Winter exterior wall condensation check: room dew point, wall surface temperature, and the insulation you need
We check whether dew forms on the inside face of an exterior wall in winter, using the dew point from the psychrometric chart and the thermal resistances of the wall, then work out the minimum insulation that prevents it. The numbers show that a plain wall needs very little insulation for this, which is why real condensation shows up at corners and thermal bridges.
The situation
Say an office or an apartment in Seoul reports water droplets and mould in winter on exterior wall corners, behind furniture and around windows. Two things need checking: what is the dew point of the room air, and how cold does the inside surface of the wall get at the outdoor design temperature? If the surface is at or below the dew point, it condenses. Even above the dew point, a surface that stays above 80 % RH for long periods grows mould. The conditions below are an example for illustration. A real check has to use the building's actual wall build-up, measured indoor humidity and the local outdoor design conditions.
Given
| Item | Value |
|---|---|
| Room | 22 °C dry bulb, 50 % RH (a humidified office or a crowded flat) |
| Outdoor design | -11.3 °C (a value commonly used in Korean practice as the Seoul heating design temperature; an example value here) |
| Wall | 200 mm reinforced concrete, λ 1.6 W/m·K → R = 0.125 m²K/W |
| Insulation (typical values) | EPS λ 0.037, XPS λ 0.032 W/m·K. Use the product's test certificate in a real design |
| External surface resistance Rse | 0.043 m²K/W (directly exposed to outdoor air) |
| Internal surface resistance Rsi | 0.11 (Korean U-value calculation, walls) · 0.13 (ISO 6946, horizontal heat flow) · 0.25 (ISO 13788, for surface condensation and mould checks) |
| Barometric pressure | 101.325 kPa (sea level) |
Why three Rsi values: the Rsi used for a U-value is meant for heat loss. Condensation checks use a larger one, because air barely moves in corners and behind furniture or curtains, so less heat reaches the surface and it runs colder.
Reading order on the chart
- Plot the room point RA (22 °C, 50 %).
- Move horizontally left from RA (constant humidity ratio) to the saturation curve: the dry bulb at that point DP is the dew point. Air next to a cold wall cools without changing its humidity ratio.
- Where the same horizontal line crosses the 80 % RH curve gives the mould limit temperature.
- Draw a vertical line at the calculated wall surface temperature. Left of DP (colder) means condensation, between DP and the 80 % point means no droplets but a mould risk, right of the 80 % point is safe.
Step-by-step calculation
1) Room dew point. At 22 °C and 50 %, the humidity ratio is W = 8.22 g/kg, the vapour pressure 1.322 kPa and the dew point tdp = 11.11 °C. The temperature at which the same air reaches 80 % RH (the mould limit) is 14.52 °C.
2) Thermal resistance of the bare wall. Rt = Rsi + d/λ + Rse = 0.11 + 0.125 + 0.043 = 0.278 m²K/W, so U = 3.60 W/m²K.
3) Inside surface temperature. The 33.3 K indoor-outdoor difference splits across the resistances in proportion, so θsi = Ti − (Ti − Te)·Rsi/Rt = 22 − 33.3 × 0.11 / 0.278 = 8.82 °C. With Rsi 0.13 it is 7.47 °C, and with the 0.25 used for corners and behind furniture it is 2.08 °C.
4) Verdict. Even on the open wall, 8.82 °C is 2.3 K below the 11.11 °C dew point, so it condenses. In a corner (2.1 °C) the surface is 9.0 K below the dew point, so far more water condenses, and spots that never dry out, such as behind furniture, stay wet. An uninsulated concrete exterior wall cannot cope with this room humidity.
5) Adding 50 mm of EPS. Insulation R = 0.05 / 0.037 = 1.351 m²K/W, Rt = 1.629, U = 0.614 W/m²K. The surface is 19.75 °C with Rsi 0.11, and still 17.29 °C with Rsi 0.25, which is 2.8 K above the 14.5 °C mould limit.
6) Minimum insulation. For the surface to stay at or above a limit θlim, Rt ≥ Rsi·(Ti − Te)/(Ti − θlim). For the mould criterion (θlim 14.52 °C, Rsi 0.25): Rt ≥ 0.25 × 33.3 / 7.48 = 1.112 m²K/W. Subtracting the concrete and the two surface resistances leaves an insulation R ≥ 0.694, which is 25.7 mm of EPS or 22.2 mm of XPS. Rounded up to 10 mm product steps: 30 mm EPS, 30 mm XPS.
What the result means in practice
| Criterion | Limit temperature · Rsi | Minimum fRsi | Minimum Rt | Insulation R | EPS mm | XPS mm |
|---|---|---|---|---|---|---|
| Condensation, open wall | 11.1 °C · Rsi 0.11 | 0.67 | 0.34 | 0.06 | 2.2 → 10 mm | 1.9 → 10 mm |
| Condensation, corner / behind furniture | 11.1 °C · Rsi 0.25 | 0.67 | 0.76 | 0.35 | 12.8 → 20 mm | 11.1 → 20 mm |
| Mould (surface RH 80 %) | 14.5 °C · Rsi 0.25 | 0.78 | 1.11 | 0.69 | 25.7 → 30 mm | 22.2 → 30 mm |
Preventing condensation and mould takes only a few tens of millimetres of insulation. Meeting an energy code U-value takes far more. For example, with a target U of 0.20 W/m²K you need 175 mm of EPS (180 mm product) or 151 mm of XPS (160 mm product), and that wall's surface is 20.4 °C even with Rsi 0.25. Statutory U-values depend on climate zone and building use, so check the current code. In practice the energy code sets the insulation thickness, and a condensation check is about finding the places where the insulation stops or thins out.
The minimum EPS thickness for the mould criterion (Rsi 0.25) changes with room humidity like this.
| Room RH | Dew point | 80 % limit | Minimum fRsi | Minimum EPS → product |
|---|---|---|---|---|
| 40 % | 7.8 °C | 11.1 °C | 0.67 | 12.8 mm → 20 mm |
| 45 % | 9.5 °C | 12.9 °C | 0.73 | 18.4 mm → 20 mm |
| 50 % | 11.1 °C | 14.5 °C | 0.78 | 25.7 mm → 30 mm |
| 55 % | 12.6 °C | 16.0 °C | 0.82 | 35.9 mm → 40 mm |
| 60 % | 13.9 °C | 17.4 °C | 0.86 | 51.0 mm → 60 mm |
Going from 40 % to 60 % RH raises the dew point from 7.8 °C to 13.9 °C and multiplies the insulation needed several times over. That is why, when a complaint comes in, you measure the room humidity (humidifier set point, ventilation rate, laundry dried indoors) before touching the wall.
Thermal bridges and corners: where condensation really happens
Everything above is a one-dimensional calculation for a flat wall with continuous insulation. Real condensation appears where heat leaks sideways: slab edges, beams and columns running out to the facade, balcony slabs, window reveals and the corner where two exterior walls meet. For these spots the surface temperature is expressed as a temperature factor.
fRsi = (θsi − Te) / (Ti − Te). Under this example's conditions, avoiding condensation needs fRsi ≥ 0.67 and avoiding mould needs fRsi ≥ 0.78. The bare wall has fRsi = 0.40, the wall with 50 mm EPS has 0.86 (both with Rsi 0.25). Even when the open wall clears the limit comfortably, a thermal bridge with fRsi below 0.78 grows mould right there. A thermal bridge's fRsi cannot be found with the one-dimensional formula. It comes from a 2D or 3D thermal simulation (the ISO 10211 method) or the manufacturer's thermal bridge data. The fixes are continuous insulation (external insulation, thermal break elements), air circulation at corners and behind furniture, and humidity control.
Common mistakes
- Checking condensation with the U-value Rsi. Rsi 0.11 is for heat loss. With 0.25 for corners and behind furniture, the same bare wall surface is 2.1 °C, not 8.8 °C.
- Treating anything above the dew point as safe. In this example, a surface between 11.1 and 14.5 °C shows no droplets but sits above 80 % RH, and mould grows.
- Checking only the lowest instantaneous outdoor temperature. A concrete wall has a lot of thermal mass, so a cold spell lasting days matters more than a minimum lasting hours. Look at the room humidity during that period too.
- Assuming a fixed room humidity. 40 % and 60 % RH need insulation thicknesses several times apart. Fix the wall without measuring and the problem comes back.
- Checking only the open wall. The open wall passes easily. Almost every condensation complaint comes from a thermal bridge where the insulation is broken.
Try this calculation on the site
Open the dew point tool with the bare wall surfaceOpen the corner surface (Rsi 0.25)Open the condensation check on the chart
The dew point tool links fill in the 22 °C / 50 % room and the surface temperature found above, and show the verdict, the margin and the highest room RH that surface can take without condensing. Change the room RH to find where condensation starts. The insulation surface condensation section on the same page is for the outside of insulation on chilled water pipes and ducts, where the cold side is inside. The chart link draws the room point and the condensation check surface temperature on the psychrometric chart. The plotting principles are in how to read a psychrometric chart, and exam-style problems are in the practice problems.
References
- ASHRAE Handbook Fundamentals 2017 (SI), chapter 1 Psychrometrics: humidity ratio, saturation vapour pressure (Hyland-Wexler) and dew point. The psychrometric numbers here were computed with this site's engine, which implements those equations at 101.325 kPa.
- ISO 6946: thermal resistance and U-value of building components, surface resistances Rsi 0.13 (horizontal heat flow) and Rse 0.04 m²K/W.
- ISO 13788: surface condensation and mould assessment. Rsi 0.25 m²K/W for opaque elements, the 80 % surface RH criterion and the temperature factor fRsi. The standard works with monthly mean conditions. This example uses the outdoor design temperature directly, which errs on the safe side.
- Korean Building Energy Saving Design Standard (MOLIT notice): internal and external surface resistances for Korean U-value calculations (0.11 and 0.043 m²K/W for exterior walls) and the regional wall U-value limits. The values here are those used in practice. Check the current notice for a design. The -11.3 °C outdoor temperature and the 0.20 W/m²K target U are example values.
- Numerical calculation of thermal bridge temperature factors: ISO 10211.