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 in low-slope roofing has been a long-standing issue for the roofing industry. Many materials traditionally used in roofing construction are highly hygroscopic, allowing substantial quantities of moisture to be built into a new roof. Membrane and edge detailing failures due to aging, workmanship, or roof misdesign permit water to enter the roofing system, potentially compromising the energy efficiency and the service life of that portion of the building envelope. The roof is exposed to a wide variety of environmental conditions that are governed by local weather and the building interior. Combinations of these conditions can cause moisture to migrate from the building interior into the roofing system.

It is estimated that energy losses through roofs in the U.S. are increased by 70% because of the loss of insulation's thermal resistance due to moisture contamination. Wet roofing must be replaced at significant cost, both financially and in terms of increased construction waste [1]. Clearly, the potential cost savings of a moisture-tolerant and energy-efficient roofing system is great.

The existing moisture control strategies that the roofing industry utilize are concerned exclusively with moisture flow into the roofing system when the roofing system is performing properly. Most often, we require a waterproof membrane be placed on the climate side of the roofing system to prevent water from penetrating into the insulation layers and deck below, yet we are unconcerned about the inevitable leak that will allow water this access. We perform condensation (or dew-point) analyses that dictate whether a vapor retarder is needed to control moisture pickup from the building interior during wintertime, yet we know that these analyses include simplifications that impact the preciseness of their predictive capabilities. When our dew-point analyses indicate that a roofing system needs a vapor retarder, we know that the vapor retarder can compromise the long-term performance of the roof by trapping leak water in the insulation layers. Today, we simply accept this compromise.

In this paper, we propose new moisture control guidelines for low-slope roofing systems. These guidelines consider the impact of wintertime control of moisture as well as the performance of the system after a leak has occurred. A new technique for assessing winter moisture uptake based on computer modelling is proposed and compared to the existing procedures. Procedures to evaluate leak prevention as well as rapid dissipation of leak water into the building interior as water vapor are discussed. The use of these new design tools are described and illustrated through examples.

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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