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Building Barrier Condensation Calculator
Climate conditions
Internal conditions
External conditions
Partition layers
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Temperature distribution in partition
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Condensation Risk Calculator for Building Partitions — Dew Point and fRsi
Professional condensation risk calculator for building partitions compliant with PN-EN ISO 13788:2013. Supports external walls, roofs, and ground floors. Calculates temperature distribution across multi-layer partitions, dew point, temperature factor fRsi, and assesses the risk of surface and interstitial condensation. Allows defining climate conditions (internal/external temperature and humidity) and any number of material layers from the database or with custom parameters. Automatically verifies compliance with fRsi ≥ 0.72 per Technical Requirements 2021. Results can be downloaded as a PDF report with temperature profile chart — ready for project documentation.
How to use the condensation calculator in 3 steps
Select the partition type (external wall, roof, or ground floor), then set the climate conditions — internal and external air temperature and humidity. You can use preset values or enter your own. Typical design conditions are 20°C and 50% humidity inside and -20°C outside.
Add partition layers in order from the internal to the external side. Select a material from the database (the calculator will insert default parameters) or enter custom thermal conductivity λ and diffusion resistance values. The calculator automatically computes temperature and vapor pressure distribution in each layer.
Check the results: fRsi coefficient, internal surface temperature, dew point, temperature margin, and condensation risk assessment. The temperature profile chart shows the temperature and dew point across the full partition thickness — interstitial condensation occurs where the dew point line crosses the temperature line.
Calculation formulas — dew point, fRsi, and Glaser method
The dew point is the temperature at which air at a given relative humidity becomes saturated with water vapor. It is calculated using the Magnus-Tetens formula:
Td = (b · γ) / (a − γ)
where γ = ln(RH/100) + (a·T)/(b+T), a = 17.625, b = 243.04, RH — relative humidity [%], T — air temperature [°C]. For typical residential conditions (20°C, 50% humidity) the dew point is approximately 9.3°C. Any surface in the room below 9.3°C will be at risk of condensation.
Temperature factor fRsi
The fRsi coefficient is a dimensionless indicator of condensation risk on the internal surface of a partition. The higher the fRsi value, the higher the internal surface temperature and the lower the condensation risk:
fRsi = (Rt + Rse) / (Rsi + Rt + Rse)
where Rt — thermal resistance of material layers [m²K/W], Rsi and Rse — internal and external surface heat transfer resistances per PN-EN ISO 6946, depending on the heat flow direction (for walls Rsi = 0.13 m²K/W, Rse = 0.04 m²K/W). Technical Requirements 2021 require fRsi ≥ 0.72 for partitions in rooms heated to at least 20°C.
Glaser method — interstitial condensation
The Glaser method (PN-EN ISO 13788) compares the actual vapor pressure distribution with the saturation pressure distribution across partition layers. Saturation pressure depends on temperature: ps(T) = 610.5 · e^(17.269·T/(237.3+T)) for T ≥ 0°C. Interstitial condensation occurs where actual vapor pressure exceeds saturation pressure — water vapor encounters layers too cold for it to pass through in gaseous form.
Types of condensation in building partitions
Surface condensation
Occurs on the internal surface of a partition when its temperature drops below the dew point. Symptoms include water droplets on the surface, damp and darkened plaster, peeling paint, and mold/fungus growth. This is the most visible type of condensation and relatively easy to diagnose.
Interstitial condensation
Occurs inside the partition when water vapor penetrating through materials encounters layers with temperatures below the dew point. Much more dangerous — difficult to detect, leads to degradation of thermal insulation, causes corrosion of steel elements, and can persist for years without visible symptoms.
How to prevent condensation?
- Thermal insulation continuity — thermal bridges (uninsulated ring beams, lintels, balcony connections, building corners, window and door surrounds) are the main cause of surface condensation. Example: an uninsulated RC column 25 cm (λ = 2.3 W/mK) gives fRsi ≈ 0.58 — well below requirements. Adding minimum 5 cm of EPS insulation raises fRsi above 0.72.
- Proper layer sequence — materials with high diffusion resistance (e.g. vapor barrier) should be placed on the warm side of the partition, and vapor-permeable materials on the cold side.
- Proper ventilation — internal air humidity above 60% drastically increases condensation risk. Air exchange must comply with standards.
- Adequate insulation thickness — for Polish climate minimum: EPS foam (λ=0.035) — 14–15 cm, mineral wool (λ=0.035) — 14–15 cm, PIR (λ=0.022) — 9–10 cm.
Standards and regulations
- PN-EN ISO 13788:2013 — hygrothermal performance of building components and building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation.
- PN-EN ISO 6946 — building components and building elements. Thermal resistance and thermal transmittance. Calculation method.
- Technical Requirements 2021 — Ministry of Infrastructure regulation on technical requirements for buildings and their location. Requires fRsi ≥ 0.72 for partitions in rooms heated to at least 20°C.
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