Image Credit: Fine Homebuilding This graph shows the thermal conductivity of polyiso at different mean temperatures. At low temperatures, the thermal conductivity of polyiso rises; that means that its performance gets worse. Note that the temperature scale refers to the mean temperature of the insulation, not the outdoor temperature. [Graph credit: Building Science Corporation]
Image Credit: Building Science Corporation This graph shows the thermal conductivity of XPS at different mean temperatures. At low temperatures, the thermal conductivity of XPS drops; that means that its performance improves. Note that the temperature scale refers to the mean temperature of the insulation, not the outdoor temperature. [Graph credit: Building Science Corporation]
Image Credit: Building Science Corporation This graph shows the thermal conductivity of a variety of insulation types at different mean temperatures. Clearly, polyisocyanurate behaves differently from all of the other insulation types shown on this graph. The temperature at which XPS starts to perform better than polyiso is about 10°C, or 50°F. At temperatures below 50°F, 1 inch of XPS will perform better than 1 inch of polyiso. [Graph credit: Building Science Corporation]
Image Credit: Achilles Karagiozes — Owens Corning Test results showing the thermal performance of 2-inch-thick samples of 4 brands of polyisocanurate at different mean temperatures. The line labeled "NRCA average" refers to the average results of tests performed by the National Roofing Contractors Association. [Credit for graph: Building Science Corporation.]
Image Credit: Building Science Corporation Results of testing of 15 samples of polyisocyanurate from 5 manufacturers, showing apparent R-value per inch at different mean temperatures. The tests were performed by the Building Science Corporation. The graph shows the highest and lowest performing examples, in order to indicate a range. The line labeled "NRCA average" refers to the average results of tests performed by the National Roofing Contractors Association. [Credit for graph: Building Science Corporation.]
Image Credit: Building Science Corporation Roxul, a manufacturer of mineral wool insulation, has published this graph comparing the performance of Roxul mineral wool insulation at various temperatures to the performance of polyiso.
Image Credit: Roxul
Researchers have known for years that most types of insulation — including fiberglass batts, extruded polystyrene (XPS), and expanded polystyrene (EPS) — perform better at low temperatures than high temperatures. The phenomenon was described by Chris Schumacher, an engineer and researcher at Building Science Corporation, at a conference in 2011: “If you measure the R-value of an R-13 fiberglass batt, you’ll get different results at different outdoor temperatures. If the outdoor temperature rises, the R-value goes down. If the outdoor temperature drops, the R-value rises. Why? Because as you move to a higher temperature, you get more radiation happening, and therefore a lower R-value. But at lower temperatures, there is less conduction, less convection, and less radiation — and therefore a higher R-value.”
Polyisocyanurate does not follow the usual pattern for other types of insulation. When tested at mean temperatures below 50°F, polyiso performs worse than it does at a mean temperature of 75°F. The reason for this declining performance, according to Schumacher, is that “the trapped blowing-agent gases start to condense at cold temperatures.”
R-value is defined by law
The standard ASTM test methods for determining a material’s R-value are performed at a mean temperature of 75°F. According to the Federal R-value Rule, the U.S. law that regulates how insulation products are labeled and marketed, R-value claims for insulation must be based on these ASTM tests. It could be argued that these test procedures tend to favor polyisocyanurate (which ends up with a labeled R-value of about R-6 per inch) over XPS (which ends up with a labeled R-value of R-5 per inch). Many builders probably specify polyiso because of its high R-value per inch, without considering the fact the the performance of polyiso suffers at low outdoor temperatures.
Achilles Karagiozis, the director of building science at Owens Corning, decided to use WUFI, a hygrothermal modeling program, to study the performance of XPS and polyiso in a variety of climates. His conclusion: in cold climates, R-5 XPS beats R-6 polyiso.
Karagiozis modeled the energy use of several wall assemblies in three cold-climate locations (Chicago, Toronto, and Minneapolis) and one hot-climate location (Miami). He looked at two different wall types (a 2×4 fiberglass-insulated wall with 1 inch of exterior rigid foam, and a 2×4 fiberglass-insulated wall with 2 inches of exterior rigid foam) and two different types of cladding (vinyl siding and brick veneer). He ran WUFI simulations (for two simulated years) for these walls, with separate software runs for two different types of rigid foam (polyisocyanurate and XPS).
Most energy modeling programs ignore the fact that the thermal performance of rigid foam varies with temperature. Karagiozis tweaked WUFI so that the program properly calculated the fact that, when the outdoor temperature drops, the thermal performance of XPS improves, while the thermal performance of polyiso gets worse.
Karagiozis obtained his information on the thermal performance of XPS and polyiso at different temperatures from Chris Schumacher, who made the measurements as part of his multi-year research into the thermal performance of walls. The graphs showing how the thermal conductivities of polyiso and XPS vary with temperature are reproduced below (see Images #2 through #6).
For more information on testing by Building Science Corporation researchers to determine the thermal resistance of polyisocyanurate samples, see Temperature Dependence of R-values in Polyisocyanurate Roof Insulation.
Karagiozis presented his findings on December 1, 2013, at the Buildings XII conference in Clearwater Beach, Florida.
“I was astonished”
I telephoned Karagiozis and asked him to explain the motivation for his study. “I listened to Chris Schumacher’s presentation at Building Science summer camp this year and last year,” Karagiozis told me. “I started asking people in the room, ‘How many of you knew about this temperature-dependency effect?’ It turned out that everybody knew about this effect but couldn’t quantify it. I asked Andre [Desjarlais], ‘Is it kind of a big effect or kind of a small effect?’ And he said, ‘It is kind of big.’ ”
Karagiozis continued, “So I looked at the ASHRAE handbook and other sources, because I wanted to translate the effect so that I could determine its impact on a building. I figured, maybe it is big, and maybe it is small. But I didn’t find what I was looking for. So I decided to integrate the Building Science measurements into the WUFI model. WUFI has been tested on hundreds of walls. It is well documented that WUFI performs very well at determining heat flux through walls. It does a really good job. So I got the results, and I was astonished by the effect this temperature-dependency phenomenon had in cold climate zones.”
Unfortunately, Karagiozis has only presented his findings in visual form, in a series of line graphs and bar graphs that compare the heat flow through XPS-insulated walls with polyiso-insulated walls. He has not yet shown how these performance differences affect the annual energy budget (either in kWh or dollars) of a typical single-family home.
The line graph below shows two (simulated) years of heat flow through one square meter of a fiberglass-insulated 2×4 wall with 1 inch of exterior rigid foam and vinyl siding in Chicago. The graph includes two lines — one for XPS, and one for polyiso. From April through October, the polyiso wall and the XPS wall perform about the same. But from November through March, the XPS wall performs better than the polyiso wall.
The bar graph below displays a two-year analysis for the same wall (a fiberglass-insulated 2×4 wall with 1 inch of rigid foam and vinyl siding in Chicago). The two years of data have been displayed as a single year, so that the two Januarys are displayed as a single month, as well as the two Februarys, and so on. The contrast between the two wall types is most striking in December.
The next bar graph (immediately below) shows the percentage increase for the heat flow of the wall with polyiso when compared to a wall with XPS. Just like the previous graph, the graph below shows a two-year analysis for a fiberglass-insulated 2×4 wall with 1 inch of rigid foam and vinyl siding in Chicago.
In December, there is about 17% more heat flow through the polyiso wall than there is through the XPS wall. In April, there is about 7% more heat flow through the polyiso wall than there is through the XPS wall. During the summer months, on the other hand, the wall with polyiso has less heat flow per square meter than the wall with XPS.
What happens if the foam is 2 inches thick?
If we consider a different wall type — a 2×4 wall with 2 inches of exterior foam instead of 1 inch of foam — then the different in performance between polyiso and XPS is even more pronounced. The graphs below show what happens to this type of wall in Chicago. (The cladding is assumed to be vinyl siding.) In the coldest month (December), the heat flow through the wall with 2 inches of polyiso is 30% greater than the heat flow through the wall with 2 inches of XPS.
Brick veneer softens the differences between the two types of foam
What happens if the house has brick veneer rather than vinyl siding? The graphs below show that the “polyiso penalty” isn’t quite as bad with brick veneer as it is with vinyl. (The graph shows the performance of a fiberglass-insulated 2×4 wall with 2 inches of rigid foam and brick veneer cladding in Chicago.)
The graphs below show the results for Minneapolis. These graphs assume a fiberglass-insulated 2×4 wall with 2 inches of rigid foam and vinyl siding.
While polyiso doesn’t perform as well as XPS in cold climates, it outperforms XPS in Miami. The graphs below show the performance of a fiberglass-insulated 2×4 wall with 2 inches of exterior rigid foam and vinyl siding in Miami.
Translating the graphs to real-world numbers
It’s good to have solid numbers on which to base our insulation specifications. Now that Karagiozis has run these simulations, cold-climate builders may need to adjust their thinking about the R-value of different types of rigid foam. In a cold climate, it may make sense to assume that the “temperature-adjusted R-value” of polyiso is about R-4.5 per inch, and the “temperature-adjusted R-value” of XPS is about R-5.5 per inch. (These numbers are for purposes of illustration; they don’t represent measured values.)
If you are a hot-climate builder who uses polyiso, Karagiozis’s simulations will be reassuring. If you are a cold-climate builder, you may be wondering whether you should switch from polyiso to XPS or EPS.
There’s a problem, however: most green builders prefer polyiso over XPS and EPS. (All brands of EPS and XPS sold in the U.S. include a brominated flame retardant — hexabromocyclododecane — that many environmentalists find worrisome. Moreover, XPS is manufactured with a blowing agent with a very high global warming potential.) Until manufacturers of XPS and EPS change the formulations of their products to make them more environmentally friendly, cold-climate builders will probably stick with polyiso, in spite of its disappointing cold weather performance.
Switching from polyiso to XPS probably won’t save you as many BTUs as you might think from a cursory glance at the graphs on this page. We don’t have firm estimates of the bottom line yet; Karagiozis hasn’t yet run any whole-building simulations to show how switching from polyiso on walls to XPS on walls would affect the annual energy budget of a cold-climate home in BTUs, kWh, or dollars.
At my request, Karagiozis calculated the annual (rather than month-by-month) differences in heat flow attributable to a switch from polyiso to XPS in the walls used for his simulations. The highest saving attributable to switching from polyiso to XPS occurs in walls with 2 inches of rigid foam in Minneapolis; it that case, the switch to XPS reduced the annual heat flow through above-grade walls by 22%. For a wall with 1 inch of exterior rigid foam in Chicago, the reduction in annual heat flow was 12%. So that’s the range: 12% to 22%.
Remember, though: this is a reduction in heat flow through the above-grade walls, not a reduction in annual energy use. It’s safe to assume that the heat loss and gain through above-grade walls represent between 25% and 40% of the heat loss and gain through a building’s thermal envelope.
That means that if a cold-climate builder switches from polyiso to XPS, the occupants are likely to see annual heating and cooling energy savings of about 3% to 9%. (The highest figure — 9% savings — assumes Karagiozis’s best case for savings in Minneapolis, for a home with above-grade walls that are responsible for a higher-than-usual percentage of the annual heat loss and heat gain thorough the building envelope. Most homes won’t see such a high percentage of savings.)
Polyiso manufacturers haven’t released the data we need
It’s important to mention one other caveat about Karagiozis’s graphs: they are all based on the performance of a single (unidentified) brand of polyiso tested by researchers at the Building Science Corporation (BSC). Every time I have asked BSC researchers to identify the brands of the insulation tested in their labs as part of their multi-year Thermal Metric project, they have declined to do so, noting that the information is confidential.
There is no way of knowing whether the temperature-dependency curve of the polyiso brand tested by BSC researchers is typical or not. Other brands of polyiso might perform a little better or a little worse at low temperatures than the brand of polyiso used to produce the graph that launched Karagiozis’s modeling exercise.
When I broached this issue with Karagiozis, he told me, “I believe that most insulation manufacturers do a range of temperature testing. That information should be public knowledge. The manufacturers should disclose that.”
It’s time for designers and builders to pressure all U.S. insulation manufacturers. We should gather in a group in front of their corporate headquarters, chanting, “Free all political prisoners! Release the temperature-dependency graphs!”
When in doubt, make your insulation a little thicker
Karagiozis’s simulations confirm the validity of the advice I gave back in 2011, when I wrote, “Many energy-savvy builders are aware that the performance of some insulation types can be degraded by 20% at very cold or very hot temperatures. If you care about this problem, the solution is fairly simple: just install thicker insulation.” If you want your wall to perform well at all temperatures, it never hurts to make the insulation a little thicker than the amount that might have been selected based on the product’s R-value label.
[Author’s postcript: As Dana Dorsett points out in Comment #9 below, the “polyiso penalty” may have significant consequences for cold-climate builders who want to specify an adequate thickness of exterior rigid foam to keep their wall sheathing above the dew point during the winter. Builders falling into this category may want to specify EPS foam instead of polyisocyanurate.]
Martin Holladay’s previous blog: “All About Attic Venting.”