Wood Is Not One Material

Saying that something is made from wood is not enough information for a technical calculation.

Different species have different density, shrinkage, thermal properties, strength characteristics and moisture behaviour. Even within one species, individual pieces vary.

That variability is normal. The important part is knowing which property a calculation depends on and using a value appropriate to the material and conditions being considered.

Moisture Is Part of the Number

Wood changes as it gains and loses moisture.

Its dimensions can change. Its mass changes. Several physical properties used in engineering and thermal calculations also depend on moisture condition.

A material value without its moisture condition or reference basis can therefore be misleading.

When moisture matters to a calculation, it belongs beside the number.

Density Connects Several Calculations

Density is more than a number describing whether a species is heavy or light.

Together with geometry, density lets us estimate the mass of a log, wall or other component. That mass affects handling and transportation, and it is one of the inputs required for thermal-mass calculations.

The density value used therefore needs the same discipline as every other material property: species, condition and source matter.

Shrinkage Has Direction

Wood does not necessarily change dimension by the same amount in every direction as it dries.

For natural-log construction, dimensional change matters because the building is assembled from large pieces of timber whose moisture content can continue changing after machining and construction.

When we use shrinkage data, the direction being described and the moisture range behind the value need to be clear.

Use Sources, Not Folklore

Wood construction has accumulated centuries of practical knowledge. That experience is valuable.

But when we publish a number, we want to know where it came from.

Material properties used in calculations should come from recognized references, published test data or measurements for which we can describe the method and conditions.

Where different sources provide different values, we should not quietly choose whichever one makes the result look best. The source and the reason for selecting a value belong with the calculation.

The Data Table

As measured and referenced data is assembled, this page will become the common material-data reference used by the other technical sections.

For each relevant species, the useful data may include:

• density and reference moisture condition
• specific heat capacity
• thermal conductivity or thermal resistance data
• radial and tangential shrinkage
• longitudinal shrinkage where relevant
• typical moisture conditions
• dimensions and mass of representative logs
• source or measurement method

A table full of numbers is easy to make. A table full of numbers that can be traced back to their meaning is more useful.