It's not uncommon to see claims in our industry that timber frame structures and the wall & roof panels used to enclose them are green and sustainable – but is this really the case? I thought it would be interesting to look a bit deeper into these questions to try to uncover the facts.
The timber frame structure
A timber frame is made from solid wood, which is sustainable … right? Well, not all timbers used for timber framing are necessarily sustainable. Sustainable forestry, in an oversimplified manner of speaking, means that the volume of wood in the forest does not go down over time as a result of logging practices, or a particular resource will not be harvested to extinction. It may go up or down in individual years, but over time the resource will be there in perpetuity.
When huge "free-of-heart-center" Douglas fir timbers are specified for a timber frame project, as is now popular, these timbers will come from enormous old growth trees, on Canada and America's west coast. These trees are a limited resource – once they are full harvested, they are gone forever. A more sustainable timber would be a smaller, smaller, possibly plantation grown Douglas fir timber, that is "boxed-heart" – meaning a smaller and smaller tree can be used.
Another factor to consider is the embodied energy in the timber, or the energy it takes to harvest, transforms and transport the timber to its final location. If a timber is harvested locally, squared up on a mill and used "green" (not dried), it has very little embodied energy. A practice now gaining popularity is to have the timbers kiln dried in a special energy-intensive kiln. Add this drying energy to the cost of transporting the west coast timbers all over North America, and the embodied energy goes up quickly. Not as much as steel or concrete perhaps, but a lot more than local green timber. We always encourage our clients here in Quebec and Ontario to use local, green, boxed-heart timbers for their timber frame projects.
Insulated wall and roof panels
A product that has become synonymous with timber framing is the foam based stress-skinned panel used to form the walls and roof envelope. Available in either EPS or urethane foam, the panel does a great job of insulating the home, so it's often touted as being very green. While it's true that lowering energy consumption is part of a green home, using petroleum based products is clearly not sustainable. When you look at the larger context and the amount of energy used by the petroleum industry, including CO2 emissions, you have to question the green label it has been given. Foam panels are also quite wasteful, as roughly 20% of the walls panels will be cut out and put in a landfill as a result of window and door openings.
A more sustainable insulation material would be cellulose, straw or even mineral wool. Cellulose is made from 85% recycled newsprint, and does a great job sealing walls and roofs against air infiltration. Straw is an agricultural by-product, and also has a great R-value. Even mineral or rock wool, which technically comes from non-sustainable minerals, can be made from recycled iron slag, which is a by-product of iron and steel making.
So if there is one message to take from this article it's this: do your homework. Don't always take the claims of manufacturers at face value, as products often have undesirable qualities that don't get mentioned in advertising tag lines.