Condensation risk calculator
Enter the room/ambient air conditions and either a duct or pipe's surface temperature directly, or its insulation build-up — get the dew point, the margin above it, and (if you want it solved instead) the minimum insulation thickness that keeps the surface above the dew point.
How to use this calculator
- Enter the room/ambient temperature, relative humidity and altitude.
- Choose how you know the surface temperature — enter it directly, or build it up from a duct/pipe's insulation.
- If solving from insulation: pick the shape, internal temperature, and the wall/insulation materials and thicknesses.
- Set a safety margin above dew point if you want one.
- Read the dew point and the margin above it.
What this calculates
Condensation forms wherever a surface is colder than the dew point of the air touching it — regardless of the air's own temperature. This calculator works out the ambient air's dew point (reusing the exact same Arden Buck/barometric-formula psychrometric engine as this site's Mollier (h-x) diagram calculator, not new physics), the temperature of the surface in question, and the margin between them. The surface temperature can come from two places: you can enter it directly if you already know it (e.g. from a datasheet or a measurement), or let the calculator derive it from a duct or pipe's own construction — the temperature of the cold air/fluid inside, the wall, and the insulation build-up around it.
Surface temperature from a duct or pipe
This is a standard steady-state 1D thermal-resistance network — the same textbook heat-transfer approach (e.g. Incropera Fundamentals of Heat and Mass Transfer) used for any insulated-pipe or insulated-wall problem, not a fitted or empirical correlation. A duct wall is treated as flat (ignoring its curvature, the same simplification standard duct-insulation guidance uses); a pipe uses the exact cylindrical-shell result, since insulation thickness on a pipe is often a large fraction of the pipe's own radius, where the flat approximation would be a poor fit.
Duct (flat, per unit area): R = 1/hint + twall/λwall + tins/λins + 1/hext
Tsurface = Tamb − [(Tamb−Tint)/R] / hext
Pipe (cylindrical, per unit length), r₁=pipe ID/2, r₂=pipe OD/2, r₃=r₂+tins:
R′ = 1/(hint·2πr₁) + ln(r₂/r₁)/(2πλwall) + ln(r₃/r₂)/(2πλins) + 1/(hext·2πr₃)
Radiation isn't modelled as a separate term — it's lumped into the "external film coefficient" tiers, the same simplification most practical insulation-thickness guidance uses rather than a full radiative exchange calculation. Those tiers, and the internal film coefficients, are typical published ranges (comparable to the surface coefficient ranges ASHRAE Fundamentals itself tabulates for still vs moving air), not a single universal constant — override either with your own value if you have one. Thermal conductivities for the wall and insulation materials are typical textbook/manufacturer midpoints; check the actual product datasheet for a critical design.
Solving for the minimum insulation thickness
Set "Insulation thickness" to solve for the minimum needed and this calculator inverts the same resistance network, in closed form (no iteration, since resistance is linear in the flat/duct case — a genuine algebraic inversion, not a search): given a target surface temperature (the dew point plus a safety margin you choose), it solves for the one unknown resistance term and converts that back to a thickness. The pipe case can't isolate the insulation resistance in a single algebraic step, because the external film resistance itself depends on the insulation's outer radius — so that one is solved by bisection instead, but it's still an exact 1D monotonic root-find (thicker insulation always means a warmer surface), not an approximation.
If the result says "not needed", the bare surface already stays above your target even with no insulation at all for condensation-control purposes — insulation might still be needed for other reasons (energy loss, personnel protection, acoustic), which this calculator doesn't address.