[Greenbuilding] sealing plywood floor?

[John Straube]

The idea attributed to John Salmen is perfectly sensible from a building science perspective and has been done by numerous people in numerous parts of the world.
By placing foam insulation between the plywood and the earth, the plywood is isolated from the earth thermally and from a moisture point of view. It acts as if it in the room above.
As the temperature of the plywood is close to the interior, it will see no more humidity variations than if it were placed on the first floor.
Wood warps when it expands differentially (top side relative to bottom side) and otherwise just expands laterally with changing RH.  Lateral changes can be managed by providing gaps between plywood and rigid objects (like basement walls).  Warping is controlled by limiting the rate at which moisture enters/leaves the plywood. Fast entry results in gradients and warping.  One reason we finish furniture and floors with vapor semipermeable finishes is to reduce this rate.
If one is placing such a floor in the basement rather than at grade in garage/workshop, the interior humidity will be higher because of the moisture that might be coming in from unfinished walls.  By using foam insulation you HAVE a vapor retarder on the floor.  In most climates, if the earth is kept cool (by the insulation) water vapor will rarely if ever move from it to the inside, even if the earth is wet (which it often is).   This can be shown on a psychrometric chart.  Hence even if the foam was not quite vapor resistant, a vapor barrier would not be needed.
If you have no insulation under the slab then yes, the slab is at the temperature of the earth, hence often colder than the air above, hence we have damp floors, wet cardboard boxes and musty smells in basements, only solveable by installing a dehumidifier.  Adding insulation below the slab (or plywood) makes all of that go away by warming the slab to room temperature.
If such an insulated floor where to be used in a basement, then the walls would also need to be treated via insulation and vapor control, to avoid moisture problems.


Dr John Straube, P.Eng.
www.BuildingScience.com

On 12-04-03 4:22 PM, Gennaro Brooks-Church - Eco Brooklyn wrote:
> The one thing I would have added to the plan was a good vapor barrier.
> If the vapor barrier can't handle the humidity then you have a larger
> problem at hand that should be dealt with before installing the floor.
> Apart from that it seems like a viable idea. I like the idea of not
> using cement, especially since I am able to salvage all the materials
> (sand, insulation, plywood).
>
> Gennaro Brooks-Church
> Director, Eco Brooklyn Inc.
> Cell: 1 347 244 3016 USA
> www.EcoBrooklyn.com
> 22 2nd St; Brooklyn, NY 11231