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



Proposed Moisture Control Strategy

The proposed moisture control strategy addresses those issues covered by the existing guides as well as topics that were previously not considered. Although the majority of moisture control problems pertain to roof leaks, none of the existing moisture control strategies address this issue. A further enhancement obtained by the proposed moisture control strategy is that the physics of the moisture control problem is treated more rigorously so that conclusions regarding the roof design can be drawn with more confidence. Reference [3] contains a substantial discussion regarding the proposed moisture control strategy. The proposed moisture control strategy can be summarized as follows:

Under normal operating conditions (no leaks), the total moisture content of a roof system shall not increase with time (Requirement 1) and condensation shall not occur under the membrane during winter uptake (Requirement 2). Moisture vapor movement by convection must be eliminated and the flow of water by gravity through imperfections in the roof system must be controlled. After a leak has occurred, no condensation on the upper surface of the deck shall be tolerated (Requirement 3) and the water introduced by the leak must be dissipated to the building interior in a minimum amount of time (Requirement 4).
The strategy contains four quantifiable and two qualitative requirements. The first two quantitative requirements echo those introduced by Tobiasson [5]. If the total moisture content of the roofing system is increasing on a yearly basis ("progressive" wetting or Requirement 1), then eventually condensation must occur in the roofing system. Additionally, we do not want to allow condensation to occur within the insulation layers of the roofing system during winter uptake ("seasonal" wetting or Requirement 2) because of the deleterious effects water has on the thermal and mechanical performance of roofing systems.

Through proper roof design and selection of materials, it may be possible to eliminate drippage into the building interior from small to moderate leaks (Requirement 3). Dripping manifests itself as condensation on the interior surface of the deck. If the rate of water vapor being driven to the deck or the deck permeance can be controlled to prevent condensation onto the deck, dripping from roof leaks into the building can be eliminated.

After (if?) all of the above criteria are satisfied, the roofing system shall be optimized to dissipate leak water into the building interior through downward drying as expeditiously as possible (Requirement 4). Any water entering the roofing system will begin to degrade the thermal and physical properties of the insulation, deck and metal components, and we therefore want to minimize their exposure time to the leak water.

The two qualitative requirements pertain to other mechanisms of moisture transfer in roofing systems that cannot be readily quantified. Moisture can move through roofing systems by air movement or infiltration and gravimetrically through cracks between insulation boards and imperfections in the insulation layer. These moisture movement mechanisms must be eliminated since copious amounts of moisture can be transferred by either of these mechanisms. The control of moisture movement by infiltration is typically satisfied for low-slope roofing systems that have fully adhered membranes. The complete attachment of the membrane to the outermost surface of the insulation prohibits the transfer of low pressures induced by winds blowing over the roofing system through the membrane. In the absence of a pressure difference, air movement through the insulation layer is typically negligible. In roofing systems that employ a loose or mechanically-attached membrane that can transfer outdoor air pressure, precautions to eliminate air movement must be considered. An air-tight deck or the addition of an air retarder should be considered.

Gravimetric flow of moisture from a roof leak to the deck through imperfections in the insulation layer must also be controlled to prevent leakage into the building interior; multiple layers of insulation or the use of a continuous layer of an absorptive material is proposed in situations where direct communication between the leak and the deck is suspected.

We have used finite difference computer modelling to demonstrate the effectiveness of moisture-tolerant roof designs in several different climate zones in the US [1]. In this paper, we offer a simpler, readily available technique for assessing the suitability of different moisture-tolerant roof designs. Algorithms, based on a large database of computer simulations, have been produced that can predict the quantifiable moisture control design requirements.

Previous Section - Moisture Control Strategies Presently Employed
Next Section - Developing the Algorithms

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