January 27, 2025
Interesting facts
How humid can the air be in a woodworking shop?
Let's start with an example. You have a workshop and want to build a workbench. You know that the workshop space must not be too humid over time, otherwise the wood will swell and the workbench may warp.
The workbench is made of wood that has been dried to a residual moisture content of 8%. The workshop has an average temperature of 15° C with an average humidity of 70%. Will that work?
The workbench will not warp only if it has the ability to maintain the existing wood moisture content. If the air is very humid, the wood of the workbench absorbs moisture and the moisture content of the wood increases. With the higher water content, the volume of the wood increases. Conversely, the volume shrinks due to further water loss to the air in extremely dry air. The wood thus adjusts its moisture content to the respective climate, this is called equilibrium moisture content.
The equilibrium moisture content depends essentially on two factors: relative humidity and temperature. There are tables today that are the result of research. According to these, the equilibrium moisture content at a temperature of about 15° C and a relative humidity of 70% is about 13%, as you can see from the table below. Therefore, these conditions are not suitable for setting up such a workbench. In such a case, it is better to use wood that has been dried to only 12 or 13%, or to store the wood in the new environment before processing until it reaches this higher moisture - if you cannot change the environmental conditions.
This can change with the alteration of a value. If I want to set up the aforementioned workbench with an equilibrium moisture content of 8% at an average temperature of 15° C, the humidity must not be higher than 45%, at 20° the value is almost the same. However, since the air at 20° can hold more moisture than at 15°, the humidity in the mentioned room will be lower. However, other factors also play a role, the discussion of which would go too far at this point (e.g., basement location, type of insulation and building material).
If possible, a workshop project should be planned long-term and sufficient measurements taken and documented at different temperatures during different seasons.
The example presented here can also be applied to furniture in living spaces.
The abbreviated table presented here is based on:
Forest Products Laboratory. 1999. Wood handbook - Wood as an engineering material.
Publisher: U.S. Department of Agriculture. Forest Service
How much a piece of furniture or a workbench warps also depends on the type of wood and the construction. Therefore, the values given should only be taken as guidelines and local conditions should be considered.
Essentially, a rule of thumb can be applied (+ - 2 or 3%):
Furniture in enclosed spaces with central heating: Equilibrium moisture content 9%
Furniture in enclosed spaces without central heating: Equilibrium moisture content 12%
Wood constructions outdoors, protected from the weather: Equilibrium moisture content 15%
Wood constructions outdoors, exposed to the weather: Equilibrium moisture content 18%
The values apply to Central Europe and may vary regionally.
All information without guarantee!
The workbench is made of wood that has been dried to a residual moisture content of 8%. The workshop has an average temperature of 15° C with an average humidity of 70%. Will that work?
The workbench will not warp only if it has the ability to maintain the existing wood moisture content. If the air is very humid, the wood of the workbench absorbs moisture and the moisture content of the wood increases. With the higher water content, the volume of the wood increases. Conversely, the volume shrinks due to further water loss to the air in extremely dry air. The wood thus adjusts its moisture content to the respective climate, this is called equilibrium moisture content.
The equilibrium moisture content depends essentially on two factors: relative humidity and temperature. There are tables today that are the result of research. According to these, the equilibrium moisture content at a temperature of about 15° C and a relative humidity of 70% is about 13%, as you can see from the table below. Therefore, these conditions are not suitable for setting up such a workbench. In such a case, it is better to use wood that has been dried to only 12 or 13%, or to store the wood in the new environment before processing until it reaches this higher moisture - if you cannot change the environmental conditions.
This can change with the alteration of a value. If I want to set up the aforementioned workbench with an equilibrium moisture content of 8% at an average temperature of 15° C, the humidity must not be higher than 45%, at 20° the value is almost the same. However, since the air at 20° can hold more moisture than at 15°, the humidity in the mentioned room will be lower. However, other factors also play a role, the discussion of which would go too far at this point (e.g., basement location, type of insulation and building material).
If possible, a workshop project should be planned long-term and sufficient measurements taken and documented at different temperatures during different seasons.
The example presented here can also be applied to furniture in living spaces.
The abbreviated table presented here is based on:
Forest Products Laboratory. 1999. Wood handbook - Wood as an engineering material.
Publisher: U.S. Department of Agriculture. Forest Service
How much a piece of furniture or a workbench warps also depends on the type of wood and the construction. Therefore, the values given should only be taken as guidelines and local conditions should be considered.
Essentially, a rule of thumb can be applied (+ - 2 or 3%):
Furniture in enclosed spaces with central heating: Equilibrium moisture content 9%
Furniture in enclosed spaces without central heating: Equilibrium moisture content 12%
Wood constructions outdoors, protected from the weather: Equilibrium moisture content 15%
Wood constructions outdoors, exposed to the weather: Equilibrium moisture content 18%
The values apply to Central Europe and may vary regionally.
All information without guarantee!