Gentlemen, Super long time lurker here...I'm finishing my basement and have installed 3" XPS insulation on the walls and already have 1" XPS under the concrete slab. Time to get studding...height from the top of the finished slab to the bottom of the floor joists is 8'4" and thus question #1, should I: 1. Put 1" XPS under the 2x4x16' SPF bottom plate, lag with a 1/4" tap-con, use a 2x4x8' stud with a 2x4x16' SPF top plate? ***I like this option*** 2. Use a PT 2x4x16' base plate, rip 1" off the 2 x4x8' studs, and double the top (or bottom) plate with 2x4x16' SPF? Question #2 I have standard 4' x 8' x 1/2" drywall, and I plan to use a standard suspended ceiling system...do I: 1. Hold it 4" off the floor and scab in a piece of plywood, wedi, duorock...etc, and cover with say 5 or 6" base trim? 2. Keep the drywall around an 1" off the concrete and fill the top piece in with plywood. I believe all these options are decent...curious what you all think! Thanks, John

Not totally sure why you're putting insulation horizontally, under what are supposedly interior walls.

Care to explain your thinking there?

As for question 2.

You don't need a full inch at the bottom. Use a piece of 3/8" or 1/2" drywall as your floor spacer.

With a suspended ceiling, unless you're doing cavity-fill insulation, you really don't need to sheath the walls up beyond the drop ceiling.
If you want to avoid horizontal seams, hang your sheetrock vertically. This'll leave you with your half-ish inch gap at the bottom and a couple inches near the ceiling.
The couple inches at the top will be covered by the drop ceiling, and thus out of sight.

If you're doing cavity fill insulation, you should STILL be set, since the cavity will still only be about 8'. You're looking for either a double top plate or double bottom plate, plus a single opposing bottom/top plate.
With regular dimensional lumber, that subtracts 4.5" from your wall cavity.

As such, you shouldn't need to "scab over" anything.

My thinking for adding 1" of the XPS under the bottom plate (horizontal) of the non-load bearing stud wall is if I ever would get some water (under an inch) the water wouldn't wick up into the lumber and drywall. I'd have to imagine it would give you a thermal break as well.

Two questions. How did you install the 3" foam, and why the 2x4 wall on top?

Anything under the bottom plate isn't going to do anything for insulation perspective. Lifting the drywall a half inch is what most people do.

You are not required to so double top or bottom plates because it is not load bearing. But do bridge between the ends of each stud in the plate.

As far as the studs go. Do it however you feel you can get it the tightest fit. A floating wall not securely connected at the top doesn't seem very good to me. Cutting each stud to length is the best.

Using 3" XPS manufactured in North America anywhere in the assembly is the opposite of "green building", due to it's extremely high CO2e lifecycle impact, most of which is from the HFC blowing agents used (since team USA didn't sign on to the Kigali Amendment to the Montreal Protocol). At any given labeled R value XPS does ~8x the damage of EPS. See:

https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

Worse yet, it may be labeled R15 @ 3" but is only warranteed to R13.5. And if you bothered to sample and test it at 50 years you might be able to collect on a warranty claim. At full depletion of the climate damaging HFC blowing agents it's performance is no better than R12.6, the same as EPS of the same density.

By contrast, EPS is blown with a much lower impact hydrocarbon, usually a variant of pentane with less than 1% the impact of HFC134a, the predominant component of the mixtures used for manufacturing XPS. Almost all of the blowing agent leaves the foam while still at the factory and is recaptured, and is not vented to the atmosphere.

It's possible to hit IRC 2018 basement wall performance in a moisture-safe manner with half that much foam-R (or less) if using fiberglass or rock wool batts in the studwall. The total amount of foam-R needed is local climate dependent, since the above grade portion of the foundation needs to follow the rules for above-grade walls with exterior foam.

None of that matters here, if the XPS is already on the wall, but really, if you can design it out, you SHOULD design it out. The carbon "payback" on energy use carbon savings for R15 XPS is pretty much "never".

Using EPS under the bottom plate of a non-structural studwall serves as a thermal and capillary break from the slab. That can be useful if the slab isn't insulated at both the edge and under the slab, since it helps keep the wood above the average outdoor summertime dew points, and less susceptible to mold in the IECC "-A" zones. In this case there's an inch of XPS under the slab, but how about the slab edge- is it tight to foundation wall? If yes putting EPS under the bottom plate of the stud may help. Don't worry about the compression ratings- it's just holding up the drywall & electrical wiring, not the weight of the house.