Observations from Recent Research and Field Studies

Although frequently discussed in recent research and reports, moisture accumulation in roofs appears to be a relatively rare phenomenon tending to occur only in the presence of one or more severe conditions. These severe conditions may include:

• Extremely cold external temperatures. It is possible that such extreme temperatures may exist only in the northernmost areas of North America.
• Extremely high internal temperature and humidity. The observations of the complete lack of moisture accumulation in 26 roofs over Target stores suggests that actual winter humidity in typical retail, office, and warehouse occupancies may be significantly lower than assumed in several recent moisture modeling studies.
• No insulation or a single layer of roof insulation above-deck. The Dregger (2012) study of uninsulated roofs in California certainly suggests that some level of above-deck insulation should be used with any single-ply roofing membrane. In a similar manner, the Hutchinson (2009) reports of condensation associated with only a single layer of roofing insulation may suggest that two or more staggered layers of insulation should be installed under all single-ply roofs.
• Unusually high levels of air movement within the roofing system. Some of the modeling studies reviewed in this article suggest that high levels of air movement within a roofing system will contribute to moisture accumulation. However, recent NRC (2014) air barrier testing of mechanically attached singleplies suggests that modeling tools may easily overestimate the amount of air infiltration affecting the roof. As a result, they also may overestimate the potential for moisture condensation in roofs.

In addition to this brief review of the key conditions that may contribute to condensation and moisture accumulation in most if not all roof assemblies, it would be important to restate the benefits and risks of vapor retarders within roofing systems. Although a well designed and installed vapor retarder can remove many of the concerns associated with moisture accumulation, they add new risks of allowing moisture accumulation from roof leaks and may severely limit important self-drying roof properties.

Recommendations for the Design Professional

Based on the review of moisture movement in roofing systems and the tools available to assess and mitigate roof condensation, the following strategies are recommended for the concerned design professional:

• Establish a conservative but realistic estimate for actual interior humidity conditions within the building. Although underestimating humidity conditions may lead to moisture accumulation within a roofing system, overestimating anticipated humidity conditions may unnecessarily point to the need for a vapor retarder and the loss of self-drying roof benefits.
• Limit unnecessary air movement within the roofing system—both in the design of the roof and in the quality of roof installation. The bad news here is that poor design and installation practices—like installing only a single layer of roof insulation and failing to close obvious gaps at penetrations and transitions in the roof assembly—will cause a high level of unwanted air infiltration that may lead to moisture accumulation. But the good news from the recent NRC (2014) study is that mechanically attached single-ply roofs using reinforced membranes significantly reduce air movement— even to the point that they could be considered air barriers by current building code criteria.
• Include a minimum amount of above deck thermal insulation beneath the roofing membrane. The Dregger (2012) study of uninsulated roofs in California suggests that some minimal level of above-deck roof insulation should be installed with every cool single-ply roofing system, even in moderate climates. Dregger further suggests that above-deck insulation be installed even when not required by the building code.
• Allow whenever possible the capability of the roof to “self-dry” in the event condensation does occur. It’s not that often that building designers are given a free gift like the self drying roof concept, and it should not be discarded without considerable analysis.
• Select roofing materials with low perm ratings and high resistance to moisture damage. As illustrated in Table 1, there are many roofing materials available with low perm ratings, especially critical roof insulations that must resist moisture for many years. Using low perm materials can reduce the consequences of moisture accumulation in the event other factors fail to remain within your design assumptions.
• Use modeling tools, but recognize their limitations. In the case of the simplest tools, recent research clearly suggests that the answers they provide will almost always be overly conservative. In the case of more sophisticated tools like WUFI, it is vital that the designer understand the underlying assumptions and adapt these assumptions to actual expected conditions.

Endnotes

1. Additional information about WUFI including a free software download may be obtained from Oak Ridge National Laboratory at http://web.ornl.gov/sci/btc/apps/moisture/ index.html

References

• ASTM E96-14 (2014). Standard Test Methods for Water Vapor Transmission of Materials. ASTM International.
• Bludau, C., Zirkelback, D., and Kunzel, H. M. (2008). Condensation Problems in Cool Roofs. Proceedings of the 11th International Conference on Durability of Building Materials and Components. Istanbul, Turkey, May.
• Desjarlais, A., Petrie, T. W., Childs, P. W., and Atchley, J. (1998). Moisture Studies of a Self-Drying Roof. Proceedings of Thermal Performance of Exterior Envelopes of Buildings VII. Atlanta, GA: ASHRAE
• Dregger, P. (2012). Cool Roofs Cause Condensation: Fact or Fiction? Western Roofing, January/February.
• Ennis, M. and Kehrer, M. (2011). The Effects of Roof Membrane Color on Moisture Accumulation in Low-slope Commercial Roof Systems. Proceedings of the 2011 International Roofing Symposium, Washington, DC, July.
• Fenner, M., DiPietro, M., and Graveline, S. (2014). Assessing the Performance of Cool Roofs in Northern Climates. Roofing Contractor, October.
• Griffin, C. W., and Fricklas, R. L. (2006). Manual of Low- Slope Roofing Systems, 4th Ed. New York: McGraw Hill.
• Hutchinson, T. (2009). Challenging What’s Cool. Eco- Structure, May.
• Lstiburek, J. (2004). Insulations, Sheathings and Vapor Retarders. Building Science Corporation Research Report No. 0412. Available http://www.buildingscience.com
• Melton, P. and Yost, P. (2014). Go with the Flows: The Promise and Peril of Hygrothermal Modeling. Building- Green, April.
• NRC (2014). Evaluation of Air-Leakage Properties of Seam- Fastened Mechanically Attached Single-ply and Polymer Modified Bitumen Roof Membrane Assemblies. National Research Council Canada (NRC) Report A1-004336.1, September 8.
• Saber, H. H., Swinton, M. C., Kalinger, P., and Paroli, R. (2011). Hygrothermal Simulations of Cool Reflective and Conventional Roof. Proceedings of the 2011 International Roofing Symposium, Washington, D.C., July.
• Tegnos Research (2012). Low-Slope Commercial Roofing : 2000-2010. Available http://tegnos.org/files/2001to2010Market_Report.pdf
• Tobiasson, W. (1994). General Considerations for Roofs. Moisture Control in Buildings. ASTM International Manual 18.

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