But a little bit of investigation has forced me to rethink this. Polyurethane foams can indeed spontaneously combust, but only for a short while after they have been applied. The exotherm from the reaction can generate quite a bit of heat and if the insulation is thick enough, that heat can have a difficult time dissipating leading to a fire. Such a scenario has been blamed for 3 house fires in Massachusetts as well.
But it should be pretty clear that spontaneous combustion is only a plausible cause shortly after the polyurethane foam has been sprayed. To blame a fire on it after 24 hours or more would be nonsense. A reacted polyurethane is just that - reacted, and it will not react any further.
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