were using a 2x4 studwall with unfaced batts on the interior side, 1" of EPS is good enough for the southern part of MI, but not the northern US climate zone 6 portions, which would require 1.5" minimum, but 2" is even better:



If you're not willing to give up much interior space, 2.5"-3" EPS held in place by 1x furring Tap-conned to foundation 24" o.c. on which to mount the wallboard works, and would deliver twice the thermal performance of 1" XPS plus batt-filled 2x2 studwall.

XPS is far more environmentally damaging and more expensive than EPS- same polymer, different blowing agent. As the HFC134a bleeds out of the XPS over time (doing it's damage), it's R value eventually reaches that of EPS anyway. XPS is generally not worth the upcharge. It's more vapor tight than EPS which is useful in some stackups, but it's a double-edged sword. When you can design it out you're usually better off.

The IRC doesn't specify the R value of foam required for dew point control in a sub-grade studwall with foam between the studs and concrete, but if you follow the IRC spec for above grade walls you're in pretty good shape- you would in fact have margin. The point of the IRC prescriptive is to avoid mold on the structural sheathing that would be between the foam and cavity insulation, but in your basement wall stackup that susceptible element doesn't exist. The condensing surface in the basement stackup is the EPS/XPS, which is very tolerant of moisture, as is fiberglass or rock wool batting. The section of the cavity that is more than a foot below grade will NEVER condense of you go with the IRC prescriptive values for above grade walls, and the above-grade section would only occasionally, but with no consequences to either thermal performance or mold issues. But at half the IRC prescriptive you could end up with frost on the upper part of the foam growing into the fiber during longer cold snaps, which would be a thermal issue.

Put an inch of EPS under the bottom plate of the studwall and the slab, and TapCon it to the slab. (You'll have to vaccum the dust out of the hole to be able to screw it in.) That forms a pretty good thermal & capillary break for the bottom plate, reducing mold & rot risk there.

The wallboard is best caulked air-tight to the framing, and painted with nothing more vapor retardent than standard latex paints. Air seal around any electrical boxes as well (can-foam the back-sides, and caulk or foam the box to the wallboard). That prevents air-transported moisture from condensing in higher quantities at the top of the stud bay.

The foundation sill & band joist need to be air-sealed tight to the wall foam, and the seams of the wall foam can be either taped with housewrap tape or sealed with duct-mastic. Cut'n'cobbled EPS at at the top of the foundation, the top of the sill, and the band joist, sealed at the edges, and carefully trimmed unfaced batts on the interior side of those areas completes it.

The "whole-wall" R of a 2x4 wall with a 20% framing fraction (typical for basements, at 16" o.c.) with R13 batts is about R9.5-R10 after factoring in the thermal bridging of the studs, adding in the R value of the concrete and wallboard. Putting in 1.5" of EPS (R6) increases that to R16, since the EPS is not thermally bridged by the studs, and meets the IRC 2012 R-value minimums for basement insulation. That's a higher performance than a wood-sided above grade 2x6 wall with R21 batts at a typical 25% framing fraction, which comes in at about R14 whole-wall. But in climate zone 6 & higher it's worth going to 24" o.c. studs and R15 batts and 2" of EPS, which brings you to about R20 whole-wall (and beats the IRC 2012 code min. for basements.)