Changes in relative humidity caused by variations in temperature, in the interior of a closed box containing wood, are studied here from the practical and theoretical points of view. In the theoretical section two solutions are given, one graphical and the other algebraic. Both depend on the fact that as long as no moisture enters or leaves the box the humidity given off by the wood must be absorbed by the air and vice versa. The experiments on different amounts of wood were carried out in an aluminium box with a Perspex front, placed in a cabinet of constant humidity (50%) with the temperature oscillating between 35 and 15°C. The relative humidity inside the box was registered electrically. From the results one can deduce the following: the change in relative humidity will not exceed about one-third of the temperature change (°C) and will be in the same direction (a rise in temperature causes a rise in relative humidity and vice versa) provided that there is no entry of outside air at a different relative humidity into the case.