The borate treatment also gives cellulose the highest (Class I) fire safety rating.
In the so-called "Big Burn" demonstration conducted in 1978 in the USA the ceiling of the fiber glass-insulated structure collapsed 21 minutes into the burn. The ceiling of the cellulose-insulated “house” finally collapsed an hour and ten minutes after the fires were ignited. In just over two hours the fiber glass-insulated building was reduced to smoking rubble.
At the three hour mark all four walls of the cellulose building were still standing and the fire had essentially burned itself out.
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