Checks whether the outer surface temperature of an insulated pipe stays above the
dew point of ambient air. Uses Magnus dew point formula and steady-state thermal resistance.
Tender-stage only. Confirm with project spec and manufacturer guidance, especially for cold services.
Does not model vapour barriers, radiation, k-vs-temperature variation, or moisture ingress. For design guidance only. Always verify with a qualified engineer and manufacturer data.
About this condensation / dew point check calculator
This free condensation check tells you whether the outer surface of an insulated pipe stays above the dew point of the surrounding air. The condition you need to satisfy to keep cold services (such as chilled water at 4–8 °C) free of surface condensation. You enter the ambient temperature and relative humidity, a safety margin, the service temperature, the pipe outside diameter, the insulation thickness, the insulation conductivity (k) and the surface film coefficient (h); it returns the dew point, the target surface temperature, the predicted surface temperature, a pass / marginal / fail verdict and the minimum insulation thickness needed to clear the target. It is aimed at UK building-services engineers and contractors sizing pipe insulation at tender stage.
How it works
The dew point is found from the ambient temperature and relative humidity using the Magnus formula with the Sonntag (1990) / WMO No. 8 constants (a = 17.62, b = 243.12 °C). The surface temperature is found from steady-state cylindrical heat flow: the insulation conduction resistance Rcond = ln(r₂/r₁)/(2πk) in series with the surface film resistance Rs = 1/(h·2πr₂), giving the heat flow per metre and hence the temperature drop across the film. The verdict compares the surface temperature against the dew point plus your safety margin. The result is indicative only. It does not model vapour barriers, radiation, k-versus-temperature variation or moisture ingress, so always confirm against the project specification and manufacturer data.
Frequently asked questions
What does the pass, marginal or fail verdict mean?
The target surface temperature is the dew point plus your chosen safety margin (typically 1–3 °C). If the predicted surface temperature sits comfortably above that target it reads PASS; if it is only just above the target it reads MARGINAL; if it falls below the target the surface is at condensation risk and it reads FAIL.
Which insulation conductivity should I use?
The calculator offers preset design k-values for phenolic (0.025), PIR / PUR (0.025) and mineral wool (0.040 W/m·K), or you can enter a custom value. Use the figure from your chosen product's data sheet, as conductivity varies by material and manufacturer.
What is the minimum thickness figure?
It is the insulation thickness, solved to 0.1 mm resolution up to a 250 mm ceiling, that raises the surface temperature enough to clear the target with a comfortable cushion, landing in PASS rather than MARGINAL, for the pipe size, conductivity, film coefficient and ambient conditions you entered.