Two units, two directions, no conversion between them
Vapour behaviour on a coating's datasheet turns up in one of two forms. One is vapour resistance, given in MNs per gram, where a lower number means the coating lets more moisture pass through. The other is water vapour transmission rate, given in grams per square metre per day, where a higher number means the same thing. Both describe how easily a wall can breathe through the paint. They just don't agree on which direction is better, and no published conversion between the two has been found.
That's the problem when you're standing with two tins and one quotes MNs per gram while the other quotes grams per square metre per day. You can't turn one figure into the other to see which coating is actually more open. The only way to compare them properly is to get both figures stated in the same unit, from the same manufacturer, for the same product. A datasheet that quotes only one of the two is giving you half the comparison.
This is a separate question from the V and W classes printed on a tin under BS EN 1062-1, which measure vapour and liquid water permeability on their own scales again. Those are covered on The Two Ratings on the Tin, and worth checking alongside whichever of these two figures your datasheet actually gives you.
Vapour resistance versus water vapour transmission rate
These are the two quantities you will meet when a tin or a datasheet talks about breathability, and they are not the same scale read two ways.
| Quantity | What it measures | Which way is better | Where you'll see it |
|---|---|---|---|
| Vapour resistance (MNs/g) | Resistance the coating offers to water vapour passing through it | Lower figure means more breathable | Found in guidance for older and solid-walled buildings |
| Water vapour transmission rate (g/m²/day) | Amount of water vapour that passes through the coating over a day | Higher figure means more breathable | Found on manufacturers' datasheets for masonry paint |
No published conversion between these two quantities has been found, so a figure given in one cannot be checked directly against a figure given in the other.
A guideline figure for lime render
The Society for the Protection of Ancient Buildings sets a guideline of up to 2.5 MNs per gram of vapour resistance for coatings on older buildings. On this scale, a lower number means a more breathable coating, so the guideline is a ceiling.
The same body gives non-hydraulic lime render at 19 millimetres a vapour resistance of approximately 1.0 MNs per gram. A render at that thickness is already well within the 2.5 MNs per gram guideline on its own, before any coating goes on top.
Using the two figures together
Checking a manufacturer's vapour resistance figure against 2.5 MNs per gram tells you whether a coating fits the guideline for the wall as a whole. The render's own figure of approximately 1.0 MNs per gram shows how much of that allowance the wall is already using, which is useful if you are also weighing up which substrate you are actually working with before choosing a coating.
What the conservation bodies ask for
The Society for the Protection of Ancient Buildings doesn't back one coating over another and names no product. What it asks for instead is a figure: the vapour resistance of whatever you're putting on the wall, stated by the manufacturer that makes it. That figure is what tells you whether the coating will let the wall keep drying out.
A gap worth naming
Historic England also publishes guidance on coatings over lime render, but its position hasn't been read for this site, so nothing here is drawn from it. If you're working on a listed or older solid-walled property, that's a second source worth checking directly. For help working out which substrate you're actually dealing with first, see Concrete, Natural Stone or Lime Render.
Where a figure like this matters, ask the supplier for it in writing before you buy.