The use of polyurethane foam insulation boards in low-sloped roof constructions has increased in recent years to upgrade their thermal efficiency. Built-up membranes applied directly over polyurethane foam insulation may experience greater incidence of blistering than membranes applied on other roofing insulation boards. The bubbling of hot asphalt during its application to the polyurethane insulation board can cause voids in the asphalt layer, which eventually can grow into blisters. This paper presents the results of a study to investigate the effect of application of hot asphalt directly onto polyurethane insulation boards. In addition, it discusses causes of blistering and methods for using polyurethane foam insulation boards in built-up roofing systems so as to reduce the risk of blistering.

Polyurethane foam insulation specimens, typical of those used in the late 1970s in the United States for built-up roofing construction, were included in the study. Most of the insulations contained a facing material. Hot asphalt was applied to the insulations with facings to determine the extent of off-gassing. Hot asphalt was also applied to insulations without facings, to facings removed from insulation, and to oven-dried insulations and facings to investigate the effect of the moisture content of the insulations and facings on the off-gassing.

In other tests, oven-dried and moist (“room dry”) felts were used in the preparation of built-up membranes over polyurethane boards. Delamination of the roofing specimens indicated many large voids in the asphalt layer between the polyurethane insulation and the first ply of felt. These voids were considered to increase the risk of blistering in built-up membranes applied directly on the polyurethane foam insulation. The data support previous suggestions to apply a thin layer of rigid insulation to the polyurethane foam before the membrane is applied.