June 2003
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A different look at polyiso

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A contractor shares his problems with polyisocyanurate insulation

by Dick Baxter
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The roofing industry has been monitoring changes in polyisocyanurate foam technology since the federal government mandated changes in blowing agents used in the manufacture of polyurethane and polyisocyanurate foam materials. The change to pentane blowing agents has raised questions about insulation values, fire resistance and dimensional stability of the foam in roof insulation. At the same time, polyisocyanurate insulation manufacturers have changed the facers on their roof insulation products. Almost by accident, my employees and I discovered facers may contribute to decreased fire resistance and increased dimensional instability of roof insulation boards.

Current facers

A recent project of ours brought the subject of roof insulation board facers into perspective. The roof assembly consisted of an intermediate rib steel roof deck, one layer of mechanically attached 25-pounds-per-square-inch (172-kPa) polyisocyanurate insulation, a polymer-modified asphalt base ply set in adhesive with heat-welded side laps and end laps, and a mineral granule-surfaced polymer-modified asphalt cap sheet set in adhesive with heat-welded laps. Roof insulation boards blown with both HCFC-141b and pentane were used on the project.

As the laps of the base ply were being heat-welded with a torch, a fire broke out under the base ply along a longitudinal insulation board joint. When the base ply was cut open, the facers on the bottom side of the insulation board were burning. The fire was extinguished, and the condition was thought to be an aberration. On the following day, fire broke out again under the base ply—in a similar location as the first fire at longitudinal insulation board joints coincident to a side lap in the base ply. Now, the condition no longer was isolated, and we notified the polyisocyanurate insulation...



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