Phenolic closed-cell foam comprises a hydrocarbon blowing agent and includes an alkali metal silicate in an amount of at least 1% by weight. The foam has an aged thermal conductivity as determined by the procedures of EN13166:2008 of less than 0.025 W/m·K. The foam is formed from a phenolic resole resin mixture having a water content of greater than 15% by weight but less than 24% by weight.