Carbon Capture And Roundwood Timber Frame Construction
At Tŷ Pren we are passionate, and I mean really passionate about roundwood timber frame buildings. This is not just because these buildings are beautiful to live in and visit, because of all the natural materials used in the construction, but because the homes we build are Zero Carbon, and in most instances carbon negative. While most of the rest of the construction industry is using petrochemical technologies, and working out how to Reduce their carbon footprint- we are looking to build more homes that Capture carbon. Pretty cool.
So How Do We Do Capture Carbon?
This is not rocket surgery or brain science. It is dead simple in fact. We all know from school that trees use photosynthesis to grow. And they suck up carbon dioxide from the air, release oxygen, and store carbon. When trees are felled or fall they decompose, this carbon is released back into the atmosphere. When a tree is burnt that process of release is accelerated. When the wood is used to make something, buried deep (as happens when fossil fuels are created) , or drowned in water, that carbon is stored and captured. Any waste wood rots and releases the carbon.
Roundwood Carbon Benefits
This is the clever thing about roundwood timber frame- or Building with Poles. There is practically no waste, and no sawmills creating excess wood waste. The off cuts are used elsewhere in the build. So while it takes craft and skill to successfully build using roundwood- the carbon benefits are quite impressive.
The Maths and Science Bit:
Before going into the specifics- we do want to qualify- at the Tŷ Pren cooperative we are builders, not scientists specialised in carbon lifecycle, and product, analysis. So any corrections, additions, or expansions on all this are really welcome. That said we are competent and have done our research best we can, thanks to Mr Google among others.
It is possible to estimate (to a mostly reliable order of magnitude) any tree’ s CO2 sequestration like this: (thanks to ecomatcher.com for this)
“Step 1: Determine the total green weight of the tree
The green weight is the weight of the tree when it is alive. First, you have to calculate the green weight of the above-ground weight as follows:
Wabove-ground= 0.25 D2 H (for trees with D<11)
Wabove-ground= 0.15 D2 H (for trees with D>11)
Wabove-ground= Above-ground weight in pounds
D = Diameter of the trunk in inches
H = Height of the tree in feet
The root system weight is about 20% of the above-ground weight. Therefore, to determine the total green weight of the tree, multiply the above-ground weight by 1.2:
Wtotal green weight = 1.2* Wabove-ground
Step 2: Determine the dry weight of the tree
The average tree is 72.5% dry matter and 27.5% moisture. Therefore, to determine the dry weight of the tree, multiply the total green weight of the tree by 72.5%.
Wdry weight = 0.725 * Wtotal green weight
Step 3: Determine the weight of carbon in the tree
The average carbon content is generally 50% of the tree’s dry weight total volume. Therefore, in determining the weight of carbon in the tree, multiply the dry weight of the tree by 50%.
Wcarbon = 0.5 * Wdry weight
Step 4: Determine the weight of carbon dioxide sequestered in the tree
CO2 has one molecule of Carbon and 2 molecules of Oxygen. The atomic weight of Carbon is 12 (u) and the atomic weight of Oxygen is 16 (u). The weight of CO2 in trees is determined by the ratio of CO2 to C is 44/12 = 3.67. Therefore, to determine the weight of carbon dioxide sequestered in the tree, multiply the weight of carbon in the tree by 3.67.
Wcarbon-dioxide = 3.67 * Wcarbon
Example of CO2 calculation
10 years old tree
5 meter tall or 16.4 feet tall
25 cm trunk or 9.8 inch trunk
Wabove-ground= 0.25 D2 H= 0.25(9.82)(16.4) = 394 lbs
Wtotal green weight = 1.2* Wabove-ground= 1.2 * 394 = 473 lbs
Wdry weight = 0.725 * Wtotal green weight= 0.725 * 473 = 343 lbs
Wcarbon = 0.5 * Wdry weight = 0.5 * 343 = 171.5 lbs
Wcarbon-dioxide = 3.67 * Wcarbon = 3.67 * 171.5 = 629 lbs CO2 sequestered in 10 years; that equals 285 kg. EcoMatcher uses an average of 250 kg CO2 sequestered per tree. “
Ty Pren mostly build using European Larch. An average three bedroom house made from a roundwood larch frame, would use 9 entire larch trees- more if you consider roof and flooring- but to take waste into consideration 9 is a safe assumption, so we estimate:
Average Dry Weight (DW) a mature European Larch= 575KG https://www.wood-database.com/european-larch/
Average Carbon weight (CW)- CW= DW / 0.5
Weigh carbon-dioxide = 3.67 * CWÂ = 3.67 * 575KG = 1.725 Tonnes of carbon captured per tree.
1.725 x 9 = 15.525 tonnes of carbon captured per average 3 bedroom build.
Lifecycle Carbon Savings?
This is of course not the whole picture- as other natural materials like straw bail and hempcrete also capture carbon. And an off grid renewable home will also produce less c02 emissions- The average family produces 3.2 tonnes per annum heating and powering their homes.
So we estimate- and this is the starting point for more rigorous research and debate- that a home built using natural materials and powered exclusively by renewables, produces 64.48 tonnes carbon less over its lifecycle than a conventionally built and powered home. With 15.525 tonnes or carbon capture.
Potential At Scale
The figures for one property are an impressive estimate. If 2000 new homes annually were built using these technologies you get into game changing territory. We are not able (yet if ever) to produce 2000 homes a year- but we are interested in creating an industry that can, and being part of that.
Needless to say the principle of capturing carbon in a build, rather than creating carbon in a build, is solid, and warrants more attention, in our opinion anyway.
If you’ve comments and want further discussion on this please do get in touch.