Addresses the danger to paintings posed by microclimates that can form between the back of a painting and the interior wall, particularly in historic buildings which are not well insulated. In cold climates, high humidity levels can form in the zone close to these walls, even to the point of moisture condensation, while in summer elevated temperatures and low humidity pose risk factors to paintings displayed on them. These extreme microclimates can vary widely from the target controlled climates in museums and galleries, and can seriously damage artworks. The article reveals a series of damage mechanisms to canvas paintings posed by these highly fluctuating microclimates. Tensile force tests were carried out on restrained samples from canvas supports, hide glue size, lead white and Naples yellow pigments at different RH levels. The results, presented in tables, show that the greatest damage occurs at RH levels above 80%. Cycling between high and low RH levels also causes cracking and damage, while moisture condensation typically runs to the bottom of paintings and can cause failure of the glue size. Unequal reactions in oil paints to these extreme levels and fluctuations are a further source of damage.