L16. Is Wood Good?
We have discussed the benefits of wood products (there are many!) and the sustainable management of forests. However, none of our discussions have really quantified the environmental impacts of wood and wood products, or compared wood to other options. Is using wood really good for the environment? Is it better than other non-renewable options?
There are two parts to this story: 1) the growing of the trees and 2) the products that are made from the trees. Although LCA covers many different impact categories - our focus will be primarily on the LCA impact category of climate change.
Carbon, Trees, and Forests
Lets begin with an overview of the carbon cycle of a tree. We start with a tree taking carbon dioxide out of the atmosphere and converting the carbon to biomass. There are three general options to what happens next.
- A Natural Death: The tree dies and the wood (tree and roots) slowly decay through fungal and microbial activity. Through this decay process, carbon in the wood fibers is released as carbon dioxide or methane by the microbial activity or absorbed into the soil biome. Eventually, over the course of several years, depending on moisture conditions and temperatures, all of this carbon is released back into the atmosphere.
- Harvest for Lumber: If the tree is harvested for lumber and the wood is used for building a house or furniture, the carbon is essentially stored (sequestered) until the end of the life of that wood product. At some point in time the lumber is put into a landfill or burned in an incinerator, which means that the carbon will eventually be returned to the atmosphere. In this scenario, any logging residue left behind (stumps and branches) and roots slowly decompose, eventually releasing all of the carbon back to the atmosphere.
- Harvest for Energy: If the tree is harvested, processed, and used for energy, the carbon stored in the wood is released back into the atmosphere more immediately during combustion. Again, any logging residue left behind (stumps and branches) and roots slowly decompose, eventually releasing all of the carbon back to the environment.
In each of these scenarios the carbon eventually gets back to the atmosphere making all scenarios carbon neutral - whatever carbon was captured by the tree is now back into the atmosphere. However, the timing of this carbon return to the atmosphere is very different depending on these scenarios. Natural degradation on the forest floor of trees, branches, stumps, and roots will occur over several years. If the wood is used in home construction that carbon may be sequestered for 100-200 years. If we convert the wood to energy, most of the carbon is goes back to the atmosphere within a year after the tree is harvested.
Why is this important?
This question of the timing of carbon neutrality is only important if our goal is removing carbon from the atmosphere. Imagine the conversion of an old forest that has been capturing carbon from the atmosphere for the last 100 years (since 1918). If these trees are harvested and used for energy the return of that captured carbon into the atmosphere could be nearly immediate. If they die a natural death or burn in a forest fire this carbon is also put back into the atmosphere very quickly. However, if it is used for construction or furniture it could remain sequestered for the next 100-200 years. If our goal is to maintain or reduce atmospheric carbon by 2030, what should we do with those trees?
So when we talk about carbon neutrality, it must be in the context of a time period. Carbon sequestration should also be discussed in terms of a time period. The term "Carbon Debt" is an accounting term that is used when discussing forest management for carbon. If today we cut a tree that has been growing for 100 years and use it for energy, we release "old" carbon to the atmosphere. This would be the carbon debt. If we replanted trees that started removing this carbon from the atmosphere - eventually we would repay this carbon debt and be carbon neutral again. Once again, it is all about accounting for carbon and the time frame we are using.
Here is a short video that David made to help explain all these terms and concepts.
Do sustainably managed forests capture more carbon?
In general, an acre of forest will capture between 0.5 to 5 tons/acre/year of CO2. This is a wide range because there are many contributing factors such as type of tree, age of tree, climate zone, soil type, and tree health (Forests and Carbon Links to an external site.). Carbon capture from a tree is all about tree growth. The general assumption is that about 25% of the mass of the tree is carbon. Each year that a tree grows it captures more carbon at a faster rate (more CO2 per year). A tree that is 40 years old will begin its life capturing just a few pounds of carbon dioxide per year but by the time it is 40 it will be capturing 500 or more pounds of carbon dioxide per year. Again the range is quite wide depending on many factors, but a good average might be 200 lbs of CO2/year/tree per year over the 40-50 year life of a tree. Remember, this sequestration is only temporary.
Regarding forest management -in natural forests, trees die and return carbon to the atmosphere, and are therefore at equilibrium with carbon capture and release. A forest that is being harvested will not have the decaying trees as part of the carbon capture equation. This makes the calculations of carbon capture complex. The bottom line is this - over the course of 200 years, both natural forests and sustainably managed forests harvested for wood products (or energy) are considered carbon neutral.
LCA of Wood Products
What does the life cycle of a wood product include? What are the boundaries? What would a cradle to grave analysis look like? A cradle to gate analysis? What about a functional unit? All of these are important questions when evaluating the LCA and it gets complicated quickly. For this lesson we will look at three examples. Cradle to gate for dimensional lumber, cradle to grave for decking, and cradle to grave for a building constructed from wood.
Just a reminder about LCA. There are typically four phases that are evaluated. These are 1)the extraction of the raw materials, 2) the production of the product, 3) the use of the product and finally 4) what happens at the end of a products life. A cradle to grave analysis looks at all of these phases.
Example 1: Cradle to Gate LCA Dimensional Lumber
For this example we are using a paper called "Cradle to Gate Life Cycle Assessment of Softwood Lumber Production from the Northeast-North Central Download Cradle to Gate Life Cycle Assessment of Softwood Lumber Production from the Northeast-North Central" written by private contractors in partnership with the USDA. Note how specific the study was. It was only for softwood lumber produced in Northeast and North Central United States and did not include shipping to the retail store.
The stated scope of the study "includes cradle to gate LCI's based on primary data for producing planed, dry dimension (framing or construction) lumber from logs using practices and technology common to the NE-NC region." Note LCI (Life Cycle Inventory) is the term used for the data collection needed for the LCA.
The system boundaries are see in Figure 1 along with all the inputs. Forest management (not shown clearly in the figure) included all aspects of the preparation of the soil, planting, and everything involved in growing the trees to a stage where they could be harvested. We saw all of the other stages (harvesting, transporting, drying planing, and packaging) in Lesson 11. Every step of the process was accounted for in this LCA. Adding to the complexity is the fact that many co-products are made throughout the process.
Figure 1. System boundaries for a cradle to Gate Analysis of softwood lumber (Source Download Source)
Attribution:
If you remember from Lesson 11, the sawmill's primary product was lumber. However, only a portion of the log was made into dimensional lumber. Remember, the bark, sawdust and other wood material was used for other things (paper, energy, etc.) This means that the energy and impacts of growing the tree, hauling a log to the sawmill, cutting of the tree, etc. must be divided among the various end products. In this study only about about 47% of the energy inputs of the sawmill were assigned to the dimensioned lumber. The rest went to pulp, sawdust, bark, or fuel used for other end products. Attribution is assigned at every step of the production process.
Functional Unit:
The functional unit for this study was 1 m3 of finished softwood lumber. This detailed analysis took into consideration all of the many inputs and outputs with an evaluation of approximately 150 different chemicals emitted to the air and 50 to the water. Table 1 lists some of the key results from the study including LCA impact categories and use of different energy sources and water. Note the contributions from forestry (growing of the trees) vs the producing of the wood (harvest, transport, drying, packaging, etc.).
Table 1. LCA results from study. Environmental performance of 1 m3 planed dry softwood lumber (Source Download (Source).
| Impact Category | Unit | Total | Forestry | Wood Production |
|---|---|---|---|---|
| Global Warming Potential (GWP) | kg CO2 equiv | 93 | 15 | 78 |
| Acidification Potential | H+ moles equiv. | 50 | 11 | 39 |
| Eutrophication Potential | kg N equiv. | 0.037 | 0.012 | 0.025 |
| Non-renewable Fossil Fuel | MJ | 1342 | 212 | 1130 |
| Solar, wind and hydro energy | MJ | 182 | 2 | 180 |
| Biomass Energy | MJ | 2586 | 0.0 | 2586 |
| Fresh Water | L | 179 | 0.0 | 179 |
What next?
This was a very complete study. However, what do the numbers really mean? How does the functional unit of 1 m3 of planed softwood lumber compare to the same volume of steel or concrete or plastic? And even so, what would such a comparison mean? An LCA using a functional unit of volume (CO2e/m3) or mass (CO2e/kg) is useful, and has been done on almost every material, but there needs to be more. What we need to know is how this information relates to something we build and use. As a fairly simple example, let's go back to the construction and use of a deck.
Example 2: LCA of Deck Boards
What is the most environmentally friendly deck material? Is an all-wood deck better than using recycled plastic or a composite made up of both plastic and wood for deck boards? What about cedar vs redwood vs chemically-treated pine? To answer these questions, we need to take some of the following factors into consideration:
- Volume of material. In these studies they choose the size of the deck (area) but in addition to the area the board thickness must be included to calculate the volume. Different materials will need to be different thicknesses to support the load.
- The Cradle to Gate LCA of the material. This includes everything that happens from resource extraction (cradle) to the factory gate (i.e., before it is transported to the consumer).
- Any additional structural components and fastening systems to hold the deck together.
- How long the material lasts or the replacement frequency.
- Any maintenance (such as painting or staining).
- Final product disposal. Landfill? Recycle? Energy production?
A study by Bolin and Smith Download Bolin and Smith compared the LCA of treated softwood decking to wood plastic composite decking (WPC). Their functional unit was 320 square feet of deck and a lifespan of 10 years. Table 2 shows the results of the study and indicates that the composite decking has more of an environmental impact than treated lumber.
Table 2. Summary of Cradle to Grave impacts. Treated lumber decking vs wood/plastic composite (Bolin and Smith).
| Impact | Units | Treated Lumber | WPC |
|---|---|---|---|
| GHG Emissions | lb CO2e/year | 114 | 330 |
| Fossil Fuel Use | MMBtu/yr | 0.24 | 3.4 |
| Water Use | Gal/yr | 12 | 34 |
| Eutrophication | lb Ne/year | 0.013 | 0.015 |
A second study by an organization called CORRIM (Consortium for Research on Renewable Industrial Materials) compared redwood decking to three other decking materials. (Final Report Summary Download Final Report Summary). In this study, the functional unit was a 100 square foot deck and a lifespan of 25 years. Results from this study indicate redwood decking had a much smaller cradle to grave impact than the plastic, wood plastic composite, and even the wood plastic composite made from recycled plastic.
Example 3: A Building as a Functional Unit
The much more challenging LCA is comparing wood to other materials for a building. Unlike a deck, there are many considerations that must be made in defining the functional unit.
A study entitled "Life cycle assessment (LCA) of wood-based building materials" from Lawrence Berkeley National Lab outlined the key components that must be included in any comparative study between wood and concrete/steel construction.
- The functional unit must account for all the materials in the building (e.g. roof, wall construction, windows). In addition, the functional unit comparison must have equivalent "use phase" energy requirements. This primarily means similar heating and cooling requirements for the building.
- Include the forest management from wood products (e.g. the assumption should be that the forests are sustainably managed). This means that over a long time period (50+ years) the system is carbon neutral, so forest carbon capture is not included in the LCA.
- The end of life of all of the building materials (e.g. landfill vs reused)
- The source of energy to produce all of the materials (e.g. solar, wind, fossil fuel, biomass)
- Any additional elements such as wood preservatives or adhesives (Glulam or CLT) should be accounted for in the study.
Carbon Neutral Consideration
There are two things that are critical to understand when thinking about the LCA of buildings. First, the use phase always has a bigger impact than the material production phase. Even considering all of the inputs in resource extraction, the production and transport of the building materials, and the construction of the building, the biggest impact of any building comes in the "use phase" during the life of the building. Things like heating, air conditioning and lighting over the 50 year life of the building far outweigh the material construction and production phase of the building.
Second, the LCA of any wood product does not include the capture of carbon from the growing of the tree. Trees are considered carbon neutral, so claims of gaining carbon 'credit' for wood buildings is not a consideration. Unfortunately, this is a confusing point. In fact, we saw this misleading claim in Michael Green's TED talk from Lesson 14. Mr. Green reported that the construction of a 20 story building would sequester 3150 tonnes of CO2 while a similar concrete/steel structure would emit 1200 tonnes of CO2. While this is true in the short term, over the long term this carbon would be re-released into the atmosphere at the end of the building's life. The carbon capture of trees is always carbon neutral in an LCA!
Five-story Office Building
| Reinforced Concrete: We have had many references to building with steel and concrete but we have never explained how it works. As we think about building materials, we know that the strength of materials varies. Wood has much different bending, compression, hardness, and stress/strain properties than concrete and steel. In general, concrete by itself is not a good building material. It is great in compression but not so good in other categories. Hence, most concrete structures include steel. When we refer to reinforced concrete - that is what we are saying - a combination of concrete and steel. |
An LCA comparing wood to concrete/steel construction is likely to look like Figure 2. This study by Robertson et al Download Robertson et al. evaluated the construction of a 5-story office building comparing engineered wood construction (glulam) to concrete/steel. The study followed the traditional LCA methods outlined above. In this study, the wood structure was better in 9 of 10 impact categories with the global warming potential being 71% lesson the wood structure. (The last category, fossil fuel depletion, reflects the energy content of the wood and adhesive in the wood timbers. It is not a negative that wood his higher.)
Figure 2. LCA of wood vs concrete. Robertson et al Download Robertson et al.
Homes in Minneapolis or Atlanta
Climate and geography can also make a difference in LCAs because of the different building requirements. A study by CORRIM compared two types of homes built in Minneapolis to two types of homes built in Atlanta. Minnesota homes are traditionally built with basements and are designed for Minnesota winters. Homes in Atlanta typically do not have basements and do not need to consider snow. Table 3 summarizes the results of the studies for the two locations and building types.
Table 3. LCA for the homes in the two regions. Note that the two homes cannot be compared - only the different types of buildings in those cities.
| GHG kg CO2e | Air Index* | Water Index* | |
| Minneapolis | |||
| Wood | 37000 | 8560 | 17 |
| Steel | 47000 | 9730 | 70 |
| % Increase | 26 | 14 | 312 |
| Atlanta | |||
| Wood | 21400 | 4890 | 7 |
| Concrete | 28400 | 6010 | 7 |
| % Difference | 31 | 23 | 0 |
* Air and water index scales have no units but compare multiple chemicals and impacts. The real value is in the % difference between the structures.
In general, this study is similar to most and shows that wood construction tends to have a lower environmental impact than concrete or steel.
Conclusions
Trees are carbon neutral. Temporary sequestration is possible, but in the end the carbon always goes back into the atmosphere. Therefore, when we build with wood, the calculations of global warming potential do not consider any carbon sequestration.
When evaluating buildings using the same functional unit, cradle to grave LCA comparisons indicate that wood is a better material than concrete or steel. Wood is indeed good!! However, the "use phase" of any occupied building is far more important than the impacts of the building construction.