The subject invention provides an open-celled polyurethane foam formed from a polyurethane spray foam system that mixes a formulated resin component with an isocyanate component. The resultant open-celled polyurethane spray foam has a density of about ½ pound per cubic foot. The resin component includes water as a blowing agent, a first aliphatic, amine-initiated polyol having a number-average molecular weight of from 150 to 500 and having tetra-functionality, and a second polyol having terminal hydroxyl groups and having a number-average molecular weight of from 3500 to 8000. A curing component having at least one primary amine group and having a number-average molecular weight of from 150 to 5000 is also included in the resin component. The spray foam system reacts a) the resin component and b) the isocyanate component in a volumetric ratio of a) to b) of from 1:1.2 to 1:5 and having an isocyanate index of from 15 to 70.