Floor and Subfloor Installation Over Concrete Q&A

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Dear NH,

What is the best way to install a plywood subfloor over concrete slab? Regular or treated plywood? Glued or nailed? Plastic for vapor barrier between plywood and slab?

LM from Hot Springs, AR

I don't think it is a good idea to install raw plywood directly over a slab if avoidable. It is too hard to attach securely and does not allow you to install insulation or a vapor barrier underneath.

Instead, you should install strips of wood on the slab as nailers, 2x4's. You can use them laid on their side to give less of a rise, or even use 1x2's. These nailers should be spaced no more than 16" apart as with a standard floor. They should be nailed and glued with construction adhesive to be sure they will not loosen later.

You can install a foam board as a weatherstrip between the nailers. A vapor barrier is also advisable. A heavy construction grade plastic tarp can be laid on the floor before the nailers are installed.

The most important consideration is whether or not you get water in this area! You also didn't mention whether or not your slab is below grade, such as in a basement. If there is a history of water problems, I advise you to either eliminate the water problem completely before considering any renovation like this, or instead go with carpet placed directly on the slab (or with padding). Water under a raised floor can be a real problem... virtually impossible to dry out and a real mildew magnet!

Dear NH,

We have recently converted 1/2 of our garage into an extra bedroom. We live in Massachusetts where we have a cold winter climate. The garage floor is a concrete slab on grade. Needless to say our heating bill for the winter was very high. The walls and roof have been insulated with fiberglass batt insulation and we are in the process of insulating the floor.

We are planning to lay 2x4's on their sides, 24 inches on center. Then we are laying 2 ft x 8 ft x 2 inch thick rigid insulation sheets between the 2x4's. Then we are applying a 6 mil polyethylene vapor barrier over the entire area which will be covered by 3/4 inch plywood underlayment, a carpet pad and carpeting. We are also considering topping off the rigid insulation (about 1-1/2 inches) with loose fill insulation as normally used in attics. Is this a good method of reducing heat loss through the floor? Should the vapor barrier be above the insulation or below on the concrete slab? Any other suggestions?

BS from Tewksbury, MA


Sounds like a good plan! I do have a couple of suggestions. First, there is no reason to use 2x4's. 2x3's are plenty adequate. Some folks even use 1x2's, though I like the increased rigidity of the 2x3's and the extra space for insulation, You should nail them to the floor (every three or four feet is plenty) with a power nailer so everything stays put! This will insure that the floor will not do anything surprising over time. Oh... be sure that the lumber you use is straight and has been acclimated to room temperature for at least a few days or longer to minimize warping and shrinkage. Don't under any circumstances use wet wood... you'll be sorry!

Though 24" on center is acceptable from a load standpoint… obviously the 2x3's will not sag since they are on the slab… using a 16" on center installation will give you a more rigid floor surface.

The vapor barrier should be right on the concrete. If you sandwich wood between the vapor barrier and the concrete, it will absorb moisture and eventually begin to rot… big time trouble!! It is better that the wood be allowed to breathe and reach a normal room moisture level.

As far as laying extra loose insulation upon the rigid insulation, my question to you is... why?? Once the oak floor is installed and the slab is essentially sealed under the floor, it is no longer cooled by the outside. The extra insulation will only give mice a great nesting place. If you feel the urge to add more insulation anyway, use additional rigid insulation instead.

As an aside, you should check with your building inspector to see if you can legally run electrical wiring under the floor. If you don't have another convenient way to run the wires, this might be a helpful timesaver.