Module 1 · Moisture, Air and Materials

1.1 Vapour Pressure — What Actually Moves Water

9 min read

Almost every wrong damp diagnosis starts with an unexamined idea about what makes water move. "It's drawing damp in" and "the wall needs to breathe" are both gestures at a mechanism nobody states. This lesson states it: water vapour moves down a vapour pressure gradient, and liquid water moves under capillary and gravitational forces. Those are different systems with different rules, and confusing them is how a surveyor ends up sealing the surface a wall was drying through.

Learning objectives

By the end of this lesson you will be able to:

State the driving force for vapour movement and distinguish it from liquid transport.
Calculate vapour pressure from temperature and relative humidity, and read the gradient across a wall.
Explain why a warmer, wetter room drives vapour outward even in winter.
Predict where a vapour gradient will deposit condensate inside a construction.

Key terms

Vapour pressure: The partial pressure exerted by water vapour in a mixture of air and vapour, in pascals. The quantity that actually drives vapour movement.
Saturation vapour pressure: The maximum vapour pressure air can sustain at a given temperature. Rises steeply — roughly doubling every 10°C in the range buildings operate in.
Vapour resistance: A material layer's resistance to vapour flow, in MN·s/g. The product of its vapour resistivity and its thickness.
Dew point: The temperature at which a given vapour pressure becomes the saturation vapour pressure. Below it, vapour condenses.

Take a room at 20°C and 60% RH, with an outside air temperature of 5°C and 80% RH. The instinct is that the inside is 'wetter' at 60% than the outside at 80% is 'drier', so moisture must be coming in. The instinct is wrong, and the arithmetic shows why.

Saturation vapour pressure at 20°C is about 2339 Pa; at 5°C it is about 872 Pa. So the room sits at 0.60 × 2339 ≈ 1403 Pa, and the outside air at 0.80 × 872 ≈ 698 Pa. The vapour pressure inside is double the pressure outside. Vapour is being driven outward through the fabric, hard, for the whole of a British winter — and it is the fabric's ability to pass that vapour, and the temperature it meets on the way, that decides whether it leaves harmlessly or condenses inside a wall.

Relative humidity told you nothing useful here on its own. Vapour pressure told you the direction and the strength of the drive. That is the difference this lesson is about.

Two transport systems, two sets of rules

A building moves water in two fundamentally different ways, and a diagnosis that mixes them up will specify the wrong repair.

Vapour transport — water as a gas, diffusing down a vapour pressure gradient through porous materials and through air leakage. Slow through masonry, fast through gaps.
Liquid transport — water as a liquid, moving under gravity, under pressure, and by capillary suction through connected pore networks. Orders of magnitude faster than diffusion where the pores allow it.

The practical consequence: a wall can be simultaneously drying by vapour diffusion outward and wetting by capillary suction from below. Measuring one and specifying against the other is a common and expensive error.

It also explains why 'breathability' is a claim about vapour resistance, not about liquid water. A lime render is vapour-open and still sheds rain. The two properties are independent, and a product that conflates them in its marketing is worth reading sceptically.

Reading a gradient across a construction

To work out where vapour is going, you need three things at each plane in the construction: the temperature, the vapour pressure, and the saturation vapour pressure at that temperature. Condensation occurs wherever the actual vapour pressure would have to exceed the saturation vapour pressure.

The method, in the order you would actually do it on a wall:

Establish internal and external temperature and RH. Convert both to vapour pressure.
Work out the temperature at each material interface from the layer thermal resistances — the temperature drop across each layer is proportional to its share of the total resistance.
Work out the vapour pressure at each interface the same way, using vapour resistances instead of thermal resistances.
Compare the vapour pressure at each interface with the saturation vapour pressure for that interface's temperature.

Where the two lines cross, you have interstitial condensation. This is the Glaser method, and its limitations are as important as its result — it assumes steady state, ignores capillary redistribution and ignores the moisture already stored in the materials. It is a screening tool, not a verdict, and Module 6 covers what to use when it is not enough.

Why the insulated wall is where this bites

Insulation does not create moisture. What it does is move the temperature profile, and therefore move the plane where the saturation vapour pressure falls below the actual vapour pressure.

Insulate a solid wall on the inside and you make the masonry colder, because it now sits outside the insulation. The vapour pressure driving outward has not changed; the temperature it meets has dropped. A wall that dried adequately for a century can start condensing at the insulation/masonry interface, where nothing can see it and nothing can dry it.

This is the single most consequential piece of physics in retrofit, and it is why PAS 2035 requires a moisture risk assessment rather than a U-value calculation. It is developed properly in Module 6; you need the mechanism now because you will meet its symptoms in Module 4.

Working the numbers without a spreadsheet

You will not carry psychrometric tables onto site, but you should be able to sanity-check an instrument. Two approximations are worth memorising:

Saturation vapour pressure roughly doubles for every 10°C rise between 0°C and 30°C. At 0°C it is about 611 Pa, at 10°C about 1228 Pa, at 20°C about 2339 Pa, at 30°C about 4243 Pa.
Dew point is approximately the air temperature minus (100 − RH%) ÷ 5, for RH above about 50%. At 20°C and 60% RH that gives 20 − 8 = 12°C, against a true value of 12.0°C.

The second of those is the one you will use most. If a surface is at 11°C in a room at 20°C and 60% RH, you do not need an instrument to tell you it will be wet — the dew point is 12°C and the surface is below it. Being able to say that on site, out loud, is what separates a diagnosis from an observation.

Common mistakes to avoid

Treating relative humidity as though it were an amount of water, and concluding that 80% outside is 'wetter' than 60% inside. Convert to vapour pressure before comparing air at two temperatures.
Assuming winter vapour drive is inward because it is wet outside. In a heated British building the drive is outward for almost the whole heating season.
Using a Glaser calculation as a verdict rather than a screen. It assumes steady state and ignores capillary redistribution, which is precisely what makes many real walls survive a result that says they should not.
Specifying a vapour-closed finish on the inside of a wall that is also being wetted from outside or below, trapping liquid moisture that previously left as vapour.
Talking about 'breathability' as if it described resistance to rain. It describes vapour resistance, and a material can be open to one and closed to the other.

Lesson summary

Vapour moves down a vapour pressure gradient and liquid moves by capillarity and gravity — two systems, two sets of rules, and most misdiagnoses live in the gap between them. Converting temperature and RH into vapour pressure tells you the direction and strength of the drive, which relative humidity alone never does. In a heated British building that drive is outward through the winter, and anything that changes the temperature profile — insulation above all — moves the plane where it condenses.

Next: why relative humidity is a ratio rather than a quantity, and what that means for every logger you deploy.