Laying plywood and rigid foam floor over concrete

[Snipe Hunter]

If you live in a humid area and the room isn't conditioned, you will have condensation on the floor. I don't know how much a foam insulator will do.

If I were doing this project, I'd build 4 x 8 mock up in the corner of the room. Lay down the moisture barrier or foam or both and cover it with a flat piece of subfloor, maybe with some evenly distributed weight to keep it flat and let it sit a few weeks and look underneath to see if it's wet. If it were wet, I'd probably not do the project.

When I inspect homes my moisture detector picks up high moisture when checking slab floors. Always worse if it's in a basement (but generally tolerable), garage, utility room or any unconditioned area. Slabs in conditioned areas show about the same moisture level as the rest of the structure.

[vernonator]

When we finished our basement we used exterior wall waterproofing material as a water barrier. Think the heavy black dimpled PVC that they put on the outside of the foundation to direct water to the drain tile. We laid that down, put AdvanTech on top (you could do your insulation first) and then flooring on that, it was all then tapcon'd in. We had 10"+ of rain in 3 hours a couple weeks back that overwhelmed my sump (and the battery backup pump above it) and water poured in via the sump pit AND all around the slab/foundation interface. Water came in for almost 2 hours and it ALL ran under the finished floors right into the floor drains (we have 6) and out of the basement. We had ZERO damage, while our neighbors are ripping out dry wall and pulling up carpeting. Well worth the few $100 the material cost.

[srv52761]

... . We had 10"+ of rain in 3 hours a couple weeks back that overwhelmed my sump (and the battery backup pump above it) and water poured in via the sump pit AND all around the slab/foundation interface. Water came in for almost 2 hours and it ALL ran under the finished floors right into the floor drains (we have 6) and out of the basement. We had ZERO damage, while our neighbors are ripping out dry wall and pulling up carpeting. Well worth the few $100 the material cost.

Do you live n Iowa?
I may have been one of those neighbors.
Just bought the house. Has a sump, but the realtor said it had never gone on, never even a drop.
Seven inches in 2 hours, we had 2" of water in the basement when we returned home (only one sump, and like a dummy, we had set a couple of boxes over the only floor drain).

[Snipe Hunter]

Do you live n Iowa?
I may have been one of those neighbors.
Just bought the house. Has a sump, but the realtor said it had never gone on, never even a drop.
Seven inches in 2 hours, we had 2" of water in the basement when we returned home (only one sump, and like a dummy, we had set a couple of boxes over the only floor drain).

Something similar happened here last month. Drain clogged in the exterior basement stairwell and came in through the door. Apparently our sump pump is in the highest spot in the basement. Didn't like the bamboo floor anyway.

[mike4244]

I have a small unseated detached garage that has an interior of approximately 20’ x 11’  The floor is a concrete slab, and I am looking to add an insulated floor and then put up either drywall or 1/2” plywood with rigid foam behind to cover the cinderblock walls.

I have seen some mentions of using powder actuated nails to fasten the 2x4’s to the concrete, and then to place the rigid foam boards between the sleepers. Since I do not have a powder actuated nail gun, I am thinking of skipping that step. My main use of the garage will be for a hand tool wood shop, as well as storage for bikes, too many surfboards, and even more fishing rods. The heaviest machine that I have is a 14” bandsaw.

Here is my plan, so please let me know if you see any issue with this based upon my intended usage.   I am planning on laying pressure treated 2x4’s flat, so that the height before plywood is 1 1/2’. I will lay them out on 24” centers with rigid foam in between. No fasteners will be used at this point. I will then lay 3/4” sheets of plywood over top of the foam / 2x4’s, and then screw the plywood into the sleepers. This should hold everything together nicely, and basically give me a floating floor. I will leave about a 1/2” gap Betweeen my floor and the wall to allow for expansion.  

Do any of you see any issues so far?

Lastly, what is the best way to adhere the wall joists to the cinderblock that current makes up the walls? My thinking here is also 2x4”s laid on the flat side against the walls with more rigid foam between on 24” centers. I will not be hanging anything too heavy on the walls, so i think 24” centers with the joists being only 1 1/2” deep will suffice.

Thank you for any and all comments.

First , lay down 30 lb felt. Adhere to concrete with a dot of subfloor adhesive 2'-0" centers or so. Lap each sheet 2". White line on the paper is 2". This is a vapor barrier.
Next rip treated 2x4's in half, these are called sleepers. Rent ,borrow or buy a roto hammer, not a hammer drill. Snap lines for your sleepers and place one on your layout.
Stand on the sleeper so it won't move and drill thru the wood and into the concrete 5" deep with a 3/16" drill made for the roto hammer . Remove drill from hole and hammer in a 16d galvanized nail. If the nail goes in too easily,then add a piece of tie wire 4" or so long in the hole before you hammer the nail. The depth of the hole is only critical to the point that it has to be deeper than the nail. The hole can be 12" deep and still work, 3" deep and it will not hold because it bottoms out.
Make sure you are on your layout because the nail isn't coming out . The wood will pull thru the nail before the nail comes out. Tap coms are an expensive way to do the same thing. If you find after a lot of holes that the nail is hard to hammer in, the carbide tip is wearing out. Replace the bit and the going will be easy again. 
1-3/4" Robertson head screws will be a good fastener for the plywood. 5/8" touch sanded plywood will eliminate underlayment plywood. You could nail the plywood with 6d nails instead of screws.
If you are familiar with light gauge metal studs, use them instead of wood. One minor thing, I think you confused the term joists with studs. Install 30 lb felt on the walls like you do on the floor. Fasten the floor plate like the sleepers. The top can be nailed or screwed to the joists running 90° to the top plate. fasten to the block with cut nails or the same as the sleepers and plates with galvanized nails. If you roto drill for the brackets, only drill into the solid part of the block. If you go into a cell the nail may not hold.
Save money and rip your 2x4's in half for the studs and plates. If you build the wall on the floor and stand it up, do not make it full height, you have to tilt it up. Usually 1/2" short will do. Then tap a shim between the plate and joist before fastening.
One other thing, the walls are made up of studs, joists carry the floor load. The rigid insulation is fine.

mike