Moisture/condensation
Another consideration when installing highly reflective roofs in cold climates is the potential for moisture damage from condensation. It is important to note a properly installed code-compliant roof system should not have any issues with moisture damage. Most of the cases of damage described in the literature can be explained by installation errors and do not indicate a systemic problem with the use of cool roofs.
Over the course of a year in seasonal climates, moisture can build up in roof systems during cool periods and dry out during warm periods. This process occurs regardless of roof color. Research indicates that although cool roofs may take a little longer to dry out than dark roofs, they also fully dry out, resulting in no net moisture buildup over yearly weather cycles. As the U.S. Department of Energy has noted in reference to the potential which has been observed in both cool and dark roofs in cold climates, “the authors are not aware of any data that clearly demonstrates [sic] a higher occurrence in cool roofs.” This finding is borne out by field tests of a large number of buildings as well as through model-based research.
A roofing manufacturer and a prominent retail store chain undertook a field performance evaluation of single-ply white roofs in service for 10 to 14 years on 26 stores located in cold climates. Two test cuts were made on each roof (and three test cuts on one roof) for a total of 53 samples, and moisture was present in two of them. One was determined to be the result of leakage from a nearby HVAC unit, and the other showed no signs of staining, mold, or deterioration which would indicate a long-term moisture problem.
In another case, researchers simulated the performance of several roofing systems—including typical, smart, and self-drying roofs for residential and commercial buildings in very cold climate regions—and found office buildings did not experience moisture accumulation problems during the simulation period (five years) using WUFI software modeling. The “smart roof” features related to venting and vapor retardation are covered in International Green Construction Code/American Society of Heating, Refrigerating, and Air-Conditioning Engineers (IgCC/ASHRAE) 189.1, Standard for the Design of High-Performance Green Buildings Except Low-Rise Residential Buildings. In a paper presented at the 2011 National Roofing Contractors Association (NRCA) International Roofing Symposium, the Single Ply Roofing Industry (SPRI) reported on a field survey and modeling studies to verify whether cool roofs were, in fact, susceptible to condensation issues. The study was designed to achieve the greatest likelihood of observing condensation within the roofs. The roofs studied all consisted of a white roof membrane (aged two to 12 years) mechanically fastened over a single layer of insulation on a steel deck without a vapor retarder. The roofs were surveyed during February and March 2010 and were located in ASHRAE Climate Zone 5. Two test cuts were done on each of the roofs. All cuts were done in the morning, to minimize the impact of any heating of the roof surface which might have occurred under the afternoon sun. In seven of the roofs, there was no evidence of any moisture in the assembly. Though moisture was observed on the top face of the insulation and/or the underside of the membrane on three roofs, researchers noted no detrimental effects due to moisture in any of the roofs. WUFI modeling was performed for the 10 roofs included in the study, with simulations conducted for both a black and a white surface in each case. Although the modeling results showed all of the roofs would be subjected to condensation in the winter months, it predicted higher levels of condensation below a cool white membrane than below a black sheet. However, in all cases, for both white and black membranes, the modeling showed the resulting moisture would dry out completely in the summer months.
Cool roofs for the future
Despite ample bad news and dire predictions about the present and future climate, there are solutions available to meet these challenges. An abundance of cool roofing materials is readily available on the market, and at little to no incremental cost, depending on the product type. Building enclosure consultants, architects, specifiers, contractors, and builders can use the Cool Roof Rating Council’s (CRRC’s) online Products Directory to identify roofing products which meet a variety of needs, including:
- Complying with local energy code requirements and/or green building certifications.
- Qualifying for financial incentives (e.g. rebates, tax incentives, property assessed clean energy [PACE] efficiency financing).
- Decreasing indoor air temperature and increasing occupant comfort.
- Lowering the building’s cooling demand and utility bills.
- Reducing energy peak demand.
- Helping to mitigate the UHI effect by lowering ambient and surface temperatures.
Each of the products featured on the directory has a CRRC rating which indicates the radiative performance (solar reflectance, thermal emittance, and solar reflective index [SRI]) of the roofing product. Knowing the radiative performance of roofing products is important for understanding the roof’s impact on building energy use, occupant comfort, and the surrounding environment.
Author
Kurt Shickman is the former executive director of the Global Cool Cities Alliance, an organization that partners with Chemical Fabrics and Film Association (CFFA) to share the latest information on urban heat islands and cool roofing research and policy. The vinyl roofing division of the CFFA was created to educate architects, specifiers, building owners, and roofing contractors on the attributes of polyvinyl chloride (PVC) as a durable, reflective, heat-weldable material for single-ply roofing systems. Representing all the leading manufacturers of thermoplastic PVC (vinyl) roofing systems in North America, the division is committed to making available sound, scientifically backed information on the environmental and functional performance of energy-efficient PVC (vinyl) roofing membranes.
