An important part of the Thermal Mass and Buoyancy Ventilation Research Project is investigating whether, if properly sized, mass timber can be used as a thermal mass. To do so, the team will build a Wood Chimney Experiment to run in parallel with the Concrete Chimney Experiment. As they work to compare the thermal mass performance of concrete and softwood, they must learn more about the thermal and structural properties of the materials. A material’s anatomy affects how it absorbs, transfers, and offloads heat. Concrete has been used as a thermal mass for ages and therefore has widely known and defined thermal properties. However, there is far less information on the thermal properties of wood. Let’s take a closer look at the composition of timber as taught to the team by colleague David Kennedy, a self-proclaimed, “Wood Anatomist Fanboy.”
The first consideration for wood is the difference between isotropic and anisotropic materials. Isotropic materials, like concrete, have consistent properties in all directions. Anisotropic materials, like wood, have orientation dependant properties. Wood has an anisotropic, cellular structure that provides its strength and its the ability to move water and minerals to the outer reaches of the branches. A tree’s main job is to transfer water and nutrients from the ground to the sky and vice versa. Therefore, wood cells can be visualized as bundles of straws, with some acting as pipelines while others store nutrients. These cells consist of tracheids, parenchyma, and epithelial cells. Tracheids are actually dead cells that function as water transporters. Parenchyma store starches that give nutrients for the tree. Epithelial cells take on the job of building the tree. These cells act vertically, while ray tracheids, ray parenchyma, and ray epithelial cells work in the horizontal direction.
As trees grow their structural properties change based on what they need. This is why different parts of the tree are composed of different types of wood depending on their age. If a cross-section is examined, two important types of wood will be present, juvenile wood and heartwood. Juvenile wood is a weaker less dense outer ring and is usually distinguished by a lighter color. This wood makes up the early growth and is not evenly distributed throughout the tree. Often, the tops of trees will be 100% juvenile wood, while the lower parts of the tree are more mature heartwood. Heartwood, the mature, older wood, is produced by converting stored chemicals that are produced by the dying parenchyma cells that have been sent to the center of the tree. This area of the tree is typically darker and denser. These different cells and wood types can result in different characteristics throughout the same tree, which must be carefully considered.
The performance of thermal mass is heavily influenced by the thermal conductivity of materials. The thermal conductivity of materials is heavily influenced by the density of materials. With the variety of conditions that are possible all within the same tree, it is important to consider how the material is cut and oriented. This is also important because of the grain directions present in the wood. Just like water flows through the tree along cellular pathways, heat will follow the grain of the wood. Therefore, the cut of the wood must be selected so that the grain in the same direction as the preferred heat transfer.
Now that we have learned more about the structure of wood and how that affects its heat transfer we can understand how the wood should be cut and oriented. Initially, the team’s wood test panels utilized the timber end grain. This end grain direction would be parallel to heat transfer. However, depending on the size of the tree and how blocks were cut, the types of wood in the panels could become inconsistent. This meaning they could be made of different mixtures of juvenile wood and heartwood. They were also pretty inefficient to make.
To find a better way, the team investigated the different ways timber is milled into boards. The TMBVRP team found that different cuts of wood could produce different grain directions. The most efficient grain direction for our panels would be to have the grain running perpendicular to the wide face of the boards. This allows the board to cover the area most efficiently, and the board thickness can be controlled to work with the app results. This grain direction can result from several milling, but we believe quarter sawing will produce the most favorable boards with the least waste. The team then made their own version of these boards, but as panels for their wood chimney.
Wood is a far more complex than the Thermal Mass and Buoyancy Ventilation Research Project Team realized! They too are now “Wood Anatomist Fanboys” and hope you’re on you’re way to becoming one. Now, Give your brain a break and look at Rosemary and Dijon being mischievious! Thank you again to David Kennedy for all the help and as usual stay tuned to see what theTMBVRP Team learns next!