Environmental Footprint of Timber vs. Stick Framing

Brian Malone, a Master’s candidate at the University of Oregon in the schools of Forestry and Civil Engineering, recently published a thesis titled : Light-Frame Versus Timber Frame: A Study in Quantifying the Differences. His objective was to compare the environmental impact of a traditionally built timber frame barn with an exact replica built as a conventional stick frame. He also compares the structural strength via a load path analysis of each, but that’s beyond the scope of this blog post. The Athena Impact Estimator, a tool often used in Life Cycle Analysis, was used to measure the environmental impact of the buildings from “cradle to gate” (taking into account the impacts of manufacturing products used in construction, transportation of products to the site, and the actual building process, but not the impacts of the building’s use after construction, or what happens to the materials at the end of the building’s life.) The four major areas considered were energy consumption, fossil fuel consumption, global warming potential, and wood fiber use and waste.

Malone chose to include nine variations on the same structure, altering the sheathing type, the inclusion of insulation, and whether timbers were kiln dried or not, to compare across a wide range of commonly employed construction techniques. Those are:

• light framing with 2×6 walls, OSB sheathing

• light framing with 2×4 walls, OSB sheathing

• light framing with 2×6, plywood sheathing

• light framing, 2×6, OSB, fiberglass insulation

• traditional (green) timberframe, 1″ white pine shiplap sheathing

• kiln dried timber frame, shiplap

• timber frame with light frame infill, shiplap

• timber frame with structural insulated panels (SIP), sans insulation (taking into account the impact of 2 sheets of OSB sheathing)

• timber frame with SIPs, insulation included

 Interestingly, the largest differences in impact in every one of the four areas of study resulted from sheathing and insulation choices, rather than choosing a stick frame versus timberframe. If we look at two building options that are enclosed in a similar way to try and reveal the true differences between framing systems- light framing with 2×6 walls and OSB sheathing, and timber frame with structural insulated panels not counting insulation value, there’s an uncanny similarity in results. This is not a true comparison, as the timberframe option includes a double layer of OSB, which contributes significantly to all impacts measured, but for comparison’s sake:

 The Athena Impact Estimator says that differences of 15% or less should be ignored, given the multitude of variables going into the estimating process. All of these values are within that range, although it would be interesting to know precisely what that extra layer of OSB contributes.

Malone reveals much interesting food for thought for those interested in timber framing in an environmentally responsible way. In all four categories, the traditionally built timberframe has the lowest environmental impact, and the timberframe with SIPs has the highest. The only fair comparison to the timberframe with SIPs is the stick frame with insulation, but even this comparison shows that in all categories except total wood usage (energy consumption, fossil fuel consumption, global warming potential), the SIP system doubles the impact.

Myriad factors contribute to this: the energy required to manufacture OSB, notes Malone, is twice that to manufacture plywood. “OSB manufacturing ….. requires wood flaking, drying and screening of flakes, blending with adhesives and pressing of the panel product, finishing, as well as heat generation and emission control for these processes.” 

Another interesting topic covered in this thesis is the difference between energy consumption and fossil fuel consumption, in the manufacturing stages and the construction phases of a building’s life. Malone points out the kiln drying certainly does increase a building’s footprint, but “over half of the total energy required to manufacture kiln-dried softwoods (and hardwoods) in the Northeast and the North Central USA is generated by burning wood biomass”….biofuels (mainly waste wood) “contribute a large amount of energy to the drying process and their increased or decreased utilization can cause (the relative percentage of fossil fuels used) to vary considerably.”  Fossil fuel consumption during the manufacturing phase is higher for products that require “resins and further mechanical processing such as chipping or pressing. The LF structure sheathed in OSB requires 37% more fossil fuels for manufacturing than the LF sheathed with plywood. ” The electricity required for these processes is, in the Northeast, supplied by 59% coal, 11% natural gas, and a small percentage of petroleum. The other non-fossil fuel sources are 25% nuclear, 3% hydroelectric, and 2% renewables.

The construction phase of a buildings life, however, is dominated by fossil fuel consumption. The construction phase is characterised by the use of diesel fuel to power cranes, excavators, and the trucks that transport building materials. 

Malone’s paper is thought provoking and relevant to our work as builders in an age of peak oil and global weirding (aka, climate change). It can be found for free here: Brian Malone Thesis . Thanks to Brian for sharing his work and being willing to answer questions.