Where can I find sustainability or environmental impact comparisons between various structural approaches?
Good question. Actually, there is a lot of information out there, but much of it is contradictory. It is not surprising that many of the studies are funded by the various industries involved and tend to support the products manufactured by the industry providing the funding. Let’s try to sort out the fundamental framework for evaluation and then I invite anyone who knows of objective studies that look at the issue comprehensively to identify them.
Looking at the 100,000-foot sustainability foot level, we want to consider people, economics and environment. The people aspect should consider both the human health impacts during the manufacturing, construction, use (indoor environmental quality), and disposal phases, as well as social equity impacts (working conditions, wages, human rights) during those phases. The economics aspect relates to weighing total lifecycle costs, first costs vs. operating costs, local economy factors and unincorporated social costs. The environmental aspect is that with which we are most familiar: materials quantity, embodied energy, pollution production, water consumption, recycled content, recyclability, capacity to save energy and water, environmental and human toxicity, and rapidly renewable content. The environmental aspects and human health aspects are incorporated into what is called life cycle assessment, an imperfect (to date) but fundamental impact and comparison tool that is dependent upon accurate databases and good assumptions for data gaps and other unknowns. Small variances in inputs can result in dramatically different results.
Looking at the 10,000-foot environmental level, specifically focusing on embodied energy (of the system materials) and capacity to save energy (of the building envelope constructed from the system), we want to consider the pre-use phase (manufacturing and construction), the use phase (operating energy of the home), and the post-use phase (demolition and disposal or re-use). A 1998 University of Michigan study determined that 94 percent of the energy consumed by a standard-construction Michigan home was consumed during the use phase. So, clearly, using materials that save considerable amounts of a home’s operating energy, even if the material’s embodied energy is a bit higher, is a good direction. Using materials that save operating energy but have a low embodied energy are even better.
Looking at the 1,000-foot level, let’s assume that we have a number of choices for building our walls that, when used in the right system configuration or thickness, all create the same home operating energy costs (and let’s assume we use one kind of roof that complements our energy goals). Here are my predictions for materials' embodied energy from lowest to highest: adobe or cob (made from on-site soil), strawbale (made with local or non-local straw), rammed earth (made from on-site soil and using a small percentage of local cement), straw-core structural insulated panels (SIPs), wood with blown cellulose or cotton insulation (local, Forest Stewardship Council-certified wood), foam-core structural insulated panels, insulated concrete forms (ICFs), autoclaved aerated concrete (AAC), masonry and monolithic masonry.
Validation of this prediction is complicated by the fact that most of these systems are thermal barrier systems while some rely on thermal mass. Also, climate conditions are going to affect results and some of the systems are less-suited for particular climates. So, I’m sure the prediction will create spirited responses. It is important to remember, though, that the overwhelming benefit will be in a reduction of operating energy over the life of the home, and any of these systems, when designed to increase thermal performance, will have a net benefit over standard code minimum practice. It is certain that a better building envelope reduces environmental impacts overall.
At the ground level, the best decision for you will be determined by results of the 1,000-foot comparison, local climate, local availability, local building codes, local code officials, and familiarity of the local construction workforce with using the different systems.
Other things to consider:
- Better design (orientation, space planning, passive solar exposure and shading) has a huge impact on energy consumption.
- The more locally sourced a material is, the better.
- If you are using SIPs, try to get them with no added formaldehyde in the OSB sheathing.
- Look for FSC-certification in wood products (SIPs or wood framing).
- If using concrete, use fly-ash content to the percentage that works from an engineering standpoint and see if you can use recycled concrete as aggregate.
- ICFs come in many styles with varying amounts of foam versus fill concrete. It would be interesting to find out the total embodied energy and home operating energy of one with a lot of foam and less concrete fill versus the opposite.
- Consider indoor air quality. When constructed properly, all the systems will have less air infiltration and, therefore, toxins introduced into the home have fewer options for leaving the home (so they are more likely to leave through our body). Make sure that systems for adequate fresh air exchanges are built into your home and avoid bringing toxins into your home.
- Quality construction and attention to detail makes a significant difference with any system. Poor installation can ruin the environmental benefit of any system. Refer to the LEED for Homes rating system and reference guide sections on nergy and Atmosphere for addressing common building envelope energy issues.
- Quality construction and attention to detail makes a significant difference with any system. Poor installation can ruin the environmental benefit of any system. Refer to the LEED for Homes rating system and reference guide sections on Energy and Atmosphere for addressing common building envelope energy issues.