wood

Preparing a Timber Pun for a Post Title

Live from HomeLab, it’s Wood Chimney Experiment preparation. The Thermal Mass and Buoyancy Ventilation Research Project team members are continuing their efforts to refine a passive cooling and ventilation system which can be deployed to public buildings in the rural South. Due to the fantastic results from the Concrete Chimney Experiment, the team is starting the Wood Chimney Experiment. They have developed an experimental method for designing and building chimneys which test the Optimal Tuning Strategy. They also have honed their data collection workflow and analysis. Now they can move on to testing how timber can work as a thermal mass. You can read about why we are using mass timber as a thermal mass here.

The first step in Wood Chimney Experiment preparation is gathering materials. The team collected sensors that the Mass Timber Breathing Wall team is no longer using. Rural Studio has been growing its scientific equipment stock which allows for reuse between research projects.  The TMBVRP team is inheriting data loggers, heat flux sensors, thermocouples, power supply, and airflow sensors. They will be using different temperature sensors, thermocouples and heat flux sensors, then are used in the Concrete Chimney Experiment. These sensors, like the GreenTeg Go Measurement System, will still deliver the proper temperature readings. This equipment is flexible and adaptable making it easily reusable between projects.

Sensors and power source for wood chimney experiment.
Reduce, Reuse, Re-sense!

Next, you might remember the team’s good friend, GeoFoam. GeoFoam is a type of dense expanded polystyrene foam usually used for earthwork under roadways. Both research teams have been able to use it as insulation for their experiments after the geofoam was donated to the Studio from a construction site. Remember, the team must cut smaller sections of GeoFoam from a huge 8’ x 4’ x 4’ block using a hot wire. The team was able to do so underneath the Morrisette Campus Fabrication Pavilion for a designated time and with faculty approval to ensure safety during the pandemic. They collected the rest of the batt insulation from storage in Brick Barn as well as materials for the structure of the experiment. Everything was hauled back to HomeLab for construction.

Next, the Thermal Mass and Buoyancy Ventilation team continued cutting down and shaping openings in the Geofoam. The top and bottom pieces of the chimney are made of two 6” thick pieces of GeoFoam that are adhered together as 1’ of insulation is needed for the proper U-Value for testing. The top and bottom pieces have cones carved out to ensure proper airflow. Resident King of Precision, Jeff Jeong, double and triple checks each piece of foam. This way the Chimney comes together like an airtight puzzle.

The base for the chimney is constructed out of 2” x 4” lumber and plywood. The legs of this base are taller than the Concrete Chimney Experiment to match its height after being raised. Another difference in the design of the experiments is the walls of the interior chimney which the wood panels will be attached to. The walls for the Concrete Chimney Experiment are, from the chimney chamber outward, concrete panels, insulation, plywood, and then more insulation. The walls of the Wood Chimney Experiment will be pine panels, insulation, ZIP sheathing, and then more insulation. Notice Dijon doing his best to help in the photos below.

Last, but not least, is pre-drilling holes for the concrete panels. The concrete panels will be screwed to the insulation, ZIP sheathing wall. There will be four walls to complete the chimney. Notice the grain direction of the panels. This edge grain allows for parallel heat transfer between the air within the chimney chamber and the pine panels. Not only is the Thermal Mass and Buoyancy Ventilation Research Project testing if timber works as a thermal mass but how the grain direction affects its efficiency as a thermal mass.

The Thermal Mass and Buoyancy Ventilation Team is excited for the Wood Chimney Experiment to come together. So are the kittens! The team would not leave you without a HomeLab mascot update. While Dijon mostly naps, Rosemary is trying to get some construction experience to build her resume. They’ve had to tell her she is not OSHA certified, but she is fine napping a safe distance from construction now. It was not a hard sell. Stay Tuned to see the completed Wood Chimney Experiment!

Wood Wise

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. 

Timber mapping diagram
Different cuts of wood result in different grain condition which cause different construction systems

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.

How heat transfer is affected by grain orientation and construction.

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!

In the field trip

The Thermal Mass and Buoyancy Ventilation Research Project Team got out of Newbern last week and into the field, sawmill, and lab!

The first field trip of last week was to Charlie’s sawmill. Charlie is a retired engineer, woodworker, and long time friend of Rural Studio, having helped with the Greensboro Animal Shelter. The team met Charlie at the Animal Shelter during neckdown week, where he was leading the project to revamp the kennels.

Charlie has a “hobby mill” he has been building up over the past years. He works mainly with salvaged wood and timbers making furniture and folk art. After the team got a tour of Charlie’s sawmill, he treated them to lunch and a brief presentation on wood. Even more than lunch, Charlie has offered the team use of his sawmill. Charlie has a passion for helping others and great deal of building knowledge, the team feels very lucky to have met him! Thank you Charlie!

Next, the TMBVRP team met up with Professor David Kennedy in the material testing lab at Auburn University’s College of Mechanical Engineering to test the thermal properties of their concrete samples. These samples were made using three different concrete mixtures, high finish, fiber-reinforced and 100% Portland cement. The objective was to find the exact heat capacity, thermal conductivity, and effusivity of each mixture. Knowing the specific thermal properties will help eliminate variables in the math when evaluating how the Optimal Tuning Theory is working.

David gave the students a crash course in scientific testing procedure. When conducting such tests, everything needs to be documented. The samples were marked, 10 of each mixture, measured for thickness and diameter, and weighed. The specific volume and density were then calculated for each sample before testing. The sample was again weighed after the test had run. Everything needs to be documented!

Next, the team will analyze the data and recode the Thermal Mass and Buoyancy Ventilation proportioning application with the specific thermal conductivity results. We’ll talk to you soon!

Panel Making

This week the Thermal Mass and Buoyancy Ventilation Research Team got to use the largest skill saw they’ve ever seen and we’ll tell you why!

In the technical workshop Sal last week, the team decided to narrow the number of materials they will test throughout the experimental cycle from four to two. The lucky two will be concrete and softwood! Concrete is often used as a thermal mass material while softwood is not which will make comparing the data collected from the separate experiments all the more interesting. The Optimal Tuning Theory calls for the thermal mass to be externally insulated which allows the thermal mass material to be much thinner than a typical thermal mass. Therefore, the concrete and wood need to be panelized.

The thermal properties of wood act most efficiently as a thermal mass when the cross grain is exposed to the air. This means that panelizing the softwood is more like creating giant cutting boards. To practice this process the team used 8″ x 8″ Cypress timbers and their matching 16″ diameter skill saw leftover from the Newbern Town Hall project. The team learned that 6″ x 6″ timbers would be ideal for their project, that way they can cut the cross-grain pieces in one cut with their 16″ skill saw without having to rip down the timber.

The concrete panels are far more straightforward, build a mold, pour the concrete, let it cure. However, the team has to think about how the panels would be attached to a larger structure. To solve this they cast PVC into the panel which will allow it to be screwed into a structure.

Voila! We have much refining to do of the panel making process, but the first two turned out well. We also have here a rendering of the habitable structural with the separate concrete and wood panel rooms. Our next step is to apply what we learned working with these materials to designing and building our first experiment. Thermal Mass and Buoyancy Ventilation Research Team out.