Moisture Control in Low-Slope Roofing: A New Design Requirement

A.O. Desjarlais and J.E. Christian, Oak Ridge National Laboratory
N. A. Byars, University of North Carolina Charlotte




Moisture Control Strategies Presently Employed

Proposed Moisture Control Strategy

Developing the Algorithms

Using the Algorithms

An Example

Comparison with Existing Methods

Conclusions/Future Work



Moisture Control Strategies Presently Employed

The only moisture control strategy that is presently used when designing low-slope roofing systems is to prevent condensation in the insulation layers of the roofing system. The National Roofing Contractors Association (NRCA) Roofing and Waterproofing Manual [2] describes three procedures to determine the need for the addition of a vapor retarder to the roofing system. These procedures are described in detail in Reference [3] and are summarized below.

For many years, the NRCA has maintained that vapor retarders should be considered when the outside average January temperature is below 40°F and the expected winter indoor relative humidity is 45% or greater. Figure 1 depicts the areas of the United States that experience an outside average January temperature below 40°F.

The American Society of Heating, Ventilation, and Air Conditioning Engineers (ASHRAE) Handbook of Fundamentals [4] is cited as the second source for vapor retarder criteria and recommends the addition of a vapor retarder if the dew-point falls within the insulation layer.

The U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) developed a series of maps of the United States to assess "progressive" and "seasonal" wetting of roofing systems [5]. In a survey, roofing professionals were asked to select which map best represented their experience and the map shown in Figure 2 with a seasonal wetting vapor drive of 2.0 kPamonth (0.6-in. Hgmonth) was selected [6].

The preceding procedures for determining the roofing system moisture control strategy have limitations. The NRCA procedure does not consider the dynamic conditions created by weather and completely ignores the roofing system itself as having an impact on moisture control. The ASHRAE guideline treats design conditions as if they are steady-state, and considers only the thermal (moisture properties are not considered) performance of the roofing system components in it's moisture control strategy. The CRREL analyses, by far the most sophisticated of the three procedures, required industry "calibration" to account for the dynamic nature of moisture flow driven by meteorological conditions and the omission of solar effects on roofs. It does not include consideration of the fact that the components of the roofing system can have an impact on the moisture control strategy.

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Building Envelope Research
Oak Ridge National Laboratory

For more information, contact the program manager for Building Envelope Research:

André O. Desjarlais
Oak Ridge National Laboratory
P. O. Box 2008, MS 6070
Oak Ridge, TN 37831-6070

E-mail Andre Desjarlais

Revised: May 26, 2004 by Juanita Denton