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



An Example

Two roofing systems are examined. The systems are identical, except for the insulation material. Both are analyzed for the climate in Chicago with an indoor relative humidity of 50% and indoor temperature of 68°F. Both have a black outer membrane and a tight metal deck. Fiberboard and foam systems are analyzed. Insulation thickness is 3 inches for both.

Requirement 1:
Both systems pass requirement 1, since the H of Chicago is 6151 which is less than or equal to 8992.

Requirement 2:
For both systems: H = 6151
P = 0.64 English perms
r = 0.1
hr = 0.5
T = 3 inches
Rd = 1.56 reps
For 3-inch fiberboard:
For 3-inch foam:
Ri = 0.071 reps
Ri = 1.39 reps
Substituting these values into Eqns. 1, 2, and 3 yield:

pvm = 0.110 psi
t = 6.4 months
pvi = 0.171 psi
These results are common for both systems. Then, using Equation (4), for fiberboard:
m = 0.215 (6.4)(0.171-0.110)/(0.211 +1.56 + 0.071)
m = 0.046 lb/ft2
The failure threshold for fiberboard is 0.20 lb/ft2. Since 0.046<0.20 lb/ft2, this system passes. Using Equation (4) again, this time for foam:
m = 0.215 (6.4)(0.171-0.110)/(0.211 +1.56 + 1.39)
m = 0.027 lb/ft2
The failure threshold for foam systems is 0.012 lb/ft2. Since 0.027>0.012 lb/ft2, this system fails.

Requirement 3:

For the fiberboard system, the condition for failure is not met, so this system passes.

For the foam system, the vapor resistance ratio is calculated:

Ri/Rd = 1.39/1.56 = 0.9.
Since 0.9 < 1.5, we fail.

Requirement 4:

Using Eqns. 7 and 8 to determine the relative time to dry for the fiberboard and foam systems:

t = 3 months (for fiberboard)
t = 7 months (for foam)
The fiberboard system dries more quickly than the foam system, but both dry in less than the maximum of twelve months.

The fiberboard system passes all four requirements and therefore represents an acceptable design for moisture control for this roofing application. The foam system would likely see condensation at the roof membrane in the winter and represents a poor design for moisture control in this case.

Previous Section - Using the Alorithms
Next Section - Comparison with Existing Methods

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