Key concepts in sustainable building - The Hempcrete Topic: Designing and Building with Hemp-lime - page 79

Traditional building materials allow moisture to pass through, keeping the structure in good 'health'.
The significance of the building fabric being vapour permeable, or 'breathable', is that
rather than moisture being kept out, as we try to do in modern construction, with damp-
proof membranes (DPMs) and synthetic vapour barriers, it was free to pass in and out of
the building fabric as water vapour. (Note that a material being permeable to water vapour
does not mean it is necessarily permeable to liquid water. Modern vapour-permeable mem-
branes - 'breather' membranes - for example, prevent the transmission of liquid water.)
It was important for the moisture content in walls to remain below a certain level, for the
thermal performance of the wall (through its thermal mass, see page 63 ) to be maintained.
However, unless something had gone wrong, appropriate moisture levels were maintained
passively by the permeable nature of the materials, coupled with the fact that the living
space was heated with a wood fire: the two combining to ensure that moisture wasn't
trapped inside the fabric of the building, where it could cause structural damage.