The effects of wooden surfaces as heat stabiliser

New studies indicate that indoor wooden surfaces can act as heat equalisers. The characteristic is based on heat released during a phase transition in humidity. The phenomenon can be used to save energy. We have compiled a review of studies related to this issue.

The interaction between wooden surfaces and indoor air has awakened growing interest. As a hygroscopic material, wood absorbs and releases moisture in an effort to achieve a moisture balance with the environment. Hygroscopic materials can reduce variations in peaks in the relative humidity of indoor air by using moisture buffering, which lowers the load on ventilation in the building. Decreasing the use of mechanical ventilation saves energy. The moisture buffering capability of wood is three times larger than concrete and brick, and twice as large as plaster.

Moisture buffering refers to a porous material’s ability to stabilise changes in relative humidity by absorbing and releasing water vapour from the surrounding air. The relative humidity of indoor air varies significantly during the day and during different times of the year. Materials that can store and release moisture decrease the extremes in variations in the relative humidity of indoor air.

Knowledge about moisture-decreasing effect is important, as it indicates that moisture produced in a space is not carried directly into ventilation systems but is partially bound by the hygroscopic material. The lower variation in moisture ensures a better micro-climate in the indoor space, where air is not too humid or too dry. In a study in which the indoor surfaces of a wooden block of flats were porous and breathable, the humidity percentage of the indoor air was lower compared to a site where the indoor surfaces were painted with paint that formed an impermeable film. The relative humidity of the air also dropped more quickly at the site that had permeable indoor surfaces.

The potential of wood as a heat equaliser is based on latent heat that is formed when moisture in the air is bound to a wooden structure. The change in temperature that this causes can be used to improve temperature comfort, lowering the load on ventilation systems and energy consumption. The temperature of the surface rises when moisture is absorbed by wood, and decreases when moistureevaporates from the wood. The wooden surface can then act as a natural heating panel or cooling element.

The studies indicated that indirect energy savings can be about five per cent for heating and about five to twenty per cent for cooling. Energy is saved when the effects of wood as a heat equaliser are taken into consideration when regulating heat and ventilation without compromising the good quality and comfort of the indoor air.

The Norwegian project Wood – Energy, Emissions, Experience (WEEE) studied the effect of indoor wooden surfaces on people’s well-being by using objective and subjective methods. One result of the overall study was that the action of wooden panels as a “heat radiator” could decrease the total consumption of energy and also improve the comfort of flats when moisture changes can be set according to the degree of use of the indoor space or the time of day.

The second part of the study examined the possibility of using wooden cladding to heat bathrooms. After a shower there was a fast reaction between the moisture formed and the wooden surfaces, which led to a rise in temperature of about three degrees Celsius. This made it possible, when necessary, to heat the bathroom to a pleasant temperature using the shower. Energy was saved when the round-the-clock temperature of the bathroom could be lowered from 23 degrees Celsius to 20 degrees Celsius. The decreased need for heat in the bathroom was a result of latent heat in wooden surfaces.

Wooden surfaces as heat equalisers have significant potential especially when used in conjunction with a well-managed heating and ventilation system. Research and design work for practical applications must still be done. Latent heat can be utilised to save energy. This should be made part of the energy balance. In the future, energy calculations should take into consideration hygroscopic materials when determining the estimated energy use of buildings.

References and more information:

Thermography measurements and latent heat documentation of Norwegian spruce (Picea abies) exposed to dynamic indoor climate. Kraniotis, D. et al. Norway. J Woos sci (2016) 62: 203-209.
Impact of air infiltration rates on moisture buffering effect of wooden surfaces. Kraniotis, D. et al. Norway. 36th AIVC Conference” Effective ventilation in high performance buildings”, Madrid, Spain, 23-24 September 2015.
The principles of sauna physics. Nore, K., Kranioti, D., Brücker, C. Norway. Energy Procedia (2015) 78: 1907-1912.
On simulating latent heat phenomena in a sauna. Kraniotis, D. et Nore, K. Norway. 2015. Norwegian Institute of Wood Technology.
Moisture buffering, energy potential and VOC emissions of wood exposed to indoor environments. Kraniotis, D. et al. 2015. 8th International Cold Climate HVAC Conference, At Dalian, PR China.
An investigation into the surface temperature changes in solid wood during sorption. Kortelainen Katja. 2015. Master’s thesis for the degree of Master of Science in Technology. Aalto University.