IN PREPARATION OF THIS CHAPTER, THE CONtents of the first edition were drawn upon. The author acknowledges the coauthors of the first edition, Catherine Langlais and Anne Silberstein. The current edition will review and update the topics that were addressed by the previous authors, introduce new technology that has been developed, and include up to date references. Predicting the thermal performance of insulating materials in the presence of moisture is a complex problem. It is possible to define the apparent thermal conductivity of a moist insulant, but measuring this conductivity is very difficult because moisture flow and phase changes affect heat transfer. On the other hand, it is important to distinguish carefully between moisture effects in service and those in laboratory conditions (during a test). Two options are possible: 1. Solve the complete set of equations to evaluate the effects of moisture. 2. Apply some kind of correction to the dry thermal conductivity.