How breathable are your paints?

Tŷ-Mawr ‌‌ posted this on 25 May 2017

Firstly the word ‘breathable’ is not very helpful as buildings don’t actually breathe! – We tend to use the term vapour permeable.

Measurements of vapour permeability. The Society for the Protection of Ancient Buildings has compiled the following advice:-

A good starting point 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.

The vapour permeability of building products.
If a manufacturer or supplier makes claims regarding the vapour permeability of a particular product it is reasonable to request that they provide details to validate this performance. As a minimum, this could be requested in the form of an MNs/g measure of vapour resistance. (This value is relatively easily obtained either via materials testing or can be converted from other measures of vapour permeability. Its suitability can be assessed using the scale provided in the preceding paragraph). In the case of multi-layered materials, the vapour resistance of all layers together including adhesives must be provided to give an accurate indication of its vapour permeability. The ability, or inability, of the seller to provide this information may well indicate the seriousness of their claims regarding vapour permeability. In the absence of this basic information, you may wish to consider whether the use of the particular material or system in question is prudent in an older building.

The m-value (“mu-value”) of a material is also known as its “water vapour resistance factor”. It is a measure of the material’s relative reluctance to let water vapour pass through and is measured in comparison to the properties of air. The m-value is a property of the bulk material and needs to be multiplied by the material’s thickness when used in a particular construction. Because the m-value is a relative quantity, it is just expressed as a number (it has no units).

You might see the reluctance of a material to let water vapour pass through expressed as an “equivalent air layer thickness”, which is usually represented as sd. As its name suggests, the equivalent air layer thickness is measured in metres.

SD values:

Ty-Mawr limewash 0.01

Graphenstone Paint 0.02

Beeckosil Mineral Paint 0.02

Clay Paint 0.02


MNs/g values – based on a thickness for 2 coats of 200µm:

Ty-Mawr limewash 0.05

Graphenstone Paint 0.1

Beeckosil Mineral Paint 0.1

Clay Paint 0.1