Many traditional building materials have characteristics that enable moisture as a vapour to pass through them (they are vapour permeable) as well as holding water in vapour form (they are hygroscopic or ‘moisture buffering’). Due to their pore structure these materials may also encourage the movement of water as a liquid via surface diffusion and capillarity (sometimes called ‘wicking’).

When undertaking work on an older building an appreciation of how all these moisture mechanisms function within the existing materials is required to avoid the application of an inappropriate material. A vapour permeable insulating material may be applied to an existing solid masonry wall without a damp-proof course and this will allow the assembly to continue to ‘breathe’. However, in particular circumstances, at the base of the wall or if it is particularly porous or exposed, for example, there may also be a need for the insulating material to provide hygroscopic buffering and/or to promote the capillary movement of water, as these may also be mechanisms by which the wall remains reasonably ‘dry’.

A good starting point for considerations of breathability is the vapour permeable nature of a material, this quality can be expressed in its opposite form as vapour resistance. Vapour resistance may be measured in meganewton seconds per gram (MNs/g). An example of a material with an acceptable level of permeability is 19mm thick non-hydraulic lime render which has a vapour resistance of approximately 1.0 MNs/g. As a rough guide ‘breathable’ materials that could be considered suitable for use in older buildings should have a vapour resistance of up to 2.5 MNs/g. However, water as a vapour is only one form of moisture threat to buildings and how materials perform in response to the possible presence of liquid water also needs to be considered.