The general restoration of the Kunsthistorisches Museum in Vienna and the new air-conditioning system in the picture gallery caused the reconsideration of the paintings hanging on the outside walls. These pictures, either in direct contact with the wall or placed too near the outside walls, are more directly exposed to daily and seasonal cycles of warming and cooling than paintings on inside walls. Pictures on so-called cold walls suffer from condensation and all the effects of increased humidity between wall and painting. On the surface itself the lower temperature causes capillary condensation, which means that water in liquid form is deposited on boundary surfaces. This causes varnish to bloom or to blanch; even delicate layers of paint show the blanching effect known as Schleißheim disease. Beyond that, nearly all deformative mechanical processes in canvas paintings are due to increasing humidity. Systematic measurement of surface temperatures of walls and paintings, performed for the first time in 1990, prove that the cold wall problem is by far more dramatic than was assumed. From the point of view of preventive conservation, the cause of deterioration should be treated first: it is to avoid condensation. The prevention of damage due to humidity is discussed subsequently.