I want to use rigid foam under t and g plywood.
My question is, do I need to apply a plastic vapor barrier on the concrete first? Also, do I need to frame out the floor with pressure treated wood sleepers 16" oc?
I've seen HOH do it this way with pressure treated wood but I was also told I could just lay the t and g on the foam and just screw to the floor.
If you do it without sleepers you need a double layer of plywood. What yo are making is a giant floating subfloor. You can get by with a 3/4" thick layer on the bottom and at least a 1/2" thick layer on top. The top layer does not need to be PT.
The process is to lay out the foam, lay out a layer of plywood, then add a second layer of plywood making sure that none of the seams align. You can screw the two layers together with 1-1/4" screws. With a minimum of subfloor thickness of 1-1/4", the load is spread out enough so that even a concentrated load in one spot will not dent the foam underneath.
The foam can be almost any thickness, if you use a high density condensed foam like blue board or pink board, you'll get about R-5 per inch. It doesn't take very much to make the floor feel warmer as the ground really doesn't get extremely cold under the slab. You also do not need a vapor barrier under the foam, especially if you tape all the foam seams as you lay it down. The foam itself will act as a barrier. A separate barrier will not hurt, but is not needed.
Once you are done with this type subfloor it is so large and so dense that it makes a suitable surface for almost any flooring material except tile. Even with two thick layers of plywood, there will be slight movements to the subfloor - too much for tile.
Fun fact: This type floor is commonly used for dance studios that are build on a slab. Makes for a sturdy floor surface that allows just a little 'bounce' to resist injuries.
Great info! Thanks Jaybee.
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