Subfloor Over Slab Advice

[ardpub]

I renovated a small ~170 sq ft detached building in back of my house to fix some water infiltration issues. It was built over a patio slab. Part of this involved raising the 3 exterior door thresholds to about 3” to top of the threshold.

Now I want to install flooring but would like to raise the flooring to be flush, or close to flush, with the thresholds (and to run some A/V cabling beneath the floor) and am debating between a sleeper subfloor of 2x4's with plywood/OSB, or rigid foam panels.

One other relevant detail, I’m in south Louisiana, about as hot and humid as it gets, but I added a mini split with dehumidifier so this room is climate controlled 24/7 (except after hurricanes, lol).

A few questions:
1- Is a sleeper subfloor the best option? Would foam insulation panels be better (see link)? At $2/sq ft, they seemed too expensive when I first started looking but now they’re actually comparable to wood--at least $350 for the sheetwood alone .

Dricore Panels:
https://www.homedepot.com/p/DRICORE...oor-Specialty-Panel-10-Pack-FG10038/316351887

2- If a sleeper is the way to go, should I use a Class I (impermeable) vapor barrier between the sleepers and the slab, or a Class II (semi-permeable), assuming moisture will inevitably make its way in to some extent?

3- Should I float the subfloor or tapcon it to the slab, which would penetrate the vapor barrier?

4- Lastly, treated wood or untreated ok?

I’m thinking 2x4’s 16” on center with 1/2” ply. 3/4” would be ideal but trying not to spend $900 on this tiny subfloor.

If I go the sleeper route, this would be the layering order:
Floated Laminate Floor
Foam Underlayment
1/2" ply or OSB, either T&G or with 1/8" gaps
Floated 2x4 sleepers
6mil poly vapor barrior (or Class II)
slab

 

[mcbane]

No idea where you are. If you get cold weather you might want to put 1-1/2” foam between sleepers. In any case I would put 6 mil plastic on the slab before installing sleepers.

Back when plywood was cheap I put a hardware floor over a slab. Vapor barrier, 1/16” pergo foam for a thermal break, then two layers of 1/2” plywood. The first layer of plywood went down with concrete nails. The second layer had offset seams and was glued and screwed (1” screws) to the lower layer. Hardwood floor was tongue and groove and fastened with the angle stapler as if it was going down on an ordinary wood subfloor. I have had no problem with this install in the 20 years since it went in.

 

[ardpub]

No idea where you are. If you get cold weather you might want to put 1-1/2” foam between sleepers. In any case I would put 6 mil plastic on the slab before installing sleepers.

South Louisiana, don't really need the insulation.

Back when plywood was cheap...

For reference, this is what's currently available locally and the costs for this tiny projcect. Foam would be more expensive unless it was a product the floor could be installed on directly. The entire rennovation, including HVAC, only cost like $4-$5k. I'm tempted to just put the laminate down right on the slab and mill some transitions that would blend with the trim to deal with the threshold height.

	Per Board	7	Project Total
7/16 OSB Sheathing	43.95	7	$337
19/32 OSB Sheathing	57.98	7	$444
19/32 OSB	59.57	7	$456
23/32 OSB T&G	63.18	7	$484
11/32 Yellow Pine Pywood	44.53	7	$341
22/32 T&G Pine Plywood	81.58	7	$625
3/4 GC Plywood	79.98	7	$613
1/2" Sumauma Plywood	30.56	7	$234
2" XPS Foam 4x8 Panels	49.41	7	$379
Treated 2x4's, 16" on center			$164
Un-Treated 2x4's, 16" on center			$198
12' Pine	12.72	9	$125
16' Pine	17.52	2	$38
			$164
12' Treated	15.27	9	$150
16' Treated	21.77	2	$48
			$198