In June 2010, Alex Wilson published a ground-breaking article, in Environmental Building News. In the article, Wilson examined the implications of the fact that the HFC blowing agents used to make extruded polystyrene (XPS) and most types of closed-cell spray polyurethane foam have a much greater global warming impact than CO2. As it turns out, the negative global warming impacts of escaped blowing agents from some types of foam insulation can sometimes outweigh the positive climate impacts attributable to energy savings.
Needless to say, the article generated a great deal of discussion. (A shorter version of Wilson’s article appeared as a blog on GBA.) As a direct result of this article, XPS became the pariah of the rigid-foam world — at least for green builders. (The problem of HFC blowing agents should not be confused with , another problematic component of some types of foam insulation. Because both XPS and EPS contain HBCD, the “greenest” type of rigid foam is polyisocyanurate.)
How insulation materials influence climate change
Insulation materials can affect our climate in at least three ways: one positive way and two negative ways.
- When properly installed, all types of insulation have the potential to slow climate change. When a builder installs more insulation than the legal minimum, the building where the insulation is installed burns less fuel and uses less electricity than it would otherwise — and burning less fuel is good for the planet.
- All insulation materials have embodied energy, because fuel must be burned to power the factory where insulation is manufactured and to deliver the insulation to its final location; burning fuel contributes to global warming.
- Extruded polystyrene foam and most types of closed-cell spray polyurethane foam contain hydrofluorocarbon (HFC) blowing agents that harm the atmosphere when they are gradually released. (The blowing agent used for open-cell spray foams is water, which does not cause any climate-change concerns.) HFCs are very potent greenhouse gases — 1,430 times more potent than carbon dioxide.
When you calculate the embodied energy of an insulation material combined with the effects on the atmosphere of escaped blowing agents, you can determine the “embodied global warming potential” of an insulation.
More insulation isn’t always better
There are many reasons to install thick insulation on your house: to improve comfort for the occupant, to reduce energy costs, and to lower your contribution to global warming.
According to Wilson’s article, it’s possible to install “too much” XPS on your walls — that is, too much from a perspective of global warming. Of course, even if you install “too much” XPS, the extra insulation will still help keep you comfortable, and it will still help lower your energy costs. But if your XPS is too thick, your wall will actually have a higher global warming potential (GWP) than a wall with less XPS.
As David White, a Passive House consultant from Brooklyn, explained at the March 2011 Building Energy conference hosted by NESEA in Boston, extra XPS can negatively affect the climate. “Let’s say I have a wall insulated with an inch of something,” said White. “We know the CO2 emissions of the building, based on the heat loss through that wall. As I add insulation, the heat loss drops but never goes to zero. The first 1-inch investment in additional insulation saves more energy than 2 inches of insulation installed later. In other words, the relationship between insulation thickness and energy savings is not linear. But there is a linear relationship for the embodied carbon dioxide and the effect of the blowing agents. If you look at the total global warming impact, including embodied energy and the effect of the blowing agents, then for some insulation materials you eventually get not just diminishing returns, but negative returns.”
Although it’s possible to estimate the global warming effects of blowing agents used to make foam insulation, the effects can’t be pinned down precisely — because we don’t know that rate at which blowing agents escape from foam and evaporate. It’s well established that gas bubbles in cured foam will escape from the edges of the foam, but the exact release rate is unknown. In his article, Alex Wilson used the best available data to estimate blowing agent release rates.
A new calculator
To help designers understand the global warming impact of HFC blowing agents, David White has developed an Excel spreadsheet that automatically creates line graphs to compare the global warming impact of different insulation types and thicknesses — for any climate. “This tool emerged out of work done by Danny Harvey, who wrote a paper on the global warming potential of insulation, and work by Alex Wilson and John Straube,” White explained. “The tool was meant to help you more specifically understand the way these concerns apply to your particular building.”
White’s spreadsheet is being distributed at no cost; here’s if you want to download it.
Adjusting your assumptions
White’s calculator allows users to adjust several variables, including:
- Climate (heating degree days);
- Heating fuel;
- Heating appliance efficiency;
- Expected life span of insulation (most users assume 50 years);
- The blowing agent release rate (White suggests using the default rate, “high”); and
- The types of insulation you want displayed on the graph.
White is forthright about the calculator’s limitations. “We are not talking about any environmental impacts other than global climate change,” said White. “Nor are we talking about costs — either the cost of insulation or the cost of energy.”
White’s calculator reveals:
- For most types of insulation, including cellulose, more insulation is always better, because more insulation reduces the GWP of a building by reducing the building’s energy use. Of course, there are diminishing returns as the insulation gets thicker — but thicker insulation always reduces a building’s GWP, as long as the insulation doesn’t include HFC blowing agents.
- The exceptions to the above rule are XPS and closed-cell spray polyurethane foam. When you plot the GWP of these two types of insulation on the Y axis of a graph, with the thickness of the insulation on the X axis, you get a U-shaped curve: at first, the GWP of the insulation decreases with increasing insulation thickness, but then the curve bottoms out and starts climbing. Beyond the bottom of the curve, thicker insulation actually causes more planetary climate damage than thinner insulation.
White explained, “In many climates, if you add XPS to an existing cellulose-insulated 2×6 wall, even the first inch of XPS increases the global warming impact of the wall.”
Better blowing agents are now being developed
Most industry experts expect HFCs to be phased out within a few years. “This is really a temporary problem until the industry shifts to different blowing agents,” said Wilson. “In Europe, XPS is already made without HFCs — they use CO2 as a blowing agent — but the R-value of European XPS is only R-4 per inch instead of R-5 per inch.”
In the meantime, concerned green builders should try to avoid the use of XPS and closed-cell spray polyurethane foam with HFC blowing agents. “Polyiso has a very low global warming potential, and is a good alternative in an above-grade application,” said Wilson. “For below-grade applications — for example, under a slab — EPS is an option. Rigid mineral wool, which is very common in Europe, has zero global warming potential. Its embodied energy is similar to fiberglass, and it can be used as a drain board on the outside of foundation walls. , a mineral wool product, is now being tested under slabs.”
Last week’s blog: “Nailing Window Flanges Through Foam.”