Let's start with an example. You have a woodshop and would like to build a solid wood workbench. You know that the shop should not be too damp for long periods of time, because the wood will swell and the workbench could warp.
The bench will be built out of wood that has been kiln-dried to about 8% moisture content. The workshop has an average temperature of 15° C, with an average humidity of 70%. Will it work?
The workbench will remain stable only if it can maintain more or less the same moisture levels. If the air is too damp, the wood will begin to soak up some of the humidity. As the level of moisture in the wood rises, the volume of the wood also increases, and the wood starts to work against its fastenings and in the joints. The same problem also works in reverse. When the air is very dry, the bench will give up some moisture to the air, and start to shrink. The wood in general adapts itself to the ambient humidity level and finds its equilibrium moisture content.
This equilibrium depends on two factors: relative humidity and the temperature. Today there are tables to predict moisture levels in wood given different temperatures and humidity levels. So for instance, looking at the table below, the equilibrium moisture level in wood at 15° C and relative humidity of 70% is about 13%. So a work bench made of wood dried to 8% will not remain stable under those conditions. In that case, it would be better to use wood that had only been dried down to about 13%, or to leave the wood in the room long enough for it to reach a new equilibrium moisture content there. Of course since wood stored in the workshop will also take up moisture, moving, say, a table from a relatively humid workshop to a heated dining room can cause similar problems in reverse. So it would be better, if possible, to see if the humidity in the shop can be reduced.
To keep the water content in the kiln-dried wood in the workbench at a constant 8%, at 15° C the humidity cannot be higher than 45%. At 20° the humidity level needs to be just about the same. Because the air at 20° can pick up more humidity than at 15°, the humidity in the work room will be lower. There are a lot of other factors that also come into play of course - if the room is in a cellar, insulation, building materials, etc. -that are too complicated to go into here.
When possible, it is a good idea to take one's time planning a woodshop in order to measure and document the temperature and relative humidity of the room during the different seasons.
This example can also be adapted to housing space.
This table can also be found, along with a lot of other good information, in Forest Products Laboratory. 1999. Wood handbook - Wood as an Engineering Material. Editor: U.S. Department of Agricuture. Forest Service
|Temperature||Equilibrium moisture content at various levels of relative humidity|
|°C||25 %||30 %||35 %||40 %||45 %||50 %||55 %||60 %||65 %||70 %||75 %||80 %||85 %||90 %||95 %|
How much a piece of furniture or a work bench warps or moves naturally also depends on the kind of wood used, and the way it is constructed. So these figures should be used only as a general guide and conditions in various areas and climates can be different.
As a rule of thumb, one can say (+ - 2 oder 3 %):
Furniture in a closed room with central heating: equilibrium moisture 9%
Furniture in closed room without central heating: 12%
Woodwork in an open area, protected from the weather: 15%
Woodwork in an open area, unprotected: 18%
These are values for Central Europe, and other climates will naturally be different.
These figures are a guide only, without guarantee.