Disclosed is a building board with increased heat and fire resistance. Boards constructed in accordance with this disclosure have lower densities while at the same time containing larger amounts of anti-shrinkage materials. In one embodiment, a board was constructed with a density of less than 1,750 lbs/msf while at the same time including anti-shrinkage additives in amounts greater than 14% by weight of the core material. The inventors have discovered that beneficial, and heretofore unexpected, levels of fire resistance can be achieved by using increased amounts of anti-shrinkage additives in lower density building boards.