Duct Condensation and Insulation Calculator

Concepts behind this calculation

Check duct insulation surface temperature against ambient dew point, screen a measured surface directly, or solve a conditional minimum thickness for a round duct or a flat duct wall.

Surface condensation screening compares the exposed surface temperature with the dew point of nearby air. The selected allowance raises the target above dew point, so the reported margin compares the surface with that chosen target rather than with a universal insulation requirement. The direct method uses a supplied surface temperature without calculating thickness.

Thermal methods predict the outer surface from steady resistances and constant supplied properties. A bulk-air cold boundary includes inside film and wall resistance; a prescribed bare-wall boundary bypasses them. Flat-face and round radial geometry are different models. A conditional minimum thickness cannot guarantee dry seams, intact vapor control, or performance at thermal bridges.

How to use this calculator

1. Choose Calculation and enter Ambient dry-bulb and Ambient relative humidity.
2. For direct screening, document Dew-point screen source or measurement note and enter Measured or prescribed outer surface temperature and Selected surface allowance above dew point.
3. For thermal modes, select Cold temperature represents and document Conditions and property source; enter Cold boundary temperature, Selected temperature allowance above dew point, Effective insulation conductivity, Combined outside surface coefficient, and any applicable inside-film and wall properties.
4. Supply Installed insulation thickness or Maximum thickness to consider, plus Bare OUTSIDE duct diameter for round modes, and review the target margin.

Calculation method and applicability

Direct screen: dew point = inverse saturation state from ambient dry-bulb/RH; target surface = dew point + allowance; margin = surface temperature − target. Geometry modes retain the supplied steady thermal resistance models.

Input definitions and units

Calculation

Geometry and thickness solving are methods of one condensation task. Choices: Flat — check installed thickness; Flat — solve conditional minimum thickness; Round — check installed thickness; Round — solve conditional minimum thickness; Direct surface temperature vs dew point.

Cold temperature represents

Bulk air adds inside-film and wall resistance. A prescribed bare outer-wall temperature bypasses those resistances; do not enter an insulated jacket temperature. Choices: Bulk duct air; Prescribed bare outer duct wall. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Conditions and property source

Record ambient design basis, conductivity at mean temperature, inside film and combined outside film basis. Required note; no material presets or source authentication. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Dew-point screen source or measurement note

Record the ambient measurement/design basis and how the surface temperature was obtained. This note is not authenticated. Applies to Calculation: Direct surface temperature vs dew point.

Measured or prescribed outer surface temperature (°F)

Use the actual or prescribed outer surface temperature at the location being screened. No insulation thickness is inferred from this mode. Applies to Calculation: Direct surface temperature vs dew point.

Selected surface allowance above dew point (°F)

User-selected screening allowance; it is not a code minimum or uncertainty guarantee. Applies to Calculation: Direct surface temperature vs dew point.

Ambient dry-bulb (°F)

Use the humid local design condition, not automatically the peak cooling-load condition. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness or Direct surface temperature vs dew point.

Ambient relative humidity (%)

Supported range 1–100%. Saturated conditions can make finite-thickness prevention impossible. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness or Direct surface temperature vs dew point.

Cold boundary temperature (°F)

35°F through ambient. Interpret according to the selected bulk-air or bare-wall boundary. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Selected temperature allowance above dew point (°F)

User-selected allowance; not a code minimum or guaranteed uncertainty bound. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Effective insulation conductivity (BTU·in/(h·ft²·°F))

Supply dry installed effective k at the relevant mean temperature. This stays constant during the calculation. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Combined outside surface coefficient (BTU/(h·ft²·°F))

Include convection and linearized radiation. Assumes radiant surroundings at ambient; document orientation, air motion and jacket emissivity. No automatic wind/emissivity correlation. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Inside air-film coefficient (BTU/(h·ft²·°F))

Only for bulk-air boundary; use a documented value for the actual section and flow. Applies to Cold temperature represents: Bulk duct air; Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Duct wall thickness (in)

Physical wall thickness, not insulation. Zero permits a negligible-wall-resistance assumption. Applies to Cold temperature represents: Bulk duct air; Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Duct wall conductivity (BTU/(h·ft·°F))

Notice the foot conductivity unit differs from insulation k. No material identification is inferred. Applies to Cold temperature represents: Bulk duct air; Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Installed insulation thickness (in)

Zero checks the bare assembly; use installed, compressed thickness. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Maximum thickness to consider (in)

Explicit search/construction ceiling. Theoretical minimum is not rounded to a purchasable size. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Bare OUTSIDE duct diameter (in)

Diameter before external insulation. Bulk-air mode subtracts twice the metal thickness for inside diameter. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Installed insulation thickness (in)

Zero checks the bare assembly; use installed, compressed thickness. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Bare OUTSIDE duct diameter (in)

Diameter before external insulation. Bulk-air mode subtracts twice the metal thickness for inside diameter. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

Maximum thickness to consider (in)

Explicit search/construction ceiling. Theoretical minimum is not rounded to a purchasable size. Applies to Calculation: Flat — check installed thickness or Flat — solve conditional minimum thickness or Round — check installed thickness or Round — solve conditional minimum thickness.

How to interpret the result

A negative margin falls below the selected surface target. A nonnegative margin applies only to the stated location, conditions, and model; zero conditional thickness does not remove other insulation or vapor-control obligations.

Worked examples

Assumptions and limits

• One-dimensional steady dry model with constant user-supplied effective conductivity and film coefficients. Not a full ASTM C680 or ISO 12241 implementation and not a certified material or code selection.
• Cooling-duct scope: ambient 35–140°F, cold boundary 35°F through ambient, RH 1–100%. Hyland-Wexler saturation coefficients are explicitly from ASHRAE 2017 SI Chapter 1, equations 5–6, with the PsychroLib triple-point branch convention. Below-freezing saturation results are labeled dew/frost point; the modeled duct remains above freezing.
• The combined outside coefficient includes convection and linearized radiation at the stated condition; ambient air and radiant surroundings are assumed at the same temperature. Wind, emissivity, sky undercooling and orientation are not automatically solved. Recheck coefficients when thickness or surface temperature changes.
• Bulk-air mode includes inside film and wall conduction. Prescribed bare-wall mode bypasses those losses; it must not use an already insulated jacket temperature or combine the two boundaries. Round dimensions are bare outside diameter and metal thickness; radial areas change with radius.
• Flat mode represents a clear flat face, not a complete two-dimensional rectangular-duct corner model. External insulation only; internal lining, multiple insulation layers, contact resistance and double-wall assemblies require a separate model.
• The minimum is conditional on the supplied constant properties and entered maximum search thickness. A zero result means this mathematical surface target is met by the bare assembly, not that energy-code insulation or vapor protection is unnecessary. At saturation or a target at/above ambient, finite thickness may not achieve the target.
• Surface control does not prevent vapor diffusion, air leakage, interstitial condensation, rain ingress or wet insulation. No condensate rate, mold prediction, fire rating, structural safety or year-round condensation guarantee is calculated.
• Worked examples are constructed using documented equations, not product defaults. ASHRAE 2025 Table 5 values are retained as observations: some do not reproduce exactly from the listed assumptions and are not claimed as passing numerical benchmarks.
• Direct dew-point mode uses the same named saturation relation and above-freezing 35–140°F range, but it does not calculate heat flow or insulation thickness. Surface measurement location, thermal bridges, seams, wet insulation, transients and sky/radiation effects remain external.

Sources

• ASHRAE 2017 Fundamentals, SI Chapter 1Directly fetched original equations 5–6 and coefficients; named historical saturation relation, not a claim to use the revised 2025 IAPWS formula.
• ASHRAE 2025 Fundamentals, Chapter 23Surface-temperature and condensation control, flat/radial conduction, combined surface coefficients and vapor-ingress distinction. Original equations directly inspected; Table 5 discrepancies retained in research.
• PsychroLib: saturation-pressure documentationDocuments use of the water triple point for switching the ASHRAE 2017 ice/water branches; no runtime dependency added.
• ASHRAE 2025 Fundamentals, Chapter 25Conductivity, film coefficients, thermal resistance and clear-wall versus thermal-bridge context.