What this calculator works out
Condensation is the most common cause of damp in British homes, and the one most often blamed on something else. It is also the only one that can be checked with two cheap instruments and a calculation, because the physics behind it is completely settled.
This calculator works out the dew point of the air in a room — the temperature at which its moisture starts turning back into liquid water — and compares it with the temperature of the cold surface you are worried about. Enter the room temperature and humidity, then either measure the surface or pick the construction and let the page estimate it.
It reports two thresholds rather than one, and the second is the important one. Liquid water needs the surface at or below the dew point. Mould needs considerably less: it will grow once the air touching a surface sits at around 80% relative humidity, which happens around three degrees before anything becomes visible. A wall can be perfectly dry to the eye and be a thoroughly good place for mould to establish itself, and a page that reported only the dew point would tell you that wall was fine.
No specialist knowledge is assumed. Every term is explained where it first appears, and the section below sets out exactly what the calculation does and where its uncertainty lies.
The room and the cold surface
Leave the outside temperature as it is if you have measured the surface directly — it is only used to estimate a surface temperature from the construction you pick.
Select Check the risk to see where a surface falls on this scale.
- Condensation forming -6 to 0
- Mould risk 0 to 3.4
- Marginal 3.4 to 6
- Comfortable margin 6 to 10
How the calculation works
Air holds water vapour in proportion to how warm it is. Warm air holds a great deal, cold air very little. Cool a parcel of air far enough and it reaches a temperature where it can no longer hold what it is carrying, and the excess appears as liquid water. That temperature is the dew point, and it depends only on how warm the air is and how much moisture it already contains.
Nothing in a house cools whole rooms of air. What happens instead is that air touches a cold surface — a window, an outside wall, the ceiling under an uninsulated loft — and the thin layer against it cools to the temperature of the surface. If the surface is below the dew point, that layer gives up its water and you get condensation. If it is above, you do not. This is why condensation appears in specific places rather than evenly across a room.
Step one: the dew point
The calculator uses the Magnus-Tetens approximation, the standard meteorological relationship between temperature, relative humidity and dew point. It is accurate to a small fraction of a degree across the range of temperatures found in houses.
γ = ln(RH ÷ 100) + (17.625 × T) ÷ (243.04 + T)
Dew point = (243.04 × γ) ÷ (17.625 − γ)Where T is the room air temperature in degrees Celsius and RH is the relative humidity as a percentage. The two constants come from fitting the curve of saturation vapour pressure over liquid water.
Step two: the mould threshold
Mould does not wait for liquid water. Mould spores are present in every house, and the common household species germinate on a surface once the air touching it sits at around 80% relative humidity for long enough. That state is reached well before the dew point.
The same equation, solved for 80% rather than 100%, gives the surface temperature at which that happens. In a room at 20 °C and 60% humidity, the dew point is 12.0 °C, but the mould threshold sits at 15.4 °C. A wall at 14 °C shows no condensation at all and is a perfectly good place for mould to grow. Across the range of conditions found in houses the gap between the two is remarkably steady, at a little under three and a half degrees.
The 80% figure is not a rule of thumb. It is the critical surface humidity given in BS EN ISO 13788, the British and international standard for assessing surface condensation and mould risk in building components, and it is what building physicists design against.
Step three: the surface temperature
If you have measured the surface, the calculator uses your figure and stops there. If you have not, it estimates one from the construction using a surface temperature factor, written fRsi. This is the fraction of the way from outside temperature to inside temperature that the inner surface sits.
Surface temperature = outside + fRsi × (inside − outside)A well-insulated wall has a high factor: its inner face stays close to room temperature however cold it is outside. Single glazing has a very low one, which is why it streams with water while the wall beside it stays dry. Because the factor is a ratio, one number describes a construction across every combination of indoor and outdoor temperature — which is exactly why building physics uses it.
You will see the figure fRsi ≥ 0.75 quoted for dwellings, from BRE Information Paper IP 1/06. It is not arbitrary. Take a room at 20 °C and 60% humidity with 0 °C outside: a factor of 0.75 puts the surface at 15 °C, which is within half a degree of the point at which the air touching it reaches 80% humidity. The whole recommendation is that one calculation, made once and turned into a design target.
Worked example: a bedroom wall in February
A north-facing bedroom in a Victorian terrace with solid brick walls, 225 mm thick and uninsulated. The heating goes off overnight and the room settles at 17 °C. Two people have been sleeping in it with the door shut, so the humidity has climbed to 70%. Outside it is 2 °C. There is a patch of black speckling in the corner behind the wardrobe.
| Input | Value |
|---|---|
| Room air temperature | 17 °C |
| Relative humidity | 70% |
| Surface | Solid brick wall — external corner or behind furniture |
| Outside air temperature | 2 °C |
Dew point of the room air: 11.5 °C
Mould threshold, 80% humidity at the surface: 14.9 °C
Working from a surface temperature of 9.8 °C, estimated from that construction
Margin above the dew point: -1.7 °C
Water is condensing on this surface. It is at or below the dew point.
Room humidity would need to be about 50% for this surface to sit clear of the mould threshold, with everything else unchanged.
The corner is below both thresholds. It is not merely at risk of mould; it is below the dew point and actively condensing, which is precisely why the speckling is there and not in the middle of the wall.
The middle of the same wall, away from the corner and with air free to circulate, has a surface temperature factor of about 0.62 rather than 0.52. That puts it at 11.3 °C — still below the dew point, though only just. The whole wall is marginal and the corner is the part that has tipped over.
What the figures suggest doing
Three levers, and running the numbers again shows how much each one is worth.
- Move the wardrobe. Letting air circulate lifts that spot from the corner figure towards the open-wall figure, worth about a degree and a half. It takes the corner from condensing to merely marginal, and no further.
- Drop the humidity. Getting the room from 70% to 55% brings the dew point down to 7.9 °C and the mould threshold to 11.2 °C. That puts the open wall clear, but the corner at 9.8 °C is still in mould territory — better, not solved.
- Raise the wall temperature. Insulated lining on the inside face, or insulated render outside, moves a solid wall from a factor of around 0.62 to 0.9 or better. Unlike the other two it holds on the coldest night of the year, and it is the only one of the three that fixes the corner outright. It also costs by far the most, and on a solid wall it needs doing properly — an internal lining that traps moisture behind it creates a new problem in place of the old one.
The realistic answer is the first two together, with the third when the money is there and after taking advice on the build-up. That is the honest shape of most condensation problems: no single cheap measure fixes them, and the cheap measures still do more than people expect.
Understanding the result
The headline figure is the margin: how many degrees the surface sits above the dew point. Four ranges matter.
| Margin | What it means |
|---|---|
| At or below 0 °C | Liquid water is forming on the surface now. Streaming windows, wet sills, damp patches that appear overnight and dry by afternoon. |
| Up to about 3.4 °C | No visible water, but the air against the surface is above 80% humidity. This is mould territory, and it is the range people miss because there is nothing to see until the mould arrives. |
| About 3.4 to 6 °C | Clear of the threshold in these conditions, without much to spare. A colder night, a bath, or washing on a rack will close the gap. |
| Above 6 °C | A comfortable margin. Condensation and mould are unlikely on this surface unless conditions change substantially. |
The gap between the first two rows is the point of the whole page. A surveyor with a surface thermometer and a hygrometer, or a landlord responding to a complaint, can measure a wall, find no condensation, and conclude there is no problem — while the same measurements say the surface has been sitting in mould conditions all winter.
This calculates conditions at the instant you measured them. Mould responds to hours and days of sustained humidity, not to a single reading, and a wall that is fine at four in the afternoon may be well over the threshold at six in the morning when the heating has been off all night. Take readings at the coldest time you can, not the most convenient one. Where a decision costs real money, or where a tenancy dispute or a health problem is involved, this is a starting point for a professional assessment and not a substitute for one.
How to measure the inputs properly
The arithmetic is exact. The inputs usually are not, and three of them are easy to get wrong.
Relative humidity
A digital hygrometer costs very little and is accurate enough for this, but give it half an hour in the room before reading it, and do not stand next to it while it settles. Humidity varies enormously across a house: a bathroom after a shower and a hallway can differ by thirty percentage points. Measure in the room with the problem.
Surface temperature
An infrared thermometer is the usual tool and is where most of the error creeps in. Two cautions. It reads what the spot emits, so a shiny or metallic surface reads low and unreliably — aim at matt paint or plaster, not at foil-backed board or a metal window frame. And it averages over a spot that widens with distance, so a reading taken from across the room is an average of the corner and the wall around it rather than the cold spot you meant to measure. Hold it close.
If you have no infrared thermometer, pick the construction that matches and let the calculator estimate. The estimate carries perhaps a degree and a half of uncertainty either way, which is enough to change the verdict near a boundary but not enough to hide a serious problem.
Room air temperature
Measure in the middle of the room, away from radiators, direct sun and outside walls. A thermometer sitting on a windowsill is measuring the window.
Leave a hygrometer in the problem room overnight, ideally one that records a minimum and maximum. What you want to know is the humidity at six in the morning when the heating has been off for hours, not the humidity at teatime. That single overnight figure explains more condensation problems than any amount of measuring during the day.
Things that affect the result
- Time of day. The largest single factor and the one most often missed. Surfaces are coldest and humidity highest in the small hours, which is when condensation actually forms.
- Furniture against outside walls. A wardrobe or a sofa pushed flat against an external wall stops room air circulating behind it, and the surface behind runs several degrees colder than the exposed wall either side. This is why mould appears behind furniture.
- External corners. Where two outside walls meet, heat escapes in two directions from an inside surface that has less area to collect it. Corners run colder than flat wall by a similar margin to furniture.
- How the house is heated. Short bursts of high heat warm the air but not the fabric. Masonry takes hours to respond, so an intermittently heated house has cold surfaces even when the thermometer says the room is warm. Steady background heat costs less than people assume and moves the surface temperature far more.
- Occupancy. Each person adds moisture continuously through breathing and perspiration. Two people in a closed bedroom overnight can raise the humidity by twenty percentage points or more.
- Drying washing indoors. A single load releases a couple of litres of water into the house. On a rack in an unventilated room this is the largest moisture source most households have.
- Trickle vents and airbricks. Closed vents and blocked airbricks raise humidity throughout the house. They are the cheapest lever on this whole page and the most commonly disabled.
- Thermal bridges. Concrete lintels, exposed beams, balcony slabs and steel elements carry heat straight through the insulation. Surface temperatures at a bridge can be several degrees below the wall around them, which is why mould sometimes traces out the structure of a building.
Common mistakes
- Treating the dew point as the only threshold. The most consequential error on this subject. Mould appears at 80% surface humidity, several degrees above the dew point, so "no condensation" and "no problem" are different statements.
- Measuring at the wrong time. A daytime reading in a heated room flatters every surface in the house.
- Pointing an infrared thermometer at glass or metal. Reflective surfaces defeat it. Read matt surfaces, and treat a suspiciously low reading on gloss paint or a window frame with scepticism.
- Confusing relative and absolute humidity. Relative humidity is a percentage of what the air could hold at its current temperature, so the same quantity of water reads as 50% in a warm room and 80% in a cold one. Cooling a room raises its relative humidity without adding a drop of water.
- Sealing the house tighter. Draught-proofing without adding ventilation reliably converts a mild condensation problem into a serious one. This is the most common way people make things worse while trying to make them better.
- Anti-mould paint as the remedy. It suppresses growth on the treated patch. The water is still arriving, and the mould reappears at the edges or somewhere else.
- Assuming a chemical damp proof course will help. It has no effect whatsoever on condensation, which is what a substantial share of damp proof courses are sold to treat.
- Using an estimated surface temperature near a boundary. When the margin lands within a degree or two of a threshold, the construction estimate is not precise enough to settle it. Measure.
When not to use this calculator
Not for a building regulations submission or a professional condensation risk analysis. Those use BS EN ISO 13788 or a full hygrothermal model with monthly climate data, real material properties and finite element analysis of the junctions. This uses one moment and one temperature factor.
Not for interstitial condensation. This page is about water forming on surfaces you can see. Water condensing inside a wall or roof build-up is a different problem with different consequences, and diagnosing it needs the whole construction analysed layer by layer.
Not as evidence on its own in a disrepair dispute. It can tell you what to measure and when, and a record of overnight readings through a winter is genuinely useful. A calculation from a website is not a survey, and a tribunal will want the survey.
Not for a health assessment. If someone in the household has asthma, a respiratory condition or a weakened immune system and there is mould in the home, that is a matter for a GP and for the landlord if there is one, not for arithmetic. The NHS publishes guidance on the health effects of damp and mould.
Not for swimming pools, drying rooms or any high-humidity space. The 80% criterion is the value for ordinary dwellings. Specialist spaces are designed to different figures.
Frequently asked questions
What is the difference between the dew point and the mould threshold?
The dew point is the surface temperature at which liquid water appears. The mould threshold is the surface temperature at which the air touching the surface reaches 80% relative humidity, which is where common household moulds will germinate. The mould threshold is always the warmer of the two, typically by a little under three and a half degrees, so a surface can be growing mould with no visible condensation on it at all.
Why does my window stream with water when the wall next to it is dry?
Glass is a far worse insulator than a wall, so its inner surface sits much closer to the outside temperature. Single glazing has a surface temperature factor of around 0.2, meaning its inner face is only a fifth of the way from outside to inside. That puts it below the dew point on most winter nights while the wall beside it stays well above.
What relative humidity should I aim for?
Somewhere between 40% and 60% suits most homes: high enough to avoid dry air problems, low enough to keep surfaces clear in ordinary construction. What matters more than the target is the reading at the coldest time of night rather than the middle of a heated afternoon. If overnight readings sit above 65% through the winter, ventilation is the first thing to look at.
Will a dehumidifier fix this?
It lowers the room humidity while it runs, which lowers the dew point, which does genuinely help — for condensation. It does nothing about penetrating damp, a leak or a bridged damp proof course, because those keep supplying water regardless of how dry the air is. Confirm the cause before buying one.
Is condensation the same thing as rising damp?
No, and confusing the two is expensive. Condensation forms on the inside surface, is worse in cold weather, improves in summer and typically brings black speckled mould. Rising damp comes up through masonry, is more or less constant, cannot climb much above a metre, and leaves a brownish tide mark with salt deposits rather than mould. The damp guide sets out how to tell all five causes apart.
Can I use this to check a rented property?
Yes, and a record of readings is worth having. Note the room temperature, the humidity and the surface temperature with the date and time, take them at the coldest part of the day, and keep them. Report the problem to the landlord in writing. Under Awaab's Law a social landlord must investigate a reported damp and mould problem within statutory timescales rather than attributing it to how the property is used.
How accurate is the surface temperature estimate?
Treat it as within about a degree and a half either way for the construction named. That is close enough to distinguish a wall that is fine from one that is condensing, and not close enough to settle a case that falls within a couple of degrees of a threshold. When the answer matters and the margin is small, measure the surface with an infrared thermometer instead.
Why does the calculator ask for the outside temperature?
Only to estimate the surface temperature from the construction, using the fraction of the way from outside to inside that the inner face sits. If you select the option to enter a measured surface temperature, the outside figure is not used at all.
Related tools
References
These organisations publish the guidance behind this page.
- British Standards Institution — BS EN ISO 13788, the standard giving the 80% critical surface humidity criterion and the surface temperature method
- Approved Documents, GOV.UK — Approved Document C on resistance to moisture and Approved Document F on ventilation
- Building Research Establishment — Information Paper IP 1/06 and related guidance on surface temperature factors and mould risk in dwellings
- NHS — the health effects of damp and mould, and who is most at risk
- Met Office — temperature and humidity records, and explanations of dew point
- Historic England — guidance on moisture and ventilation in traditionally built and solid-walled buildings
External guidance changes. Check the current position at the source before relying on it for a decision that matters.