Can I Install Hardwood Over Unheated Floor?

Wood Movement

• The reason that wood swells in hot or humid weather is that it absorbs moisture into its cells. This causes it to swell quite noticeably across the grain, but very little along the length of the grain. The result is buckling in floors where the planks have been laid too close together. If you install hardwood on a floor in an unheated building during the winter, you will be putting it down when it is at its smallest, thus increasing the risk of buckling and warping during hot weather when it begins to swell.

Heat and Humidity

• Wood swells during hot weather, but it's the humidity that causes it to swell, not the heat. Warm air holds more water in it than cool air, which is what causes humidity. In a heated building, the temperature difference between hot summer weather and cool winter weather is noticeable, but nowhere near as extreme as in an unheated building. The result is a greater risk of flooring damage through seasonal changes in an unheated building. Installing hardwood over a floor that doesn't have in-floor heating but is in a heated building shouldn't present any particular problems.

Spacing

• If you are laying hardwood in circumstances where you are worried about it swelling, shrinking, buckling or warping, the best way to defend against this is to give the wood somewhere to move to. In interlocking, floating floor systems, this is done by leaving a space between the edges of the flooring and the walls. This space is covered by the baseboards, but it still allows the flooring to swell and shrink without buckling. When using solid hardwood planks, you can leave a small space between each board. This will create a somewhat rustic look and leave spaces for dirt to get into, but it will greatly reduce the risk of damage to the flooring.

Drying

• Most commercially available wood has been kiln dried, after which it usually has a moisture content of around 6 or 7 percent. If you buy wood from a mill or lumber yard in your local area, be sure to ask if it has been kiln or air dried. Wood that has been air dried should be left for at least a year before it is used. As a rule, wood that is kiln dried is more appropriate for finish uses, such as hardwood flooring, and is less likely to present problems when put down on an unheated floor.