I am looking for information about an on-grade plywood floor method.  Its use would be in lieu of a slab-on-grade concrete floor, floating within a concrete grade beam.  As I understand it, the material sequence might be, from the top down:

finish flooring
plywood subfloor
plastic vapor barrier
XPS foam (amount climate dependent)
coarse gravel for leveling, moisture drainage, and radon collection
undisturbed subsoil

My questions are whether there would be any issues with ground moisture adversely affecting the plywood, and whether typical loadings on the plywood floor would compress the foam?  "Blueboard" Styrofoam has a compressive strength of 25psi, or 3600 lbs/sq ft, which is as much as some soils.  But Styrofoam will compress up to 5% of full loading, so Dow suggests a max loading of 1/3 of maximum rated compression.  What thickness would the plywood need to be to insure that any loads (walls, bookcases, etc) were distributed sufficiently to not overload the foam?

Comments from anyone who has tried such a system, with good or bad results, or any informative web links, would be appreciated.

I've installed plywood (Advantech) over XPS foam on a second floor where I could screw it through the foam to the old subfloor. I'm trying to wrap my head around the idea of installing the plywood over plastic where you can't fasten it to anything. What holds it in place? Any plywood will warp over time; Advantech less than most, but leaving it loose means that it will not lay flat. At least lay down 1x3 strapping or 2x4's so the floor can be fastened together to act as one unit. Leaving aside questions of moisture, 3/4" T&G Advantech would work in your situation.

I'd suggest installing the poly under the XPS, overlapping it at joints, taping the seams and run it up the wall and fasten it to the walls. In other words, like you would in a crawl space - that wil give you the best moisture barrier, then install your XPS, 2x4s and plywood. Keep in mind that any settling of the underlying fill, and any corresponding movement of the XPS will transfer to your floor so you may end up with some movement when you walk on it.

HouseListener,

Where do you plan to build this floor on grade?  Is this ok with the code officials?  Are termites a problem in your area?

The ground moisture issues are pretty much taken care of by the XPS (assuming an inch or more), but I'm with Bob- rigidity-wise I don't see this as working.

The poly definitely goes under the XPS, but if you're doing multiple layers of XPS of an inch or more you can skip the poly (except as required by code for radon control, etc.)

See http://www.polycorecanada.com/produ...ion-floors to see one way to do what you're talking about.  I talked to the company about it.  It is a couple of dollars more per foot than just concrete.

Bob, Thanks for mentioning Advantech. I'd not heard of it. It does look like a good product in my proposed application. You've asked a good question about what holds the plywood in place. My thoughts were that the plywood would fit fairly snugly between the foundation or walls, and thus not be able to slide sideways. Further, any internal walls would tie the plywood to the outer walls. The T&G of the plywood would help prevent curling in the central portion of the floor, but maybe not so much along the edges. I'm thinking that making the subfloor of two layers of plywood, laid at 90 degrees, would be a good idea. The thicker floor would minimize the issues of fill settling, as well as distributing high point loads of furniture, etc.

Alton, I'm building in VT. As far as I know there are no termites here, yet, though I'm keeping on high-alert for signs of global warming In the rural areas of Vermont there are no enforced building codes, except for State approval of well location and septic system design.

Dana, if the poly goes over a gravel base and under the XPS, and if part of the purpose of the poly is to control radon gas, is there a problem with the poly being punctured by the gravel? Why not put the poly above the XPS, or maybe even between layers of foam?

TB, the PolyCore floors look interesting. Being a Canadian product, is it available in the US, and at what cost? Their horizontal "studs" would solve the problem of point loads compressing the foam, and would prevent the plywood from warping. I had trouble finding a good description of a cross-section of the floor system. Do I understand correctly that there is perhaps 1" of foam under the studs, and then foam between the studs? What keeps the foam under the studs from compressing? I see there are a number of grades of EPS, but haven't figured out which grade PolyCore uses.

Thanks, all, for your great comments.

The standard sleeper floor panel is 4' x 11'. It consists of 4" thick foam. The studs are 1' on center starting 6" from the side. The studs can be flush with the foam or raised 3/4". The raised studs would allow radiant tubing installed. They quoted me about $5 per square foot for the products several months ago.

One concern that I would have is that the plywood or Advantech edges should be free to float.  If not, I am afraid that the floor will rise up when the moisture level increases in the warm months.

Would it be a good idea to place sand on top of the gravel to protect the vapor barrier?

Have you compared the cost of this type of floor with a concrete slab?  In my area, concrete suitable for a floor is down to $80 a cubic yard.

By the way, you might get a better price for the XPS board from DiversiFoam instead of Dow Chemical Styrofoam Blueboard (XPS) retail.

Alton,

As I imagine this plywood/foam system I think the edges of the plywood would be able to float -- in fact I haven't come up with an elegant way to tie them to the foundation or exterior walls, other than through the partition walls. But perhaps it is not needed to tie them to the exterior walls?

I'm not sure I want to put sand on top of the gravel. I understand that the sand can actually wick water up, which is what I'm trying to prevent with the gravel.

Around here concrete is $108 per cubic yard, except during the winter when there is a surcharge of $6/c.y. for heating the water to 180*F to keep the concrete from freezing during transit.

I called Polycore Canada to find out more about their foam/sleeper/plywood floor-on-soil system. Here's what I found out:

4" EPS foam, Type 1, 11psi
Studs (their word, I would call them sleepers) are 2-1/2" high
Studs can either be flush with upper surface, or proud of upper surface for PEX tubing heating
Studs can be slid out of foam so that foam and studs can be trimmed to length
Studs 12" OC
3/4" T&G plywood glued and screwed to top of studs
Floor system designed to be used in conjunction with their basement wall system. Floor studs take inwards compression of walls from backfilling. Floor system can be used independently of their wall system.
Edge Panels are 3-1/2' x 4'
Filler Panels are 4' x 11'
Typically floor assembly: subsoil, compacted and leveled sand, vapor barrier, PolyCore floor, plywood subfloor, finished floor.
For shipping panels are placed on pallets. An 1,100 sq ft floor will fit on two pallets, 4' x 11' x 7'
Flush panels cost $5.51/sq ft, Proud Panels cost $6.01/sq ft., FOB Edmonton, Alberta, Canada
Buyer is responsible for arraigning shipping.

http://www.polycorecanada.com/products/residential-foundation-floors

Just curious - what is your goal? For the cost of the Polycore system and plywood you could pay for the necessary XPS and concrete and have money left over, so I'm curious what you see as the benefit?

Bob, I don't think the PolyCore system will be cost effective for me. I posted the above information to show that there is at least one commercial company who is selling a plywood/foam floor system similar to what I initially proposed, and to provide anyone else who stumbles into this thread with a synopsis of the PolyCore floor.

I am shying away from a concrete floor because it is harder and colder than a wood floor, and is more difficult to install as a one-man DIY project. My latest idea is to lay down 4" of XPS foam, 3/4" plywood, 2x sleepers 12"OC, 3/4" plywood, all glued and screwed together. That should make a very stiff floor panel that could easily distribute point loads to the foam. It would also create a plenum to run wiring, water lines, radiant heat tubing, and maybe even forced air for heat, or perhaps just for ventilation.

I agree that concrete is difficult to install as a DIY and that it is hard. the "cold"ness is fixable, however by (first) decoupling it from the ground and the foundation and certainly by installing radiant heat. Your assembly sounds like it will work and it addresses the concerns that I and others have expressed. I'd be nervous about trying it in someone else's home, but since it is your home, go for it & good luck.