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



Comparison with Existing Methods

Since the existing methods of moisture control concern themselves exclusively with controlling winter uptake by assessing the need for a vapor retarder, we can compare the results from Requirement 1 (preventing condensation from occurring under the membrane) to the existing methods. Recall from the previous section that the roofing system containing wood fiberboard prevented condensation and therefore did not require a vapor retarder. However, the system containing foam allowed condensation to occur below the membrane, indicating the need for a vapor retarder.

According to NRCA guidelines, both roofing systems require a vapor retarder because they satisfy the requirements of the January mean temperature being less than of 40°F and the interior relative humidity is above 45%.

Following the ASHRAE protocol, we find that the winter design climatic condition for Chicago is -8°F. The dew-point for the building is 9°C (49°F). Knowing the thermal resistances of all of the roofing system components, we compute deck temperatures of 63 and 65°F for the fiberboard and foam insulations, respectively. Since the dew-point falls within the roof for both systems, a vapor retarder is required in both instances.

According to the CRREL guidelines, the roofing systems in Chicago require a vapor retarder; the CRREL map (Fig. 2) requires an indoor relative humidity of approximately 40% before a vapor retarder is required for this location.

The three existing procedures require vapor retarders for the combination of climate and interior environment specified by the example. Since none of these procedures consider the hygric performance of the roofing system components, they are incapable of discerning differences in the two examples. The proposed moisture control strategy is based on hygric performance and indicates that a system containing hygroscopic insulation materials will prevent condensation from occurring because the system can absorb greater amounts of water vapor prior to the onset of condensation.

Previous Section - An Example
Next Section - Conclusions/Future Work

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 15, 2001 by Diane McKnight