What the V and W ratings measure
A tin of masonry paint usually carries two letters most people walk straight past: V and W. Both come from BS EN 1062-1, the European standard that rates how a coating handles moisture, and they measure two different things moving in two different directions.
V: how easily moisture escapes
The V rating describes water vapour permeability, how easily moisture already inside a wall can pass back out as vapour. V1 is the open end of that scale, the setting that lets the most vapour through.
W: how much rain gets in
The W rating describes liquid water permeability, how much rain the coating lets through from the outside. This scale runs the other way round. The low end of W is the one that keeps the most rain out, so a low number here is the one doing more work for you.
A solid wall, one with no cavity to carry damp away once it is in, generally needs both at once. Open on V, so moisture already in the wall can leave. Closed on W, so rain is not driven in to replace it. Checking a datasheet for its V class alongside its W class tells you more about how a coating will behave on a wall like this than either figure read on its own, and it matters most on the substrates covered on the page about concrete, natural stone or lime render.
V and W side by side
Both ratings appear together on the same datasheet, and they run in opposite directions, so it helps to read them side by side.
| Rating | What it measures | Which end is better | What to check on your wall |
|---|---|---|---|
| Water vapour permeability (V) | How easily moisture inside the wall can pass out through the coating | The open end lets more vapour escape | Worth checking closely on a solid wall, a lime render or an older property |
| Liquid water permeability (W) | How much driving rain the coating keeps from soaking in | The closed end keeps out more rain | Worth checking closely on an exposed elevation or near ground level |
This compares what the V and W scales measure and which direction is better, as set out in BS EN 1062-1. It does not give the numerical thresholds between classes.
Why no class numbers are printed here
BS EN 1062-1 sets out numbered classes for both the V scale and the W scale, and a datasheet will sometimes print one of those numbers next to the letter. This site does not print the thresholds that separate one class from the next.
Why the numbers are left out
The standard itself sits behind a paywall, so the class boundaries cannot be checked against the document that defines them. The free sources that do quote numbers do not agree with each other, either on the unit each class is measured in or on how many classes each scale actually has.
A number copied from a source that disagrees with the next source is not a fact, it is a guess wearing a fact's clothes. Stating the direction a scale runs in, and naming the gap where the thresholds should be, tells you something you can act on. A threshold borrowed from a source that might be wrong does not.
What to do instead
- Use the direction of the V and W scales to compare two tins.
- Where a class number is printed on a datasheet, ask the manufacturer which edition of BS EN 1062-1 it was tested against.
- Treat a class number from a retailer page as a claim to be checked against the manufacturer's own datasheet.
The substrate guide covers which end of each scale matters most for a given wall, without relying on the missing numbers in between.