Are There Downsides to Reflective Roofs in the North?
Cool roofs have many benefits: most important is the reduction in the air-conditioning load on buildings, which can save a significant amount of money. The concept sounds like a win-win situation, but are there any downsides to installing cool roofs? In Part 1, I examined the so-called “northern heating penalty” and showed it largely doesn’t exist. There are many reasons that non-reflective roofs don’t contribute to overall energy savings in northern climates. In this second part, I’ll examine the claim made by some industry members that white roofs contribute to moisture build-up in roofing systems.
During the 1960s, a concept known as the self-drying roof was promoted. It was assumed by most roofing professionals that condensation of interior humidity on the underside of roofing membranes did occur during winter months in the North. However, during warmer, summer months, that moisture was assumed to dry out. Reflective roofs are cooler than darker non-reflective membranes, so maybe condensation under the membrane doesn’t dry out during summer months. Let’s look at the arguments:
Do reflective roofs lead to moisture build-up?
- For approximately two decades, Target Corporation has installed, almost exclusively, white PVC membranes on their stores. Fenner, DiPietro, and Graveline did a case study and investigated the roofs on 26 stores. These were located throughout the states of Connecticut, Illinois, Massachusetts, Michigan, Minnesota, New York, Washington, and Wisconsin. They took test cuts on each roof and found that, apart from some minor leaks, there was no evidence of any moisture build-up or damage. The cuts were all done in August and September, and showed that had any condensation been present, it had been dried out toward the end of the summer.
- A team from the Single Ply Roofing Institute (SPRI) and Oak Ridge National Laboratory (ORNL) did a survey of ten reflective roofs located in climate zone 5. In seven of the cases, no moisture or damage to the polyisocyanurate foam insulation or facer was noted. In three of the cases, condensation was apparent on the backside of the highly reflective membrane. Minimal damage had occurred to the polyiso foam insulation, which was described by the observer as “stained,” and in one case “wrinkled,” but still laminated to the insulation. The foam itself was dry, with no rust on the steel roof deck. The conclusion from the study found that although there were signs of moisture condensation in three of the 10 roofs observed, minimal effect had occurred to the roofing assembly that would affect its integrity, insulating value, or performance. No detrimental effect to the roof systems was noted.
What are the other factors that can lead to moisture build-up?
- Dr. Jim Hoff, vice president of research for the Center for Environmental Innovation in Roofing in Washington, D.C., and president of TEGNOS Research Inc., a consulting organization dedicated to expanding the understanding of the building envelope, has written about condensation in roof systems for the design professional. In his view, moisture condensation in roofs is a relatively rare phenomenon that only occurs in some unusual circumstances:
- Extremely cold external temperatures.
- Extremely high internal temperatures and humidity.
- Unusually low amounts of above-deck roof insulation.
- Unusually high levels of air movement within the roofing system.
Recommendations to reduce the risk of moisture build-up
So, even though moisture build-up is rare and only happens in unusual circumstances, there are several common-sense approaches you can take to limit the movement of moisture up into the roof system:
- Specify roof insulation to meet current code requirements. This helps to move the dew point down into the insulation layers, where airflow is close to zero.
- Use essentially non-porous insulation. For example, polyiso is preferred compared to mineral wool.
- Use two layers of insulation, like polyiso, with staggered joints.
Impact of membrane attachment method
Mechanically attached single-ply membranes can be prone to fluttering in strong winds. This is shown in the following diagram:
Even if double layers of insulation are installed in this type of system, the membrane fluttering brings building interior air up to below the membrane. When outdoor temperatures are low, this can cause condensation. In contrast, fully adhered systems prevent billowing and really limit the amount of interior air in contact with the membrane.
By making sure the insulation is installed as a double layer with staggered joints, it can be improved even further as shown here:
In this situation, the interior air can be kept away from the coldest parts of the roof system, and in some cases below the dew line.
The clear conclusion is that very reflective roof membranes do not cause a northern heating penalty or lead to moisture build-up. However, design of the roof system for Northern locations should take into account measures to limit exposure of interior air to the underside of the membrane. It is important to use the services of a design professional whenever there are concerns about moisture within a building.
In Part 3 of this series, I’ll examine the role of air barriers in a roof system.
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