Choosing Insulation for Carbon Value

Why More is Not Always Better (Part 1)

HFC blown-in SPF during the installation process. Courtesy images.

Catherine Paplin, R.A. AIA, Steven Winter Associates, Inc.

Insulation is the single most important material choice to maximize an enclosure’s thermal resistance. Combined with a good continuous air seal, the highest insulation value makes the greenest enclosure – reducing a structure’s carbon footprint. It may come as a surprise, then, that some of the most commonly used insulation materials are so carbon-heavy to manufacture and install that they wipe away the carbon savings they are supposed to provide.

Embodied Carbon, Operational Carbon, and Climate Change

Almost 40% of the total carbon dumped into the planet’s atmosphere each year is attributed to buildings. We have overwhelmingly focused on operational carbon – the carbon that buildings emit as they are being used, but embodied carbon – the carbon that goes into constructing buildings – is typically far greater than the energy saved in the first decades of operation. However, if a building stands for decades or centuries, its operational carbon will eclipse its embodied carbon over its lifetime. Therefore, when the building’s carbon Life Cycle Assessment (LCA) is calculated, operational carbon savings will be more important than embodied carbon saved and spent in the long run.

XPS Insulation Boards.

Why does embodied carbon deserve equal weight with operational carbon? Because 28% of global carbon emissions from buildings is pegged to embodied carbon–50% in the first 30 years of a building’s life. In effect, every new building is in “carbon debt” upon completion due to the huge amount of carbon emitted in order to construct it. So, for the climate to benefit from the energy savings provided by a well-insulated and sealed enclosure and a high efficiency energy system, the building needs to be durable and last for a very long time. The problem is, we cannot afford to make choices that have us paying off a carbon debt decades into the future.

All this is not to suggest shifting focus away from reducing operational carbon and primarily (or exclusively) to embodied carbon. On the contrary, the lower the operational carbon emissions, the quicker the carbon debt is paid off. Thermal insulation is therefore critical. Conversely, the lower a building’s embodied carbon, the more effective its operational efficiency really is. The two values are inseparable. When design seeks to minimize both types of emissions, the resulting building can radically reduce carbon emissions.

Thermal Value of Insulation

The primary purpose of thermal insulation is to provide resistance to heat transfer through the building enclosure, typically quantified as R-value. The higher the R-value, the better the insulating property. With respect to commercial construction in the NYC area, the three most common materials are:

1. Plastic foam (highest R-value) – sprayed or board, expanded or extruded;

2. Mineral fiber – board, batt or sprayed;

3. Organic fiber – batt or sprayed.

Embodied Carbon Value of Insulation

Unfortunately, two of the plastic foams that provide the highest R-value – closed-cell polyurethane spray (SPF-HFC type) and extruded polystyrene board (XPS) – also have the highest embodied carbon (by a lot). This is because they’re both made with a hydrofluorocarbon (HFC) blowing agent that has over 1000 times the global warming potential (GWP) of CO2.

Considering this, it’s easy to conclude that HFC blown plastics should be phased out and taken off the market, but we’ve become so dependent upon SPF and XPS, because they do so many things in addition to providing high R-value.

Insulation’s Other Values

XPS board is waterproof, serves as a vapor barrier, and if taped and sealed, is an air barrier. Therefore, this insulation type can take the place of a vapor barrier or an air barrier in a wall assembly. And, with various levels of compressive strength, it has real structural value as well. Moreover, XPS board is often used as a protection board for membranes and is the go-to insulation for foundation waterproofing due to its long-term ability to stand up to hydrostatic pressure. And, it is dimensionally stable with good R-value retention over time.

Closed-cell SPF is also highly water resistant and is an even better air barrier. When installed at a minimum 1.5” thickness, it is also a Class I vapor retarder. Sprayed on SPF can provide structural rigidity in some wall systems. It is also dimensionally and thermally stable.

None of the other insulations, even the non-HFC blown plastics, completely replicates all the characteristics of HFC blown SPF and XPS. Furthermore, plastic foams generally are highly flammable and are not the best fit for every application. So, how can we make up for the loss of the combination of helpful qualities that has made us so broadly dependent on them?

Stay tuned for Part 2 to find out.