Use And Care Manual
Q: Since the R-value of polyiso is severely compromised in cold temperatures,
how does polyiso ci compare to XPS ci in a typical wall assembly?
A: Together with Building Science Corporation (BSC), the Owens Corning building science team used WUFI, a well-respected
hygrothermal modeling program, to analyze the performance of XPS and polyiso in a variety of U.S. climates using BSC
measurements. Three cold-climate locations in Chicago, Toronto, and Minneapolis were measured along with one hot-climate
location in Miami.
Exterior wall types (each with brick veneer cladding) were evaluated, including:
• 2x4’ wood-framed, berglass-insulated wall with XPS continuous insulation
• 2x4’ wood-framed, berglass-insulated wall with polyiso continuous insulation
Using the test data from BSC, the Owens Corning building science team ran WUFI simulations to take into account the actual
thermal conductivity behavior of polyiso at different temperatures.
The WUFI results for Chicago (Figure 3) are expressed in terms of heat loss and heat gain on a month-by-month basis. As
shown, both XPS and polyiso wall assemblies perform about the same during the summer. However, in the winter months, it’s
clear that walls built with XPS perform better than those constructed with polyiso. On an annual basis, the heat flow results
show that 2 inches of XPS is 21 percent more efficient than polyiso (1 inch of XPS is 12 percent more efficient than polyiso).
This testing conclusively found that the XPS (with a published R-value of 5.0 per inch) performs 30 percent better than the
polyiso (with a published R-value of 5.6-6.0 per inch) at 50°F mean temperatures. In fact, measurements taken in Chicago
during peak cold temperatures in December found that a wall with two inches of polyiso ci enabled 30 percent more heat flow
through the wall versus the same wall with two inches of XPS.
Figure 3. WUFI results for Chicago climate featuring heat flows
(R10 board for both XPS and polyiso)
35%
30%
25%
20%
15%
10%
5%
0
-5%
-10%
1 2 3 4 5 6 7
Month
8 9 10 11 12
XPS vs Polyiso
500
0
-500
-1000
-1500
-2000
-2500
1 2 3 4 5 6 7
Month
8 9 10 11 12
Heat Loss, Btu/ft
2
Polyiso
XPS
Loss
Gain
Heat Flows - XPS vs. Polyiso
Difference in Heat Flow -
Polyiso Heat Loss or Gain vs. XPS