When we winterized a summer home by spraying it top, bottom, and sides with urethane foam, there were many who wagged their heads, saying: "If that ever catches on fire, it will go up like gasoline." Yet if I apply a flame to the stuff, it burns exactly where the flame is and no farther. Isn't it true that a well-insulated house will contain most of the heat of an internal fire, rapidly build up to a tinder point, and burst into flame? R. B. Beaman Rockport, Mass.
Building codes usually have guidelines on the use of foam substances in buildings. The codes specify that a product, often gypsum board, be used over the foam so as to inhibit flame.
In terms of heat content, urethane has something like 11,000 Btu per pound and polystyrene, 18,000.
You're right in that a well-insulated house will contain much of the heat of an internal fire. Where the situation becomes dangerous, according to the National Bureau of Standards, is when openings develop in the building, such as broken glass.
Then the onrush of fresh air will fuel the fire and cause it to burn with great intensity.