Surface Condensation
Visible water droplets or frost forming on an internal surface when its temperature drops below the dew point of the indoor air.
What is surface condensation and how does it differ from condensation in construction?
Surface condensation is water forming visibly on an interior surface, typically a window pane, a cold corner, or a balcony slab edge, when the surface temperature falls below the dew point of the indoor air. The key distinction from interstitial condensation is location: surface condensation is on the inside face that you can see and touch, whereas interstitial condensation forms hidden inside the construction layers. Surface condensation is not a structural defect in the way that trapped internal moisture is, but it creates an immediate hygiene risk: the wet surface becomes a substrate for mould growth within days, especially in low-ventilation zones like window reveals and corners.
In Slovak residential practice, surface condensation is one of the most common comfort complaints, particularly in winter. It affects new and old buildings alike. The difference is that a well-insulated new building with triple glazing rarely shows it, while an older home with single windows will sweat readily during cold weather if indoor humidity is not carefully managed.
When does water start forming on internal surfaces?
Condensation forms when a surface cools to the dew point temperature of the surrounding air. The dew point is the temperature at which air becomes saturated and can no longer hold all its moisture as vapour. At a typical indoor condition of 20°C and 50% relative humidity, the dew point is about 9°C. Any surface colder than that will collect condensation.
The relationship between air temperature, relative humidity, and dew point can be visualised on a psychrometric chart, a standard tool in building physics. For practical purposes, use the table below to estimate when condensation risk appears:
| Indoor condition | Single glazing starts to sweat | Double glazing (old spacer) | Triple glazing or insulated corner (fRsi 0.75) |
|---|---|---|---|
| 20°C, 40% RH | +4°C surface | +8°C surface | +12°C surface |
| 20°C, 50% RH | +9°C surface | +13°C surface | +16°C surface |
| 20°C, 60% RH | +12°C surface | +16°C surface | +18°C surface |
| 20°C, 70% RH | +14°C surface | +17°C surface | +19°C surface |
These temperatures show why single glazing is vulnerable in winter. The external surface temperature often falls below +4°C, triggering condensation even at modest indoor humidity. In contrast, triple-glazed frames maintain higher internal surface temperatures, suppressing condensation across nearly all normal domestic humidity levels. The difference can literally be 20°C between the two.
Why do the coldest surfaces collect all the condensation?
Moisture in the air will deposit on whichever surface is coldest. In most homes, this is the window frame or glazing, but it can also be an uninsulated corner where two exterior walls meet, the internal face of a balcony slab that penetrates through the building envelope, or a structural column running on an external facade. The reason is simple: heat flows out through the building envelope, and the weakest points (those with the highest U-value or the poorest thermal connection) become the coldest.
A single-glazed window in a 20°C room can have an internal surface temperature of only 2 to 4°C on a winter day when outside air is minus 10°C. The triple-glazed equivalent will be 15 to 17°C. The difference is huge: the single window will condense at any indoor humidity above 30%, while the triple window will stay dry until indoor humidity reaches 70 or 80%.
Thermal bridges create the same effect. An uninsulated concrete slab running straight through an external wall in an older building becomes a cold band that collects condensation. In a new building with continuous external insulation, the same slab stays warm and dry. This is why thermal-bridge mitigation during design is so much cheaper than fixing surface condensation afterwards through constant ventilation.
What is the temperature factor (fRsi) and why does it matter?
The temperature factor (fRsi), also called the internal surface temperature factor, is a single number from 0 to 1 that predicts how warm the internal surface will be relative to the indoor and outdoor air temperatures. An fRsi of 1.0 means the surface stays at room temperature (perfect insulation). An fRsi of 0 means it stays at outdoor temperature (no insulation). An fRsi of 0.75 means the surface is 75% of the way from outdoor to indoor temperature.
The formula is straightforward: fRsi = (T_surface - T_outdoor) / (T_indoor - T_outdoor). For example, if outdoor air is minus 10°C, indoor is 20°C, and the surface is 13°C, then fRsi = (13 - (-10)) / (20 - (-10)) = 23/30 = 0.77.
Why does this matter? Because from fRsi and the indoor condition, you can calculate whether condensation will form. If you know the indoor temperature and humidity (measured with a hygrometer or estimated from lifestyle), you can find the dew point, then check whether the calculated surface temperature stays above it. Building codes in the EU, including Slovakia, typically require fRsi ≥ 0.75 in residential spaces to keep condensation risk low under normal occupancy. This is one of the driving reasons why modern building standards mandate triple glazing and continuous external insulation; they raise fRsi above the threshold.
How much moisture does a typical household produce?
A family of four in an occupied home generates 10 to 15 litres of water vapour per day. About 2 litres comes from breathing and skin evaporation. The rest comes from cooking (steam from pots), showers and baths (often 1 to 2 litres per shower), laundry, indoor plants, and other daily activities. In winter, when windows are shut and heating is on, all this moisture stays indoors and the relative humidity climbs steadily unless removed by ventilation.
A poorly ventilated older home with a family inside quickly reaches 60 to 80% relative humidity by evening. When outdoor air is cold, the cold surfaces (windows and corners) condense. The solution has two parts: remove some of the moisture (ventilation, extraction fans, changing behaviour) and raise surface temperatures (insulation). Relying only on insulation without ventilation often pushes the problem elsewhere; humid air stalls in attics or unheated spaces. Relying only on ventilation is expensive in heating costs. The most durable solution combines both.
| Moisture source | Typical daily output (litres) | Notes |
|---|---|---|
| Breathing and skin (4 people) | 2 | Constant, unavoidable |
| Cooking and kitchen | 1 to 2 | Peaks during meal preparation |
| Bathroom use (showers, baths) | 1 to 2 | Concentrated load, short duration |
| Laundry and drying | 1 to 2 | Avoidable with line drying outdoors |
| Plants, humidifiers, wet surfaces | 1 to 3 | Controllable behaviour |
| Total (occupied home) | 10 to 15 | Requires ventilation or condensation risk |
How do you prevent surface condensation?
Prevention follows a hierarchy of effectiveness:
- Insulate the coldest surfaces. Replace single glazing with triple, upgrade window frames to warm-edge spacers, add external insulation to walls, and break thermal bridges where structural elements penetrate the envelope. This is the longest-lasting fix because it raises the surface temperature so condensation risk becomes low even at moderately high humidity. A home with triple glazing and 10 cm of external insulation will rarely show condensation regardless of occupant behaviour.
- Ventilate reliably. Install mechanical extract fans in kitchens and bathrooms. Run them during and for 30 minutes after use, not constantly, as that wastes heat. If the home has good airtightness (required in a passive house or new-build to standard), a heat-recovery ventilation system that exhausts damp air and recycles the heat is ideal. In older homes, passive stack vents or trickle vents in window frames are the minimum.
- Manage moisture production. Shut the kitchen door while cooking, run the extractor during and after. Hang wet laundry in a room with an open window or in a space that vents to outside, never in the living room. Do not over-water indoor plants. Avoid humidifiers. These steps sound trivial but can reduce indoor humidity by 5 to 10 percentage points without any capital cost.
The order matters. Building insulation first is most cost-effective because it reduces the ongoing ventilation load. Adding ventilation to an uninsulated home means exhausting expensive heated air. Changing occupant behaviour alone (just ventilating) works in mild climates but is tedious and expensive in cold winters. The combination of improved insulation (fRsi ≥ 0.75) with standard extract ventilation and basic moisture awareness solves surface condensation in almost all Slovak residential settings.
Is surface condensation a sign of a moisture problem in the structure?
Surface condensation on the inside face is usually not a structural moisture problem; the water does not penetrate far into most materials. However, it is a warning sign in two ways. First, if surface condensation appears regularly, the indoor air is chronically humid, which means either the home is damp (leaks, rising damp, capillary absorption) or the occupants are not ventilating adequately for their activities. A home that is dry but occupied by a large family will show temporary surface condensation on the coldest surface on winter mornings; this is normal and harmless. A home that sweats constantly even when lightly occupied suggests a moisture source that should be investigated. Second, persistent surface condensation breeds mould, which is a separate health hazard. Even if the structure itself is sound, mould on windows and walls is never acceptable indoors.
Frequently asked questions
- Why do windows sweat more than walls in my home?
- Windows have a lower surface temperature than walls because they conduct heat out more easily (their U-value is higher). Since the coldest surface condenses first, all the moisture in the air tends to deposit on glazing. Single-glazed windows sweat readily; triple-glazed ones rarely do, even in humid rooms.
- What indoor humidity level causes surface condensation?
- It depends on the coldest surface temperature and the indoor air temperature. At 20°C room air, condensation begins at about 60% relative humidity on single glazing but may not appear on a triple-glazed window until 80% or higher. A psychrometric chart or hygrometer shows you exactly when you are approaching the risk zone.
- Does surface condensation damage the building?
- The water itself does not penetrate far into most internal surfaces, but it creates a wet film where mould can grow within days. The real risk is mould colonisation, especially in corners, window reveals, and other low-air-velocity zones. This is why surface condensation, though physically minor, is a serious comfort and health issue.
- Is better insulation the only solution?
- No. Insulation prevents condensation by raising surface temperature, but you can also reduce the moisture load in the air through ventilation, correct kitchen and bathroom practices (run the extractor during and for 30 minutes after use), and by not over-watering plants or drying laundry indoors without extraction. Often the best fix combines all three: modest insulation improvements, reliable ventilation, and behaviour change.
- Why is the temperature factor (fRsi) important?
- The fRsi number predicts surface temperature: an fRsi of 0.70 means the internal surface is 70% of the way between the outdoor and indoor air temperature. This single number lets you calculate whether condensation will form for any given indoor conditions. Building regulations typically require fRsi ≥ 0.75 to limit condensation risk in normal domestic use.
- Can I fix surface condensation with only ventilation?
- If the air is very damp, yes: opening a window or running an extract fan removes moisture fast. But relying on this alone means opening windows in winter, which loses heat and money. The best strategy is to insulate the coldest surfaces (windows, corners, balcony slab penetrations) so they stay warm, then use ventilation only as needed, not as the primary solution.