How to Calculate Dew Point and Condensation Risk: UK Guide

Air holds a limited amount of water vapour, and that limit rises sharply with temperature. Warm air at 20 °C holds far more moisture than cold air at 5 °C. The dew point is the temperature you have to cool a given parcel of air down to before it becomes completely saturated and water starts to drop out. A room at 20 °C and 60 percent relative humidity has a dew point of about 12 °C. Cool any surface in that room below 12 °C and the air touching it gives up its moisture as liquid water.

That is the whole mechanism behind a misted bathroom mirror, a wet window in January, and the damp patch behind the wardrobe on an outside wall.

Takeaway: the dew point is just the temperature at which the air in your room runs out of room for water.

The standard way to get the dew point from temperature and humidity is the Magnus formula. Using the World Meteorological Organization coefficients, a = 17.62 and b = 243.12 °C:

γ = ln(RH ÷ 100) + (a × T) ÷ (b + T)

Dew point = (b × γ) ÷ (a − γ)

For a room at 20 °C and 60 percent humidity, γ works out to roughly 0.87, and the dew point comes out at about 12.0 °C. Push the humidity up to 70 percent and the dew point climbs to around 14.4 °C, because the wetter air needs less cooling before it saturates. This is why a steamy bathroom fogs surfaces that a dry bedroom never would.

Takeaway: raising humidity raises the dew point, so the same cold wall becomes a condensation surface the moment the room gets damp.

Knowing the dew point is only half the job. The other half is the temperature of the surface the air lands on. A wall's inside face is always colder than the room air in winter, and how much colder depends on its U-value. The worse the U-value, the colder the surface, and the closer it drifts toward the dew point.

You can predict that surface temperature. For condensation and mould work, BS EN ISO 13788 uses an internal surface resistance of 0.25 m²K/W, which is deliberately higher than the 0.13 you use for a heat-loss U-value. The higher figure represents a sheltered corner or behind furniture, the coldest realistic spot rather than the open average:

Surface temperature = internal temp − (U-value × 0.25 × (internal temp − external temp))

Takeaway: use 0.25 for a condensation check, not 0.13; the point is to find the coldest believable surface, not the typical one.

Take a common UK problem: a solid nine-inch brick wall in a Victorian terrace, no insulation, U-value around 1.5 W/m²K. It is a cold January evening in Manchester, outside temperature 3 °C. The bedroom sits at 20 °C, and with the door shut and someone sleeping in it, humidity is 65 percent.

First the dew point of the room air, at 20 °C and 65 percent: about 13.3 °C.

Now the wall's inside surface temperature:

Surface temp = 20 − (1.5 × 0.25 × (20 − 3)) = 20 − 6.4 = 13.6 °C

The surface sits at 13.6 °C, just above the 13.3 °C dew point. By the strict test, no liquid condensation forms. Homeowners see no dripping and assume the wall is fine.

It is not fine. The humidity right at that 13.6 °C surface works out at roughly 97 percent. Anything above 80 percent for a sustained period is enough for mould, and 97 percent is a black-mould factory. The temperature factor, fRsi, is 1 − (1.5 × 0.25) = 0.625, well below the 0.75 benchmark UK dwellings are expected to meet under BRE guidance.

Takeaway: this wall passes the no-dripping test and fails the mould test badly; the two are not the same thing.

Line that wall with 60 mm of PIR insulated plasterboard, a Celotex or Kingspan board from a merchant like Travis Perkins, and the U-value drops to roughly 0.30 W/m²K. Rerun the surface temperature:

Surface temp = 20 − (0.30 × 0.25 × 17) = 20 − 1.3 = 18.7 °C

The surface now sits at 18.7 °C, miles above the 13.3 °C dew point, and surface humidity falls to about 70 percent, safely under the 80 percent mould line. The fRsi rises to 0.925. Same room, same weather, completely different outcome, and you knew it would work before lifting a board.

Takeaway: insulation fixes condensation by warming the surface, not by blocking water; that is why a vapour-open wall can still be bone dry.

The calculation above covers surface condensation, the kind you can see. There is a second kind, interstitial, that forms inside the wall where warm vapour pushing through the layers meets a cold plane and condenses out of sight. It is more dangerous because it rots timber and soaks insulation with no visible warning. Screening for interstitial risk needs a full layer-by-layer BS EN ISO 13788 assessment, which is a job for accredited software or a surveyor, not a surface check. If you are insulating a solid wall internally, this is the risk that makes the vapour control layer and the choice of insulation matter.

Takeaway: a surface check keeps the room mould-free; an interstitial check keeps the structure sound, and internal wall insulation needs both.

Every calculation so far assumed the flat middle of the wall. In a real house the flat wall is rarely where mould starts. It starts at the junctions: window reveals, the line where an external wall meets the ground floor, the back of a built-in cupboard on an outside wall. At those spots the geometry funnels heat away and the local fRsi drops far below the 0.93 you might calculate for the open wall. A wall that screens as low risk across its face can still grow mould in every reveal.

Takeaway: solve the flat wall first, then treat the cold junctions as separate problems, because they fail on their own terms.

In practice, most UK damp surveyors do not start with a formula, they start with a meter. A surface thermometer or a thermal camera finds the cold spots, and a Protimeter or similar hygrometer reads the surface humidity directly. The maths tells you what should happen; the meter tells you what is actually happening once you factor in how the occupants heat and ventilate the place. I have seen more than one "rising damp" job turn out to be a 19 °C living room with the trickle vents taped shut and washing drying on the radiators, no defect in the wall at all. The calculation is the starting point for that conversation, not the end of it.

Takeaway: run the numbers to size the problem, then confirm with a meter before anyone starts hacking off plaster.

Plug your own room and wall figures into the Dew Point Calculator UK to see the dew point, the predicted surface temperature and the mould risk in one place. If you want to work out the U-value first, the U-Value Calculator UK builds it up layer by layer.

Disclaimer: This guide is for general information. Condensation and damp problems can have several overlapping causes. For a Building Regulations submission, a persistent damp problem or any interstitial condensation assessment, use accredited BS EN ISO 13788 software or a qualified damp or thermal surveyor, and confirm the current Approved Document C and L (or the equivalent for your jurisdiction).