Balsa

3-8 usd/kg
Circularity potential
Very high
Strength
Medium
Production energy
High
Stiffness
Low
Embodied CO2
Ultra low
Density
Ultra low

Balsa is a tropical hardwood, and one of the fastest growing woods due to its low density. There is a large variation in density, from around 50 kg/m3 up to 380 kg/m3. This is due to the growth structure of the cells that act as the main load-bearing elements in the tree. These cells are filled with water when the tree is alive, and become a thin-walled cellular structure when the timber is dry – this is computer simulated as honeycomb. As a result, the mechanical properties are very dependent on density: as the density increases, so does strength, stiffness and hardness. It is possible to extrapolate the properties of balsa from the cell wall strength and stiffness, 120 MPa and 35-40 GPa, respectively, multiplied by relative density. Cell wall density is c. 1,470 kg/m3, thus 100 kg/m3 balsa has a relative density of 0.068. This is equal to strength and stiffness of c. 8.7 MPa and 2.38-2.72, respectively.

The relatively high moisture content of balsa means that it needs to kiln dry for longer, which increases its carbon footprint compared to other hardwoods. However, it is still preferable to plastic in terms of energy, carbon and circularity.

Balsa is not traded as a commercial lumber, like other hardwoods. However, it is consumed in sufficient quantities for plantations to exist. The majority now comes from Ecuador, and it is available from certified sources. It is very soft compared to most other woods, prone to insect attack and not very durable at all. It wasn’t until glass fibre reinforced epoxy (EP-GF) became available to surfboard makers and boatbuilders that balsa could be used in applications like this – it has to be sealed in completely to avoid and moisture getting to the wood.

As with all woods, the angle it is cut from the trunk will affect its physical properties.
– A-grain is plain-sawn. The slice is made parallel to the axis of the tree so the grain runs in long arcs and swirls. It is flexible across its width and prone to warp and cup. This type is ideal for skinning model aircraft, such as around the fuselage or wings.
– B-grain is randomly sliced.
– C-grain is quarter-sawn. The cuts radiate out from the centre of the log and so the growth rings are aligned lengthwise in straight lines. It will be stiffer and is suited to lightweight structural parts in models aircraft, such as fuselage, stringers and spars.
– End-grain is used as a core material, such as in surfboards, boat hulls, boards, skis and wind turbine blades. The blocks are mounted onto a flexible membrane, which allows the core to conform to simple geometries. It has impressive mechanical properties for its relatively low density, superior to most manufactured materials.



Balsa wood with a density of less than 95 kg/m3 is considered light and the most popular. Less than 85 kg/m3 is considered ultra light and “competition” or “contest” class used in performance sports products and model aircraft.

Balsa used as a core material for composite structures is typically 100-300 kg/m3 – medium to high density. It is utilised in composite wind turbine blades, lightweight aircraft (most notably the de Havilland DH.98 Mosquito) and sports products (table tennis bats). Lightweight balsa makes an ideal core material for things that float, like surfboards and boat hulls.

It is compatible with all regular composite production techniques include laminating (such as to make plywood or sandwich panels), adhesive bonding, compression moulding, contact moulding (hand or spray), pre-preg processing, resin transfer moulding (RTM) and vacuum infusion. When used in conjunction with glass fibre reinforced phenolic resin skins, it is capable of passing aircraft and rail smoke and fire tests.

It is very easy to work and will not dull blades, making it a useful model making material, such as for planes, bridges and architecture. It does not have good nail or screw holding properties, and so is typically glued.


Design properties
Cost usd/kg
5-8
Embodied energy MJ/kg
10-20
Carbon footprint kgCO2e/kg
1.5
Density kg/m3
50-100
Tensile modulus GPa
1.4-2.4
Tensile strength MPa
4-8.7
Modulus of rupture MPa
5-10
Shear modulus GPa
0.14-0.19
Compressive strength MPa
2.5-7.5
Hardness Mohs
1
Janka hardness kN
0.1-0.2
Poissons ratio
0.23-0.66
Thermal conductivity W/mK
0.48-0.66
Temperature min-max °C
-200 to 150
Thermal
insulator
Electrical
insulator