Thermal inertia of Stora Enso CLT in comparison to other building materials such as masonry or concrete
For a good living comfort it is desired to keep a constant temperature (no large variations throughout a daily cycle) and that within a certain comfort zone. Thermal inertia describes the certain inertia that is present, when the surrounding temperature around a mass is different than the temperature of this mass and the mass is adapting the surrounding temperature. In a system where the mass adopts the temperature quickly, the thermal inertia is low and vice versa.
A typical scenario that describes thermal inertia is an old, historic stone building (church, residential building, castle, etc.) in summer. While the exterior temperature in summer is varying significantly during 24 hours, the internal temperature remains on a rather constant level.
In a building this thermal inertia can be used in a way, that during daytime, the mass of a building is absorbing energy from the surrounding environment. Warm air in a room is being cooled by its surrounding walls and floors. The walls and floors are absorbing the heat, keeping the temperature in the room at a comfortable level. In the evening hours, when the temperature decreases again, the energy that is being stored in the walls and floors can be released again and be discharged by ventilation. This effect can provide a comfortable climate inside a building in hot summer days, because a certain cooling effect is taking place and the temperature amplitudes are not as high as in buildings with low thermal inertia.
To analyze this effect, the Stora Enso research and development team did a dynamic thermal analysis of a building over an entire year with weather data from the city of Vienna, Austria. A reference building was taken. Influence from floors above and below have been eliminated by choosing the right boundary conditions. For a comparative analysis, 5 different wall compositions have been analyzed. Each of them had thermal insulation to the outside, with a certain thickness, so that all wall compositions have the same U-value (thermal insulation). This provision assures an equal heat loss for all compared wall compositions and therefore makes a comparison possible. The following wall types have been studied:
- Light Wood Framing
- CLT wall - thin (10 cm CLT)
- CLT wall - thick (20 cm CLT)
- Masonry wall (light weight hollow blocks made from burnt clay)
- Concrete wall
All the other boundary conditions were kept the same and over the duration of one year, the internal operative temperature (mean value of internal surface temperature and internal air temperature) was recorded on an hourly base. Now the hours within a year can be counted, where the internal operative temperature was exceeding a certain benchmark temperature. In our study, 27 °C have been chosen. The amount of hours, in which the internal operative temperature is exceeding the benchmark temperature for a given system, is displayed in the diagram.
Figure: Discomfort, expressed in hours per year when an internal operative temperature of 27 °C is exceeded
This study shows that a building with light wooden framing is overheating easily and a building with lot of thermal mass, as in a building with concrete walls, overheating is less of a topic.
Compared to its competitive systems, CLT is doing very well. A thin CLT wall (10 cm CLT) is performing slightly worse than a masonry block wall, with a difference of 0.2 %. The thick CLT wall (20 cm CLT) is clearly outperforming the masonry wall with a difference of 8.1 %. On average the performance of a CLT wall is closer to the performance of a concrete wall, than to a light framed wall.