Condensation Risk & Dew Point Calculator

Condensation and Mould Guide

How Condensation Forms?

Air contains water vapour. Relative humidity (RH) is the percentage of the maximum water vapour air can hold at a given temperature. When air contacts a surface cooler than the dew point, the air immediately adjacent to the surface is cooled below its dew point — water condenses. Dew point (Magnus formula): T_dew ≈ T - (100-RH)/5. At 20°C and 65% RH: dew point ≈ 20 - (100-65)/5 = 20 - 7 = 13°C. Any surface below 13°C will have condensation in this air. Cold bridging, or thermal bridges, creates localised cold spots — often around window lintels, wall ties, or uninsulated junctions — where condensation forms even when the rest of the room is comfortably above the dew point.

What's the key thing to understand about Mould Growth Conditions?

Mould grows when: surface RH exceeds 80% for extended periods (not the same as air RH — surface RH depends on temperature). Temperature is between 0-35°C (optimum 20-25°C). Nutrients are available (most building materials qualify). The relationship: if air is at 20°C/65% RH and a surface is at 12.6°C, the surface RH = 100% — condensation and mould will form. NHS definition of 'Category 1 Hazard': visible mould is a health hazard under the Housing Health and Safety Rating System. Black mould, commonly Stachybotrys or Aspergillus species, can pose a genuine health risk to those with asthma, allergies, or weakened immune systems, and should be treated seriously rather than just wiped away cosmetically.

What should I know about Sources of Moisture in Homes?

Daily moisture generation in a typical 4-person household: breathing and perspiration: 1.5L/day. Cooking: 3L/day. Washing and bathing: 1L/day. Drying clothes indoors: 5L/day (highest single source — avoid if possible). Total: up to 10-12L per day of moisture is generated — without adequate ventilation, this drives humidity up and condensation risk increases. Structural sources: rising damp (ground water wicking up walls — typically visible as a 'tide mark' below 1m). Penetrating damp — rain entering through a defect in the building's exterior, such as cracked render or a damaged roof — is distinct from condensation and needs a different repair approach entirely.

What should I know about Reducing Condensation Risk?

Increase ventilation: extractor fans in kitchen and bathroom (minimum 15 L/s during cooking/bathing). Trickle vents open on window frames. Consider mechanical ventilation with heat recovery (MVHR) for very airtight homes. Reduce moisture sources: dry clothes outside or use a vented tumble dryer. Put lids on pots when cooking. Vent tumble dryer directly outside. Heat more consistently: cold bedrooms in the morning are common sites of condensation — steady background heat prevents surfaces falling below the dew point where moisture condenses.

What's the key thing to understand about Why Condensation Forms and Where It's Most Common?

Condensation occurs when warm, moisture-laden air comes into contact with a surface cold enough to cool the air below its dew point — the temperature at which the air can no longer hold all its water vapour, causing excess moisture to condense out as liquid water on that surface. This is why condensation is most commonly seen on single-glazed windows, cold external walls (particularly in corners and behind furniture pushed against an external wall, where air circulation is restricted and the wall surface tends to run colder), and in poorly ventilated bathrooms and kitchens, where everyday activities like cooking, showering, and even drying clothes indoors generate large amounts of water vapour into the air relatively quickly. Modern, well-insulated, and increasingly airtight homes — built partly to reduce heating costs — can paradoxically be more prone to condensation and related mould problems than older, draughtier homes, precisely because reduced natural air leakage means moisture generated indoors has fewer ways to escape unless adequate mechanical or trickle ventilation is specifically provided to replace the natural ventilation that's been designed or sealed out. Persistent, unaddressed condensation is the most common cause of mould growth in homes, which is a health concern as well as a cosmetic and structural one, making early identification and management of condensation risk worthwhile beyond simply an aesthetic nuisance.

What should I know about Practical Steps to Reduce Condensation?

Reducing condensation effectively means addressing both sides of the equation: reducing the amount of moisture generated and released into indoor air, and improving ventilation to remove the moisture that is generated before it has a chance to condense on cold surfaces. On the moisture-reduction side, using extractor fans while cooking and showering (and leaving them running for a short period afterward to clear residual humidity), covering pans while cooking, drying laundry outdoors or in a well-ventilated space with an extractor running rather than on radiators indoors, and avoiding unflued gas or paraffin heaters (which release a significant amount of water vapour directly into the room as a byproduct of combustion) all reduce the moisture load that needs to be managed. On the ventilation side, ensuring trickle vents in window frames aren't blocked or taped shut, allowing some air circulation behind furniture placed against external walls rather than pushing it flush against cold surfaces, and briefly ventilating rooms after high-moisture activities all help excess humidity escape rather than settling on cold surfaces. For persistent problems despite these measures, improving insulation on the specific cold surfaces where condensation recurs (addressing the root cause — a cold surface below the dew point — rather than only managing the moisture load) or installing a whole-house mechanical ventilation system can address underlying issues that simple behavioural changes alone can't fully resolve, particularly in airtight modern homes.

What Is Dew Point and Why Does It Matter?

Dew point is the temperature at which air, cooled without adding or removing moisture, becomes saturated (100% relative humidity) and water vapour begins condensing out as liquid. It depends on both temperature and humidity — warmer, more humid air has a higher dew point than cooler or drier air. Condensation forms whenever a surface's temperature drops to or below the surrounding air's dew point, because that surface cools the air immediately touching it below the point where it can hold its moisture as vapour. This is why a cold external wall, a single-glazed window, or an uninsulated pipe are classic condensation spots: they are often the coldest surfaces in a room, so they're the first to drop below the air's dew point even when the room's average temperature feels perfectly comfortable. Knowing a room's current dew point (calculable from its temperature and relative humidity) tells you exactly how cold a surface has to get before condensation risk begins.

Worked Example: Checking Condensation Risk on a Cold Window

A bedroom at 20°C and 65% relative humidity has a dew point of roughly 13.2°C (using the standard Magnus approximation for dew point from temperature and humidity). If the bedroom's single-glazed window's internal glass surface is sitting at around 8°C on a cold night (a realistic figure for single glazing against a near-freezing outside temperature), that surface is well below the 13.2°C dew point, so condensation is effectively certain on that glass. Compare the same room with a modern double or triple-glazed window, whose internal glass surface might stay closer to 15-16°C even on a cold night thanks to its better insulating performance — now the surface temperature is above the 13.2°C dew point, and condensation should not form on the glass under those conditions. This is exactly why upgrading glazing (or fixing cold bridging elsewhere) reduces condensation without needing to change humidity or heating at all: it simply keeps the coldest surfaces above the air's dew point.

How Much Moisture Does Everyday Household Activity Add to the Air?

Ordinary daily activities add a surprising amount of water vapour to indoor air, and this moisture has to go somewhere — either out through ventilation, or condensing onto the coldest available surface.

What's the Difference Between Surface and Interstitial Condensation?

Surface condensation — the kind that fogs a window or beads on a cold wall — is visible and happens where warm moist indoor air meets a cold surface directly. Interstitial condensation is far less obvious: it forms inside a wall, roof, or floor construction, where warm moist air has diffused into the structure (through gaps, or simply through permeable materials) and then cools below its dew point somewhere within the layers, condensing invisibly inside the construction rather than on a visible surface. Interstitial condensation is more serious precisely because it's hidden — it can rot timber structure, corrode metal fixings, and degrade insulation's performance for months or years before any visible sign (a damp patch, a smell, or eventual material failure) appears. It is also why vapour control layers and correct insulation sequencing (warm side vapour-tight, cold side able to breathe where appropriate) matter so much in building construction — getting the layer order wrong can trap moisture inside a wall rather than letting it dry out.