Dew Point in Houses: Why Walls Get Wet and What to Do

In this guide

What dew point means inside a home
How to work out the dew point from temperature and humidity
What humidity level keeps a house safe
Where dew point problems show up in a house
Cold bridging: how it lowers surface temperature
Preventing cold bridging and condensation
Condensation or rising damp?
Our perspective on dew point diagnosis
Survey and treatment across Dorset, Hampshire and Wiltshire
FAQ
Sources

The dew point in a house is the temperature at which the water vapour in the air turns into liquid. When a wall, window or ceiling is at or below that temperature, condensation forms on it. The dew point rises as indoor humidity rises. So the fix is to keep surfaces warmer than the dew point, to keep indoor humidity down, or both. Cold bridges are the places where surfaces fall below the dew point first.

Proofterior

Find the Cause of Damp and Mould

Proofterior diagnoses damp, mould, and timber issues across Dorset, Hampshire, and Wiltshire, identifying root causes to guide tailored treatments.

Visit

Key Takeaways

Dew point depends on air temperature and humidity. Condensation appears wherever a surface is colder than the dew point of the room air.
At 20 °C and 60% relative humidity, the dew point is about 12 °C. Any surface at or below 12 °C in that room will collect water.
Mould risk begins before liquid water appears. BS 5250 links room humidity above 70% for several days to mould on external walls.
Cold bridges, such as lintels, corners and patchy insulation, are the usual starting points for dew point problems.
Ventilation, steady heating and continuous insulation work together. A surveyor should diagnose persistent damp, because condensation is often mistaken for rising damp.

What dew point means inside a home

Air holds water as vapour. Warm air holds more than cold air, which is why condensation forms when water vapour meets cold surfaces. The dew point is the temperature at which a given sample of air can hold no more vapour. Cool the air below that point and the excess becomes droplets.

The Met Office describes the dew point as the temperature at which condensation occurs, depending on the humidity and pressure of the air. Outdoors, this produces dew on grass. Indoors, it produces water on a window, a patch of wet paint or a black mould spot in a bedroom corner.

Two points are often missed:

Dew point is not the same as relative humidity. Relative humidity says how full the air is at its current temperature. Dew point is a temperature, and it stays the same when the room warms or cools, provided no moisture is added or removed.
Air is not the problem on its own. The surface decides where the water lands. The RICS guide notes that the dew point changes with humidity, air temperature and the temperature of surrounding surfaces.

How a cold surface pulls the air below its dew point

Air touching a cold wall cools. As it cools, its relative humidity climbs. Once the air layer against the wall drops below the dew point, liquid water drops out. Ashfield District Council lists three contributing factors: high humidity in the indoor air, low wall and surface temperatures, and poor ventilation.

Three conditions that contribute to indoor condensation

This is why condensation tends to appear in the same places repeatedly: behind wardrobes on external walls, in corners, around window reveals and on north-facing walls. The air in the room may be comfortable, but the surface is not.

How to work out the dew point from temperature and humidity

You do not need a laboratory. A basic thermo-hygrometer gives room temperature and relative humidity. From those two numbers the dew point follows.

The National Physical Laboratory explains that there is no simple direct formula. The conversion goes through the actual vapour pressure and the saturation vapour pressure, using the Magnus formula, which is valid between -45 °C and +60 °C. The air-Q FAQ gives a working version and a worked result: at 20 °C and 60% relative humidity, the dew point is 12.0 °C. The equations used in UK practice are set out in BS 5250:2011, which draws on BS EN ISO 13788.

Typical room readings

The values below come from applying the Magnus formula given by air-Q and NPL, rounded to one decimal place. They are indicative, not survey results.

Room temperature	Relative humidity	Approximate dew point	Surface temperature that will collect water
20 °C	50%	9.3 °C	9.3 °C or lower
20 °C	60%	12.0 °C	12.0 °C or lower
18 °C	70%	12.4 °C	12.4 °C or lower
21 °C	75%	16.4 °C	16.4 °C or lower

The pattern is clear. At the same room temperature, a rise in humidity from 50% to 75% pushes the dew point up by several degrees. A single-glazed window or an uninsulated external wall on a cold night can easily sit at 10 °C or below, so a normal-feeling living room can condense moisture on those surfaces.

A quick rule of thumb

For a fast estimate, subtract the shortfall from 100% humidity divided by five from the room temperature. At 20 °C and 60%, that gives 20 minus 8, or 12 °C. This approximation is close to the Magnus result when humidity is above about 50%, and is useful for a first check. For a diagnosis, use a calibrated hygrometer and a surface thermometer, or have a surveyor take the readings.

What humidity level keeps a house safe

Condensation is one measure. Mould is a second, and it starts earlier. BS 5250:2011 says the average relative humidity at internal surfaces should not exceed 80% over the coldest month, as 80% is the limit for mould growth. The same standard notes that in winter the face of an external wall can be about 10% more humid than the room air. If room humidity stays at 70% for long periods, the wall surface can support mould even though no droplets are visible.

The practical range:

Below 40%: air becomes dry. Prof Cath Noakes of the University of Leeds, quoted by the BBC, puts the ideal at roughly 40% to 60%, with very low humidity linked to respiratory viruses.
40% to 60%: the comfortable and safe band for most homes. The Passivhaus UK guide gives 50% plus or minus 10% as good practice.
Above 70% for days: BS 5250 states that surface humidity on an external element is likely to pass 80%, and mould is likely if that lasts two or three days.

Where dew point problems show up in a house

Internal walls and corners

External wall corners have more surface exposed to the cold than flat wall. They are colder than the room and more likely to drop under the dew point. Wardrobes and beds pushed against external walls also block warm air from reaching the surface.

Windows, inside and outside

Internal window condensation is the familiar form. External condensation is a different dew point effect. Pilkington explains that it is more likely in spring and autumn, when the outer pane can fall low at night while the dew point is comparatively high. It is more common with modern low-emissivity glass because heat is retained inside, leaving the outer pane colder. The Glass and Glazing Federation describes the same cause: a mix of high humidity and clear, cold conditions.

External condensation on a window is a sign of efficient glazing. It does not point to a damp problem inside.

Inside the wall and the roof space

Condensation does not need a visible surface. The SPAB notes that it can be interstitial, occurring within the thickness of building materials. Interstitial condensation occurs when the dew point lies within the insulation layer. Our guide to interstitial condensation causes, risks and fixes covers the signs, and the page on loft condensation causes covers roof spaces. A wet ceiling is not always a leak, and our roof leak or condensation guide explains how to tell which one it is.

Cold bridging: how it lowers surface temperature

A cold bridge, or thermal bridge, is a spot where heat passes through the building envelope faster than through the surrounding construction. Typical locations are lintels above windows, concrete floor edges, wall-to-roof junctions, reveals and gaps in insulation. The surface on the inside is colder there, so it reaches the dew point first.

The NHBC notes that as homes become better insulated, thermal bridges become more significant for heat loss and for the likelihood of condensation in specific places. These local cold spots sometimes occur where a critical element was left out through lack of communication between the designer, the assessor or the builder.

The measure used for this is the temperature factor, fRsi. The BRE’s PAS 2030 specification states that fRsi should be not less than 0.75 to avoid the risk of mould growth. A UCL dissertation reports that thermal bridges are common in retrofitted building envelopes and often produce low surface temperatures that make buildings susceptible to mould and condensation. That is why retrofit work needs to be designed around continuity of insulation, not just the thickness of it.

Preventing cold bridging and condensation

The TPI condensation guide names three primary controls: more ventilation to remove moist air, more insulation so surfaces do not fall below the dew point, and consistent heating so the structure does not become cold. Each one acts on a different side of the same equation.

1. Lower the moisture in the air

Open windows several times a day for at least five minutes, and run extractor fans for at least 15 minutes after showering or cooking, as the Energy Saving Trust advises.
Keep trickle vents open and clear. The NHBC advises leaving them open when rooms are occupied, even in winter with the heating on.
Put lids on saucepans, avoid drying laundry on airers or radiators, and vent tumble dryers outside, all from the same NHBC guidance.
Fit extractors in kitchens and bathrooms that cope during peak periods.

The RICS guide says windows do not need to stay open all the time: about 5 minutes for every hour spent in a room is enough. It also recommends keeping internal temperatures between 18 °C and 21 °C. For more on how ventilation works as a system, see our page on condensation control through ventilation.

A dehumidifier removes some moisture, but as Prof Noakes explains, it does not remove mould, and it will not solve leaks or water ingress.

2. Keep surfaces warm

The SPAB advises that heating can keep surfaces above the dew point, especially if run constantly at a low level instead of intermittently. Lagging cold pipes can also prevent surface condensation. Short bursts of high heat followed by cold spells let walls drop under the dew point overnight.

3. Insulate without creating new cold spots

Insulation raises the temperature of the inner surface, which moves it away from the dew point. It must be continuous. The Designs in Detail guide to external wall insulation notes that the dew point is pushed further out away from the inside of the home, but patchy insulation around windows, door frames or pipes can leave cold spots where condensation and damp patches appear indoors. The same article recommends breathable systems that let vapour pass through.

The NHBC recommends designing out cold bridges and positioning insulation exactly as drawn, and explaining the ventilation strategy to the homeowner. Insulation also changes where the dew point sits inside the wall. One company article reports that where insulation reduces natural airflow, Approved Document F calls for mechanical or trickle ventilation, and that a vapour-control layer on the warm side stops moist air reaching a cold surface behind the insulation.

Signs that ventilation is not enough

The Energy Saving Trust lists ongoing condensation, mould, lingering smells and high readings on moisture monitors as signs a home needs better ventilation. Too much ventilation can also be a problem. The Passivhaus UK guide says optimal humidity is reached by matching ventilation to moisture generation, and that over-ventilating leaves air uncomfortably dry.

Condensation or rising damp?

A damp patch on a wall at or near the dew point is not necessarily rising damp. The SPAB notes that condensation is often misdiagnosed as rising dampness, and used to justify unnecessary damp-course treatment. Moisture meters often show diffuse damp on plaster, particularly on north-facing walls, which is commonly misread.

Some pointers:

Condensation tends to appear on the coldest surfaces, worsens in cold weather and clears when the room warms and ventilates.
Rising damp rises from the ground, usually shows a tide mark at low level and does not depend on the room’s humidity.
Both can occur in the same property.

Our guides on damp versus condensation and rising damp versus condensation set out the visual differences. For a recurring problem, see how to fix damp that keeps coming back.

Our perspective on dew point diagnosis

Most damp complaints reach us as a stain or a smell, not as a dew point calculation. The stain tells little on its own. A wall can look the same whether the cause is condensation, penetrating damp, a failed damp-proof course or a cold bridge at a lintel.

Our approach is to find the cause before proposing treatment. That means recording room temperature and humidity, checking surface temperatures where the problem sits, looking at the building fabric and ventilation, and then writing the findings in a report. Where condensation is the cause, the remedy is usually ventilation, insulation or both, applied where the surface is falling below the dew point. Where it is not, the remedy is different, and we do not treat a moisture problem that is not there.

All of our diagnostic and remedial work is carried out by our own specialists, not subcontractors, and our work follows industry standards. Our treatment work carries a 25-year insurance-backed guarantee. Condensation and ventilation, mould removal, insulation and energy efficiency upgrades sit within the same team, so the diagnosis and the fix stay connected.

Dylan

Survey and treatment across Dorset, Hampshire and Wiltshire

For homes in Dorset, Hampshire and Wiltshire, Proofterior offers a complimentary property survey and a detailed written report. The survey looks at where moisture is coming from and why surfaces are reaching the dew point. Treatment follows from the findings: condensation and moisture control, mould removal and air quality, energy efficiency upgrades and other work from the full service list.

Proofterior

Homeowners, landlords, property managers and estate agents can book a complimentary property survey to find out whether a damp problem is condensation, a cold bridge or something else.

FAQ

Is relative humidity 100% at the dew point?

Yes. At the dew point the air is saturated, so relative humidity is 100%. The Met Office describes this as the temperature at which condensation occurs. Air cooled below this temperature can no longer hold its water as vapour.

Does opening windows reduce humidity?

Usually, yes, when the outside air is drier than the room air. Short, frequent openings work well. The RICS guide says about 5 minutes for each hour in a room is enough, and the Energy Saving Trust advises opening windows several times a day. Extractor fans and trickle vents handle the rest.

What is the healthiest humidity for a home?

A range of about 40% to 60% is widely given. Prof Noakes puts the ideal between 40% and 60%, and the Passivhaus UK guide gives 50% plus or minus 10%. Sustained readings above 70% raise the risk of mould on external walls.

What is cold bridging and how do I prevent it?

Cold bridging is heat loss through a part of the structure that is colder than the surrounding wall, such as a lintel, corner or gap in insulation. Prevent it with continuous insulation, correct positioning and steady heating. The NHBC says to design out cold bridges. A survey can locate existing ones.