Moisture in the Roofing Assembly

Moisture trapped within a roofing assembly can be a significant cause of insula­tion board and membrane detachment in adhered roofing systems. Adhered roofing assemblies rely on adhesive to bond insulation boards to the substrate and to each other, and to bond the roofing membrane to the insulation. The or­ganic facers on insulation materials make up a significant portion of the princi­pal bonding surface within the assembly. Organic facers are sensitive to moisture, deteriorate when wet, lose cohesive strength, and can fail in service with minimal uplift pressure well below the designed uplift pressure for the as­sembly. Our empirical observations and computer modeling identified moisture within the roofing assembly as a primary cause of roofing failure in the three cases described above, as well as at numerous other buildings that we have investigated.

After placement, a concrete roof deck or topping slab slowly releases mois­ture as it cures and dries. Concrete must be allowed to sufficiently dry before roofing materials are installed in order to prevent detrimental residual moisture within the roofing assembly. Roof deck configurations often limit drying to the interior, and it can take months for concrete several inches thick to dry suffi­ciently through the concrete top surface to reduce the risk of roofing assembly failure due to trapped moisture. Our field measurements and computer model­ing in the above-noted case studies indicated that the average relative humidity of the concrete roof deck substrates was likely higher than is acceptable prior to roofing assembly installation, and the concrete substrate was the primary source of residual moisture within each roofing assembly.

A vapor retarder installed on top of a roof deck limits vapor diffusion into the roofing assembly on roofs with significant seasonal periods with a vapor drive to the exterior (as in most of the United States). A properly selected and in­stalled vapor retarder will limit the flow of residual moisture from a concrete deck into a roofing assembly, where it will become trapped after roofing mem­brane installation. The use of a vapor retarder also limits vapor diffusion into the roofing assembly from other sources. A vapor retarder might be required in order to separate roofing materials from sources of moisture or allow the instal­lation of roofing materials over substrates with excess residual moisture to ena­ble timely installation of the roofing assembly.

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Other sources of moisture within the roofing assembly can have a negative effect on the performance of a roofing system. Case 3 included a leak into the as­sembly from a cooling tower, which increased moisture within the assembly and exacerbated the deterioration of the insulation facers and the loss of adhe­sion. Other sources of increased moisture in the roofing assembly can include leaks under the assembly during construction or the installation of wet roofing materials.

For case 1 and case 2, we also noted air flow into the roofing assemblies dur­ing our investigations. Although additional roofing assembly moisture load via vapor diffusion from the building interior through the roof deck was likely mini­mal, the moisture load and condensation potential within the roofing assembly likely increased from moisture-laden air from the building interior entering the roofing system. Consequently, in addition to a vapor retarder, a continuous air barrier might also be required as part of the roofing assembly (i. e., air seals at the building perimeter wall to roof deck transition, roof deck penetrations, etc.) in order to limit the flow of moisture-laden interior air into the roofing system. A continuous air barrier installed as part of the roofing assembly will also limit exterior air flow into the roofing system.

Each of the case studies presented above also included defects in the appli­cation of the insulation board adhesive that contributed to the roofing failures, as noted in our previous work. We concluded that both the moisture within the roofing assembly and insulation board adhesion defects were independently sufficient to cause detached insulation boards and roofing membrane. The resulting roofing assemblies were unable to resist reasonably anticipated wind forces, and they required repairs.