Polyurethane and polyisocyanurate rigid foams for roofing insulation

1. A polyurethane foam having a density less than 1.8 pounds per cubic foot and having a NFPA 101 Class B rating in accordance with the requirements of ASTM E-84 and which passes an FM 4450 calorimeter Test comprising the reaction product of a foam forming composition comprising:

a) at least 55% by weight based on total weight of foam-forming composition of an organic polyisocyanate,

b) an isocyanate-reactive composition comprising at least one polyether polyol or polyester polyol with a nominal hydroxyl functionality of at least 2.0,

c) a blowing agent composition comprising:

(1) greater than 0 to no more than 10% by weight, based on total weight of foam-forming composition, of one or more hydrocarbons having an LEL less than 2% by volume in air,

and

(2) greater than 0 and no more than 5% by weight, based on total weight of foam-forming composition, of a hydrocarbon having an LEL greater than 2% by volume in air,

and

(3) greater than 0 and up to 1% by weight, based on total weight of foam-forming composition, of water,

and

d) at least one halogen-free flame retardant;

and wherein the polyurethane foam-forming composition contains no halogenated flame retardant or halogenated blowing agent.

2. The foam of claim 1 in which the one or more hydrocarbons having an LEL less than 2% by volume in air comprises n-pentane.

3. The foam of claim 2 in which the hydrocarbon having an LEL greater than 2% by volume in air is methyl formate.

4. The polyurethane foam of claim 1 , with the proviso that the polyurethane foam-forming composition contains no ammonium polyphosphate.