A useful example can be found with a large warehouse or light industrial building in a climate with hot summers. If the lighting and equipment loads are moderate, ventilation requirements are minimal, and there are few windows, then much of the cooling load will be associated with gains through the roof assembly. When the roof insulation is exposed to higher temperatures (as would be typical under a solar-heated roof surface), it delivers lower thermal performance (i.e. more heat gain) than expected based on the label R-value. This is true for all types of insulation material.
At the exterior surface the R-value might be reduced by 20 per cent. However, over the thickness of the roof insulation the average reduction in R-value might only be 10 per cent since the exterior layers protect the inner layers by keeping them closer to the indoor temperature.
The corresponding increase in heat flow would result in an approximate 10 per cent increase in energy consumption related to the roof assembly. Whether or not this has a significant impact on building operating costs will depend on the specific climate, the building construction and operation, and various other interrelated factors (e.g. thermal mass and equipment efficiency).
It was stated earlier most materials exhibit a decrease in R-value for hot temperatures, and an increase in R-value for cold temperatures. It seems obvious to ask whether any unexpected increase in energy consumption during warm weather is offset by unexpected reductions in consumption during cold periods. Again, the net performance will depend on specific climate, building construction, operation, and other issues. All the relevant factors (including insulation temperature dependence) can be accounted for using appropriate computer models (e.g. EnergyPlus and WUFI-Plus).
Even in those cases where the summer loss in performance is offset by the winter bonus, there may be other building performance considerations. Several design questions might be considered: will the brief reduction in R-value have a meaningful impact on the required HVAC system capacity? If not, does it result in interior surface temperatures that adversely affect thermal comfort?
Further, temperature dependence does not always result in better performance under colder temperatures. The tested polyiso insulation materials exhibited lower than expected R-values at higher and lower temperatures. For some time, polyiso board insulation has been the most commonly used low-slope roof insulation. In these applications, it is the only insulation in the assembly—as a result, the thermal performance is less than expected during both winter and summer conditions.
Polyisocyanurate is also increasingly being used as a continuous exterior insulation over insulated stud spaces in residential and commercial wall assemblies. In these applications, the thickness of a continuous insulation is typically specified to:
- minimize the impact of thermal bridging through the framing; and
- reduce the potential for air leakage condensation by controlling condensing plane temperatures.
The former is an energy consideration while the latter is a building durability concern.
For a practical illustration, a residential wall assembly with a 2×4 wood frame with RSI-2.29 (R-13) fibreglass batt insulation in the stud space and 19 mm (3⁄4 in.) of polyiso insulation on the exterior provides a nice example.
Assuming the polyisocyanurate insulation is rated as RSI 1.06/25 mm (R-6/in.), without accounting for temperature dependency, if the wall is subjected to conditions of 22 C (72 F) on the indoor side and –18 C (0 F) on the outdoor side, then the temperature at the condensing plane (i.e. the inside surface of the polyisocyanurate) is predicted to be –8.5 C (17 F), as illustrated in Figure 7.
In contrast, if it is assumed the polyiso exhibits a temperature dependence similar to that shown in Figure 6, then the predicted condensing plane temperature will be –12.1 C (10 F) as illustrated in Figure 8. In this case, the temperature dependence of the material is particularly significant because the entire thickness of the insulation is on the assembly’s cold side. That is to say none of the temperature-sensitive insulation is protected by itself or another material. To bring the condensing plane temperature back to the values originally expected, the thickness of the polyiso exterior insulation would need to be be increased to 38 mm (11⁄2 in.), as illustrated in Figure 9.
Specifying more insulation is also a good option when designing roof assemblies using polyisocyanurate. A good rule of thumb for both roofs and walls is to use NRCA’s recommendation to specify polyisocyanurate insulation by its desired thickness—not its label R-value. Ideally, the thickness would be specified on the basis of annual energy simulations and hygrothermal calculations using a measured temperature-dependant R-value like that illustrated in Figure 6. When material-specific, temperature-dependant R-values are unavailable, designers will have to make some assumptions. For polyisocyanurate roof insulation materials, NRCA recommends using an in-service R-value of 5 per inch thickness (i.e. RSI-0.88/25 mm) for heating-dominated climates and 5.6 per inch thickness (i.e. RSI-0.99/25 mm) for cooling-dominated climates.
Another option is to use a hybrid insulation approach. Adding a layer of less-temperature-sensitive insulation outboard of the polyiso, protects the polyiso from extreme temperatures and gets the most value from both insulation layers. An example hybrid assembly is shown in Figure 10.
Temperature dependence can result in assemblies that do not function as expected or intended. In the case of those materials exhibiting strong temperature dependence, the consequences could be significant. Fortunately there are solutions, and as knowledge of this phenomena increases, more solutions will no doubt be developed.
Christopher Schumacher, M.A.Sc., is a principal with Building Science Consulting Inc. (BSCI), a consulting firm specializing in design facilitation, enclosure commissioning, forensic investigation, and training and communications. Its research division, Building Science Laboratories (BSL), provides a range of R&D services. Schumacher’s presentations on temperature-dependent R-values include the Westford Building Science Symposium in 2011 and the Rock-toberfest Rockwool Symposium in 2014. He has also written on this topic for buildingscience.com. He can be reached by e-mail at firstname.lastname@example.org.