Gwynne Anthony 2020

ART506 Dissertation 2019. Anthony Gwynne . 14 3.0 Background 3.1.0 Vapour permeable (‘breathable’) construction 3.1.1 Breathability of traditional buildings In the context of building materials, and with reference to traditional and historic buildings, the generic term ‘breathable’ is used to describe the ‘vapour permeability’ of the building fabric: the extent to which building materials are able to transmit moisture as a liquid and as a gas (water vapour). Breathable materials should be sufficiently permeable to enable absorbed and surface moisture to evaporate during ambient drying conditions. Loss of moisture from the material needs to occur rapidly enough to ensure that there is no accumulation of moisture which can lead, over time, to significant deterioration of building fabric, development of mould growth, infestation, or other form of decay. A breathable material, by definition, cannot be impermeable and should not trap moisture (www.spab.org.uk ). 3.1.2 Moisture movement mechanisms 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 (absorbtion as a liquid via surface diffusion and capillarity (sometimes called ‘wicking’) (www.spab.org.uk ). 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’ (www.spab.org.uk ).