Prepare extruded polymer foam comprising at least 50 weight-percent alkenyl aromatic polymer, a brominated flame retardant providing 0.8 to 1.4 weight percent bromide, 0.05 to 0.5 weight-percent hydrobromic acid scavenger, at least 1.5 weight-percent graphite and less than 0.1 weight-percent C—C and O—O labile organic compounds using a blowing agent composition comprising at least 0.3 weight-parts per 100 weight parts polymer and that consists of blowing agents having an ozone depletion potential of zero and a global warming potential that is less than 1000.