Building Science 101: Low-carbon construction
By Michael Maines
There’s a lot of buzz these days about low-carbon construction methods among environmentally-minded designers and builders. The traditional focus on reducing the amount of energy it takes to operate a home – reducing energy bills in the process – is still important. But as awareness of the climate change crisis continues to grow, new attention also needs to be given to the materials we use to build our homes – especially their embodied carbon, a measure of the energy and emissions required to manufacture, transport and install these building materials.
When it comes to global warming emissions, “carbon” is shorthand for carbon dioxide, used as a basis to compare the effects of other carbon compounds in our atmosphere.
According to climate scientists, we don’t have 100 - or even 50 - years to wait for carbon-polluting products to recoup their environmental toll, even if they save operating energy in the process. At best we might have 30 years, but many experts say that within 10 to 15 years we need to be carbon-neutral as a society in order to mitigate the worst effects of climate change. Considering that building construction is responsible for over 40% of carbon-equivalent emissions annually, the building industry presents a clear opportunity for improvement.
Some building materials and systems come with a higher carbon toll than others, so if we as a society are going to reach our carbon reduction goals, we need to seriously consider the materials we use to build our homes. Here are some tips for building with low embodied carbon:
AVOID XPS (EXTRUDED POLYSTYRENE) RIGID INSULATION, THE FAMILIAR BLUE, PINK OR GREEN FOAM. Its blowing agents are 1,400 times worse than carbon dioxide as a global warming agent. Seek out less-bad EPS (expanded polystyrene, which is white, or sometimes gray). It’s available in dense versions, usually by special order, that meet or exceed XPS’ long-term performance. Or use polyisocyanurate (yellow foam with foil or other facers) for foundation interiors or above-grade, when necessary.
IF YOU USE CLOSED-CELL SPRAY FOAM, LOOK FOR A PRODUCT WITH LOW GLOBAL WARMING POTENTIAL. Almost all spray foam manufacturers now have at least one option using an HFO (hydrofluoroolefin) blowing agent, which is about 1,000 times better than conventional HFC (hydrofluorocarbon) blowing agents from a global warming perspective. (They have some installation and performance advantages as well.)
USE LESS CONCRETE. It requires a lot of energy-intensive heat to cook limestone at high enough temperatures to liberate the calcium oxide that is the main ingredient in “Portland cement,” the “glue” that, when combined with mineral aggregates (and sometimes other “admixtures”) forms concrete. But more importantly, the chemical process also releases large amounts of carbon dioxide. Worldwide, production of concrete is responsible for 10% of total man-made global warming emissions, with about 60% of that due to the chemical process of creating Portland cement and 40% from the fuel used. There are ways to reduce the impact of using concrete: up to 50% of the Portland cement can be replaced with fly ash, an industrial waste product, with no reduction in strength. Carbon CureTM is an admixture that injects carbon dioxide, also captured from industrial processes, into the concrete mix, which strengthens the concrete and is gaining ground across the US.
While it may seem a bit futuristic to build a home without any concrete, if you’re planning a project with a foundation, you could consider a crawlspace (aka short basement) or an insulated slab-on-grade instead. Even better, build on piers – particularly helical piers, tough metal posts drilled into the ground that can support a house or deck. While not yet widely used, they have a long, successful track record in the US and Canada.
USE MORE WOOD BUILDING PRODUCTS FOR SIDING, INSULATION, AND STRUCTURE. Wood is a low-carbon material, as long as it’s harvested sustainably; healthy forests are some of our most important allies in our fight against climate change.
Though other products have taken over the siding market, cedar shingles or pine boards are traditional and low-maintenance, especially when installed over a rain screen gap, which allows the wall and the siding to dry, resulting in much longer-lasting finishes than when they are forced to hold water in conventional installations.
Wood fiber-based insulation, such as fiberized cellulose – often called “dense-packed cellulose” – is effective and has a low carbon footprint. Today’s high-quality cellulose does not settle if installed properly. (And, it’s pest-resistant, thanks to its mineral borate additive.) Insulating an average-sized house with cellulose keeps the recycled newsprint equivalent of about 40 years’ worth of daily papers out of landfills or incinerators.
Rigid wood-fiber insulation, still relatively new to the US, is made from raw fiber, not newsprint, and has an even lower carbon footprint – some reports say it is carbon positive; the more you use, the better for the environment. It is an effective substitute for rigid foam above grade, and it’s vapor-open, usually a benefit over foam because vapor-open assemblies can “breathe,” allowing wall and roof assemblies to stay safe and dry. Wood-fiber insulation is currently imported from Europe, but plans are in the works for a plant based in Maine.
Use CLT for mid-rise buildings. Long used in Europe, CLT – or cross-laminated timber – built with sustainably-harvested softwood, can be cost-effective when replacing typical steel-and-concrete construction, particularly for 6- to 12-story buildings. Maine doesn’t have a lot of candidates, but nearby regions do, and Maine is poised to support the growth of that market from our vast forests. Coincidentally, a CLT plant is also planned for Maine.
KEEP BUILDING SHAPES SIMPLE. Simple shapes are easier to air-seal and insulate, perform better in harsh weather, and require fewer materials and less maintenance than more complicated assemblies.
IF YOU’RE ADVENTUROUS AND REALLY WANT TO BUILD IN AN ENVIRONMENTALLY-BENIGN MANNER, CONSIDER STRAW BALE CONSTRUCTION. Though not particularly high in R-value, and prone to moisture damage when not done with the right details, there are ways to build safely and effectively with straw, sequestering carbon in the process.
Prioritizing embodied energy over operating energy may be new for many of us in the high-performance design and construction fields but, assuming the vast majority of climate change researchers are correct, we need to shift to that point of view – we simply don’t have time to recover the debt created by products with large carbon footprints, even if they save operating energy in the process. Fortunately, we have the technology and the know-how; we just need a paradigm shift to make it happen quickly.