In Part 1 (see issue #60 of Green Energy Times), we discussed XPS (extruded polystyrene) board and closed-cell polyurethane spray foam insulation, which are produced with blowing agents (HFC, hydrofluorocarbon-based) that put more carbon into the air during construction than they save during building operation. If we don’t use these insulations, however, what do we use?
The answer in many cases is new materials. Honeywell developed a new blowing agent, a hydro-fluoro olefin (HFO), and manufacturers (e.g., Demilec and Carlisle) are coming to market with a closed-cell polyurethane spray foam that uses this agent. These spray foams have an excellent R-value and performance qualities, as well as air and vapor barrier capability, conformance to irregularities and penetrations, etc. However, they also exhibit downsides – high flammability, potential off-gassing post-application, and they are petroleum products.
Soy-based plastic spray foams (open- and-closed cell) are lower-carbon, less toxic alternatives. These are water-blown and do not off-gas. However, soy foams are made with urethane, so they are petroleum products. The closed-cell versions do act as air and vapor barriers, but they have a lower R-value. To date, these foams are only used in the residential single-family market and there is some controversy over their long-term dimensional stability.
Several expanded plastic insulations are blown with low- or zero-GWP (global warming potential) agents. Foil facing or vacuum wrapping has been used with these (as well as with mineral and glass fiber insulation) to provide a vapor barrier. So far, nothing replicates HFC-blown XPS insulation board, particularly for subgrade applications. Will it be possible to develop an HFO-blown version of XPS board? Can mineral or organic fiberboard materials be wrapped or treated to perform as effectively as XPS?
Mineral fiber (wool), available in board, batt, and sprayable form, is a possibility. These products are fire-resistant, routinely used as part of firestopping assemblies; can achieve the same compressive strength as XPS board; are unaffected by water; and, can be used in layers to create continuous insulation on the exterior of buildings (beneath cladding or stucco). However, mineral fiber insulations are not air or vapor barriers.
Batts, either fiberglass or mineral fiber, are less useful because of their inefficiency. For example, in metal framing, batt garners 40% of its nominal R-value at best. Moreover, mineral fiber is made from rock and fiberglass is silica-based – both are energy-intensive to manufacture with high embodied carbon costs.
Organic fiber-based insulations (wood, hemp, straw) have been used in single-family and smaller commercial buildings. The El Dorado is that with responsibly managed forestry or crop growth and low- or no-fossil fuel burning processing and transportation, these materials could be carbon negative (removing carbon from the atmosphere). This is a line of development to be pursued, although organics are not applicable to commercial construction. Insulation developed from recycled and reused materials, such as post-consumer cellulose, denim and plastic are also in this category.
Doing the Math
Lots of options, but without accounting for air and water barrier effectiveness, there is a potential for disaster, primarily in the form of condensation and consequential mold growth. Understanding the complex interaction of materials and systems and the hidden dangers and best options for any condition is key. Carbon Footprint Analysis (a sub-set of comprehensive environmental, or Life cycle assessment, LCA) accounts for carbon emissions associated with a building or material from cradle to grave. Building professionals can take advantage of computer applications, such as Athena, LCA One-click, Tally, and most recently EC3, to assess and compare the carbon value of building materials and processes. Considering relationships allows us to make better decisions at design conception, inception and throughout design and construction.
The Greenest Building is the One That’s Already Built
Whenever we build new, we design assemblies and choose from a variety of materials.
Yet, most new buildings – even operational net-zero buildings – rack up carbon debt, which is compounded if an existing building is destroyed to make way for the new. To quote Carl Elefante, “The greenest building is the one that is already built.” From an LCA/embodied carbon point of view, the logical conclusion is that conservation and reuse of existing buildings should be prioritized.
Older buildings represent an opportunity for carbon savings, especially if they are retrofitted to lower their operational carbon. HFC-blown XPS insulation products may be the best solution because of their unique combination of thermal, moisture and air management benefits. Not demolishing a building and not building new may actually offset the carbon expenditure of the insulation material. Tools for quantifying embodied carbon are increasing our understanding of embodied carbon and our decision-making processes.
We Can Do It!
It is easy to become overwhelmed by the enormity of climate change, but we should take a moment to recognize how far we have come, what we have achieved, and how much we already have put in place. It is a matter of persistence, heart and will – we can do it.
Catherine Paplin is a Senior Building Enclosure Consultant for Steven Winter Associates, Inc.