One word, two scales, opposite directions
Plenty of tins print the word breathable, and the word by itself is not a measurement. A manufacturer who backs the claim with a figure is using one of two different scales, and the two run in opposite directions.
- Vapour resistance, stated in MNs per gram. Here a lower number means the coating lets more moisture through.
- Water vapour transmission rate, stated in grams per square metre per day. Here a higher number means the coating lets more moisture through.
A tin that carries neither figure is making a claim that can't be checked against another tin on the same shelf. Two products can both say breathable and behave quite differently once the wall behind them is wet through, which is the condition the coating actually has to deal with over a winter.
The same confusion sits inside the ratings printed under BS EN 1062-1, where the V and W scales also run in different directions for different properties. The two ratings on the tin page sets those out separately.
Why this matters on a solid-walled or older building
A solid wall, one built without a cavity, has no gap behind the outer face to carry moisture away before it reaches the inside. Whatever damp arrives in the brick, stone or render, whether it is driving rain from outside or condensation and rising damp from inside, has to pass back out through the wall itself. That is what makes the coating on that wall a working part of the building.
Where the 2.5 figure comes from
The Society for the Protection of Ancient Buildings (SPAB) sets a guideline figure of up to 2.5 MNs per gram for vapour resistance on this type of wall. Vapour resistance is measured in MNs per gram, and a lower number means the coating lets more moisture vapour pass through it. SPAB's figure is a ceiling: a coating above it is resisting more vapour movement than a solid wall of this kind is built to tolerate.
What happens when a coating sits above that figure
A coating that traps moisture does not fail at the paint film first. The water stays in the masonry behind it, and over repeated wet and cold spells that can mean frost damage to the face of the brick or stone, render parting from the wall, and decay in timber lintels or joist ends that sit embedded in the wall. On a lime render in particular, the render exists to let the wall breathe, and a vapour-resistant coating over it works against the material, which is covered on the page about concrete, natural stone and lime render.
Before choosing a coating for a solid-walled or older property, it is worth putting the manufacturer's stated vapour resistance, in MNs per gram, against this figure.
What to ask when a tin just says "breathable"
A tin that says "breathable" without a number is asking you to take its word for it. The word describes an effect, not a measurement, and two coatings both labelled breathable can let through very different amounts of moisture.
What you need from the manufacturer or the trade counter is the figure behind the word, in one of the two forms actually used for this property.
What to ask for
- The vapour resistance figure, stated in MNs per gram. On this scale a lower number means more vapour gets through.
- Or the water vapour transmission rate, stated in grams per square metre per day. On this scale a higher number means more vapour gets through.
These two are not interchangeable, and no published way of converting one into the other has been found. Asking which of the two the manufacturer is quoting, rather than accepting a figure with no unit attached, tells you whether you are even comparing like with like against another tin on the shelf.
The follow-up question
Ask which standard or test method the figure was measured against, and whether the datasheet carries a date. A figure with no test behind it is really just the same word in different clothing.
It is also worth asking whether the coating has been tested on the substrate you actually have. A figure measured on dense concrete does not tell you how the same coating behaves on a lime render, and that is the substrate where the wrong answer costs more than the finish. For how to tell which one you are dealing with, see Concrete, Natural Stone or Lime Render. The V and W classes printed on some tins measure something different again, and that scale is covered separately on The Two Ratings on the Tin.