Building with Wood

For several decades there has been a debate in the building industry as to whether wood frame or steel frame construction is more sustainable—wood being a renewable material, while steel has recycled content, often incorporating 70-80 percent old automobiles. Perhaps the debate is finally being decided due to a panel technology called cross-laminated timber, CLT for short. Developed in Europe in the 1990s, it is only recently gaining popularity here.

A CLT panel usually consists of 3, 5, 7 or 9 layers of kiln-dried boards stacked in alternating directions, bonded with structural adhesives and pressed to form a solid, straight, rectangular panel. Surprisingly CLT has good fire-resistant properties: it is hard to ignite and once lit resists fire spread. Because the layers are oriented perpendicular to each other, the CLT panels are exceptionally strong, stiff, stable, relatively light weight and able to handle load transfer on all sides. They can be used for walls, floors and roofs in a single building system, or used interchangeably with other wood products.

Most commonly CLT panels are 40-60 feet long but can be as much as 100 feet. They are up to 18 feet wide and any thickness up to 20 inches. These panels are widely used in Europe, Australia, Canada and Japan. The possibility of large panels is revolutionizing how 10, 20 and 30-story buildings are being built. Currently an 18-story, 400 student residence (174 feet high) at the University of British Columbia is the largest CLT structure, but a 24-story tower is under construction in Vienna and a 35-story building in Paris is in the works. The most ambitious proposal to date is London’s CLT framed, 80-story Oakwood Tower.

Not only are CLT panels frequently made using small-diameter trees, but also can use less desirable wood from pest damaged trees, or even trees killed by wildfires, without compromising the panel’s overall integrity. These small, less-than-perfect inputs to panel manufacturing are leading to better utilization of forest resources. Pulling out small and medium sized timber, as well as dead trees, contributes to healthier forests.

Processing these culled trees into CLT panels, which then get incorporated into buildings, sequesters carbon from the atmosphere. CLT not only emits less carbon dioxide during the manufacturing phase but the finished buildings then help sequester carbon for longer periods. Scientists estimate that buildings made with these materials result in a 25-30 percent reduction in global warming potential compared to those made with traditional materials—concrete, masonry and steel.

Because they lend themselves to design versatility, fast installation, reduced waste and good thermal and seismic performance, CLT can reduce construction costs by up to 50 percent. Perhaps the biggest advantage, however, is sequestering carbon while creating healthier and more resilient forests.

Nature Knows Best - Carbon Farming

For the past 20 years, since Ed Mazria calculated that buildings use almost 50% of energy consumed in this country, I have been on a quest to make buildings super energy efficient, while producing any additional energy needed from site-generated renewables. But now I am coming to believe that there is another option for reducing greenhouse gases that is equally, if not more, important.

A carbon experiment on a small number of ranches in Northern California is establishing a simple, benign way to remove carbon dioxide from the air. UC Berkeley scientists have measured the impact of a one-time spreading of compost ( ½ “ layer) on rangeland and found, to their surprise, that it boosted the soil’s carbon sequestration capacity on average one ton per hectare per year for each of the 8 years they have been testing. They forecast that this sequestration process will continue for at least two decades.

Grazing is the largest land use on the planet and most grazing lands are degraded. If this one-time thin application of compost were applied to a quarter of California’s rangeland, the soil would absorb ¾ of California’s greenhouse gas emissions for the year, and since the effect is cumulative, it would keep doing so for a number of years.

For centuries humans have been bleeding soil-stored carbon into the atmosphere through plowing, overgrazing and poor agricultural practices. These recent compost applications can reverse this and create what scientists call a positive feedback loop. Plants pull carbon dioxide from the air through photosynthesis and transfer a portion of the carbon to the soil through their roots and soil microorganisms, then turn that carbon into a form commonly known as humus. This process improves soil fertility, boosts plant growth and captures even more carbon, while enhancing the soil’s ability to absorb and retain water.

Where is all this compost going to come from and who is going to pay to have ranchers spread it on their land? The compost has to come primarily from cities. San Francisco composts 700 tons of residential and commercial organic waste everyday—the largest such operation in the world. This can become a great resource for ranches. As for compensation, key efforts are being made to incorporate soil carbon offsets in California’s cap-and-trade system as a way to make it profitable for ranchers to restore their land.

Although only recently recognized as a carbon sequestration strategy, “carbon farming” has already captured the attention of the White House and officials as far away as Brazil and China. This inexpensive, low-tech harnessing of natural systems holds the potential to turn the vast rangelands of California and the world into a weapon to reverse climate change.

What Makes a Home "Green"?

Multi-year drought, distressed forests and intense wildfires bring awareness of the local impacts of climate change. Efforts to address climate change have long focused on cutting greenhouse gas (GHG) emissions and more recently on sequestering carbon. But because the impacts of climate change are now upon us, we need to add to our efforts resilience planning. What is the focus of these efforts related to our buildings?

In reviewing many “green building” programs, the most common characteristics are: energy efficiency, water conservation and selecting materials that are sustainable and used in a resource-efficient way.

Because energy use is strongly linked to GHG emissions, reducing energy consumption in homes tends to decrease damage caused by burning fossil fuels. There are many ways to reduce energy use in homes: install more efficient appliances, select higher performance windows, or add thicker insulation.

Water conservation makes sense in most regions of the country, excluding only those where fresh water is abundant. In California, water is directly linked to energy, for close to 20 percent of our energy use statewide is consumed in transporting and treating water. Residential water conservation emphasizes careful selection of plumbing fixtures and appliances but also promotes rainwater collection.

Choosing materials for a green home involves selecting materials that are naturally renewable (for example, wood from sustainably harvested forests), have recycled content, or are harvested/manufactured regionally. These materials also need to be incorporated in structures as efficiently as possible.

Other green building criteria worth considering are:

·         Designing and building homes that last longer. Although rarely thought about, designing to make future changes easier helps save materials. Selecting materials that require less cleaning and less maintenance are also aspects of durability.

·         A green home is small. Scaling down is unpopular with Americans, but greatly reduces impact.

·         A green home provides superior indoor air quality and promotes human health. Avoiding materials and furnishings that involve toxins and outgassing is key. Good natural and mechanical ventilating is also important.

·         Because transportation involves our biggest consumption of fossil fuel, selecting a location that is close to work, shopping, schools and public transportation could be the most impactful of all criteria, but is rarely addressed.

Resilience focuses on livable conditions in buildings after a disaster; on backup power; and on access to potable water. These strategies are being woven into “green building” programs but expanded to community scale. Photovoltaics with on-site storage can provide basic services and information during power outages. We know how to build tight buildings out of non-combustible materials to resist wildfires. Mechanical ventilation with HEPA (high efficiency particulate air) filters can handle smoke and ash from wildfires. Rainwater collection, composting toilets and recycled water are other resilient strategies.

Choosing the Right Building Materials for a Low-Carbon Future

Many have advocated for tight houses with low energy needs and highly efficient mechanical systems as one of the best ways to address climate change. California now requires all new houses to be zero-net-energy, although natural gas is still allowed. A new study by Chris Maywood, director of the Endeavour Sustainable Building School in Ontario, Canada, questions this singular focus on cutting building energy use. His research concludes that curbing the emissions resulting from the harvesting, manufacture and transport of building materials, what he calls a building’s up-front embodied carbon emissions (UEC), is significantly more important than the contribution from running a building on clean, renewable energy.

Zero-net-energy makes a big impact on carbon emissions, but most of that impact is in future years as a result of on-going zero energy consumption in the building’s operation. On the other hand, if UEC is zero or even provides net carbon storage, the impact on climate is immediate and powerful. Emissions that are avoided today do more to slow climate change than emissions that are averted in the future.

What materials make the difference between a high UEC structure and one that stores carbon? A high carbon house would use such common construction materials as standard concrete, extruded polystyrene insulation, brick cladding, steel frame-vinyl windows, tile and carpet flooring and concrete or clay tile roofing.

A carbon storing building would utilize concrete where much of the Portland cement has been replaced by fly ash or other substitute materials. It would be insulated with cellulose and wood fiberboard. Sustainably grown wood would be used for the framing, for wood flooring, for wall paneling and for wood windows. Linoleum might also be on the floors. Sun baked Mexican clay tiles (low embodied energy) would be a good roofing choice for our high-fire area. The study recommends plant-based building materials such as straw panels, hemp fiber board, and rice straw medium-density fiberboard, because they prevent the release of stored carbon for the life of the building.

Because the report only focused on materials that are available, code-compliant and affordable, architects and builders can make major carbon reductions with only minor adjustments to what we already do. The optimum, obviously, is to create carbon storing buildings that also run on renewable energy.

The conclusions in this report are a major revelation to the building industry. The analysis relates only to new construction, but the same team is undertaking a new study covering retrofits. Maywood’s hunch is that there will be even bigger climate-impact opportunities with remodels.