The insulating properties of wood, best exemplified by log houses

Wood has an interesting behavior when in contact with a heat source. If you put an open flame under a pile of dry wood, it will ignite and burn, giving off heat. If, however, we stir with a wooden spoon in a pot boiling or frying something in hot oil, the spoon's tail will not heat up leaving us to enjoy our cooking. Both manifestations are related to the thermal properties of wood. These properties are responsible for wood's ability to be a good natural thermal insulator, but also for its use as an energy source, which has become increasingly popular recently.

The thermal properties of wood

The thermal properties of wood are heat value, thermal conductivity, specific heat and thermal diffusivity.

Calorific value is the amount of heat given off by 1 kg of wood during burning. Wood is a water-containing material, and its calorific value depends on its water content, the drier the wood. This is because some of the heat released is used to evaporate the water inside. It is anhydrous, completely dry wood that gives the maximum calorific value. However, the actual value is much lower because wood intended for burning has a moisture content that is sometimes higher than 12-14%. Read more about the calorific value of different types of wood here.

Thermal conductivity is the heat conducting property of wood. It is expressed by the coefficient of thermal conductivity λ (lambda) and represents the amount of heat that passes through a flat wall with a surface area of 1 m² and a thickness of 1 m for one hour, at a temperature difference between the two sides of the wall of 1°C. In short, the coefficient λ tells us how good an insulator wood is. Its value depends on the species, density, orientation with respect to the central axis, humidity. How it varies according to these elements, we see immediately.

Specific heat is the amount of heat required to raise the temperature of 1 kg of wood by 1°C. It increases with increasing temperature and humidity. The specific heat of wood is much higher than that of metals, i.e. for the same amount of heat received, the temperature of wood rises much less than that of metal. That's why we have no problems when we use a wooden spoon to stir in food, whereas we have to abandon the metal spoon quite quickly because it gets hot.

Thermal diffusivity is the ability of the material to increase its temperature at a certain rate under the action of temperature. The coefficient of thermal diffusivity is basically the ratio of its ability to conduct heat to its ability to store it. In wood this ratio is much lower than in metal, which explains its insulating properties. The thermal diffusivity of wood depends on humidity, temperature and direction in relation to the central axis. The coefficient of thermal diffusivity is important in the drying process of wood, in hot-gluing, steaming and pressing.

The more porous and less dense it is, the better thermal insulator it is

Wood is a natural thermal insulator due to its cellular structure. The voids inside are responsible for this property because they do not contribute to heat transfer from one part to another. Therefore, the more porous, less dense wood is, the better insulator it is. Its insulating properties make it a good building material, with lower energy consumption where it is used. For example, the thermal conductivity coefficient of construction timber is 0.13 W/mK, while for reinforced concrete and masonry it is 2.30 W/mK.

Insulation capabilities do not only depend on wood density but also by species, moisture content, direction relative to the central axis and temperature. The higher the water content, the higher the conductivity and the less insulating the wood. This is because water fills the voids, thus aiding heat transfer. In the longitudinal direction, the conductivity of wood is 2-3 times higher than in the radial and tangential directions. Being along the grain, heat is conducted more easily, wood being less insulating in this direction. Conductivity also increases with increasing temperature, when expansion in the radial and tangential direction is greater. Due to this expansion, the amount of heat transferred in these directions is greater, which leads to a decrease in insulating properties.

As I said, the insulating capacities of wood also depend on the species. Different densities and cell structure make some species more suitable than others for flooring, cladding or even house construction. By studying these properties, we find that Canadian poplar (0.090 W/mK) and fir (0.110 W/mK) have much better insulating properties than oak (0.197 W/mK) or plum (0.179 W/mK). Find out more here a very interesting study on the thermal insulation properties of more than 30 wood species.

Why log houses are the best example of wood's insulating properties

The most eloquent examples of the insulating properties of wood are houses in bușteni. They are constructed entirely of unprocessed resinous trunks, with no other insulation layers. The only insulating element used is between the logs, and it contributes in particular to a better connection between the logs. If it is a well-made house and the wood has been dried beforehand so that it has not been worked too hard and there are no cracks between the logs, the house keeps warm in winter and cool in summer, and you can feel this as soon as you enter the house.

Insulating your home is provided only by resinous logs which, due to their low density, have a lower heat transfer coefficient. The heat storage capacity is high so the wood stores heat outside during the day and releases it inside at night. If the wood has been left to dry before construction and the house has been left to settle after construction, the moisture in the wood will be low and heat transfer will be slow.

Although wood is a good insulating material, it is no more effective than insulation materials

With the exception of log houses, where the thickness of the log is sufficient to provide the necessary thermal insulation, at wooden frame houses insulation material is needed. The insulating properties of wood are not sufficient and additional layer is needed to achieve an energy efficient house. The advantage of these houses is that much of the insulation is put into the walls, resulting in a well-insulated house with a larger floor area.

The insulating properties of wood are also used to make highly effective insulation materials. Wood fiber mattresses and panels not only have very good insulating properties (0.038 W/mK), the advantage of being natural brings obvious additional benefits.

I hope you find the information below interesting and useful. As always, additions are welcome. And if you have any questions or queries, please leave them below in the space provided. I will certainly reply.