Laying plywood and rigid foam floor over concrete

[Joe Doyle]

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.

[jasfrank]

I would lose every little thing I owned down those half inch gaps along the walls. And then what about moisture getting out from under that floor? I'd think it would want to buckle and be very warped, etc, unless you put a moisture barrier down first.

[EricU]

sorry I didn't read the whole thing.  If you are putting up walls, the way to do this is to put down XPS, then plywood, then the walls on top.  XPS has the compressive strength of concrete and will not deteriorate in water. You can shoot fasteners through the plywood and foam into the concrete.  Putting sleepers next to the concrete doesn't make sense.

 I am contemplating doing this in my basement, but the walls are already up.  I would just leave a gap and cover it with trim

[Stwood_]

Treated lumber is notorious for bowing/twisting on it's on.
I would tie it down to the concrete.
Hammer drill and the right sized bit, and use tapcons, which can be removed, and will not tear up the concrete like a powdered nail gun.

[srv52761]

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?

I have seen floors laid similar with no issues.  
Concrete is like a sponge, only slower and harder, that is why you use pt lumber.  Depending on the type of insulation board you use, the moisture may be an issue, hate to see mold develop or the R-value of the insulation compromised.  If you are not going to glue the sleepers, I would start with a sheet of 6 or 8 mil plastic.    
3.5" of every 24 will not be insulated, that is about 14%.  A thermal break is a money saver
24" centers is a lot for 3/4" ply.  I am assuming you are depending on the compressive properties of the insulation board to offset that.  If that is the case, consider eliminating the sleepers altogether.  Move up a level on the qualty of the Insulation boards.  Lay them, tape the seams and glue the ply on.  Better total r-value, less work, don't have to worry about a wild 2x4 twisting and lifting the floor.  If you go with the good stuff you could eliminate the plastic.

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.

Again, eliminate the 2x4s.  Glue the insulation to the wall, then 1/2" ply over that.  You could secure the top of the ply with a cleat to the ceiling rafters if you are worried about it pulling off with weight of hanging cabinets or such (but it won't) 
Alternately, maybe a 2x3 stud wall with a 1/2" gap between the wall.

Thank you for any and all comments.

Assuming you are insulating as you plan to cool or heat.  If so, the extra cost in insulating now to the best you can is cheap compared to the cost of energy for years to come.  Don't skimp.

Edit: Obviously I type too slowly.  Everything I mentioned was already pointed out by someone else.  Oh, well... Maybe multiple posts saying similar will add some validity to the ideas.

[Phil Thien]

I agree that treated will move.

Plywood will too.

How about cheap chip-based sub over foam without any sleepers at all?

[Snipe Hunter]

I've been using AdvanTech 23/32 t&g subfloor for about 10 years now anywhere I'm concerned about moisture or deflection. It's extremely moisture resistant and moves very little compared to OSB. Also dead flat as opposed to OSB. Has something to do with the resin in it. I've used it under concrete pan showers, kitchens and when I build my shed, it will go on the floor. It holds nails well too. It's rough on saw blades but I don't really care about that. Some Lowes stores carry it. HD doesn't. It's heavy/dense and I've actually seen sparks from it when using an air nailer. I used some scrap for sides of my utility trailer and it lasted about 6 years outside in high humidity, rain, snow etc before it started to break down. OSB t&g wouldn't have lasted 6 weeks. It's a little pricey but I think it's reasonable for what you're getting. I've had more than one tile installer recommend it to me. I think it would serve your purpose well. Something to consider. I wouldn't lay plywood or anything without a sturdy framework or it will warp.

Also, you can get single shot powder actuated nailers under $40.00. I generally use cut-nails because I'm cheap but that's just me. You don't need many if you're gluing your bottom plate to the floor. If the room is insulated, you shouldn't have many (if any) condensation issues. 

AdvanTech

[Joe Doyle]

Guys,

Thank you for the replies.   I want to confirm that this space will not be conditioned in any way, although during the winter I will use one of those portable oil-filled radiators to bring up the ambient temperature in the garage a bit and also use one of those radiant heaters that heats objects instead of the air.  That and lots of warm clothes.

I looked at that AdvanTech stuff just now, and I can get it locally for not a lot of money.  That may be the way to go over top of the rigid foam insulation.  I guess using something to adhere the sleepers to the concrete slab make sense, as I did not take into account any warping of the wood.  Will a construction adhesive be enough to hold the sleepers flat, or do I need to use a mechanical fastener like a nail or the tapcon to make sure that it stays put?

I am planning on a moisture barrier as well, but forgot to put that in my original posting.  I live on an island in southern NJ with the ocean on one side and the bay on the other, so I do need to consider moisture.  Would it make sense to paint the concrete slab with something like epoxy first, or am I good with PT sleepers that are fastened to the ground with adhesive (and possibly nails / screws) and the use 6 mm poly sheeting to cover the sleepers and foam before I lay down the floor surface (Advantech or something similar)?

Thanks again.

[crokett™]

If you use construction adhesive, the wood sleepers will fail before the glue does. the trick is keeping the sleepers flat while the glue sets. Only fasteners will do that. if the sleepers are slightly warped they'll stay that way if you only use glue, unless you use fasteners or some kind of weight to flatten them out.

[Cooler]

Gym floor/horse stall mats.  Very durable, with a good level of insulation.  Easily installed and replaired.

Not the thin stuff you find in the big box stores.  This is 3/4" thick and much denser.

https://www.greatmats.com/products/34-in...-tiles.php