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Surface Condensation Risk Calculator

Cold ducts in humid attics, chilled-water pipes and cold walls sweat when their surface falls below the dew point. Enter the conditions and the insulation to see the surface temperature and the R-value needed.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

The humid air the surface is exposed to — e.g. attic or crawl-space air around a cold duct, or room air at a window.

Air or water inside the duct or pipe, or the outdoor temperature for a wall or window.

Insulation plus any solid layers (duct wall, glass). EPA Indoor airPLUS asks for at least R-6 on ducts outside the thermal boundary. Use the label R-value.

Result

Stays dry

Surface temperature
73°F
Dew point of the surrounding air
60.2°F
Margin above dew point
12.8°F

The surface stays at 73 °F, 12.8 °F above the dew point.

Minimum R-value to stay above the dew point
0.24h·ft²·°F/BTU
Surface film R used
0.68h·ft²·°F/BTU
Show the working
  1. Temperature drop across each layer is proportional to its R-value
    surface = 75 − (75 − 55) × 0.68 ÷ (0.68 + 6) = 72.96 °F
  2. Dew point of the surrounding air
    60 % at 75 °F → 60.19 °F
  3. R needed so the surface stays at the dew point: R = R_film × (T_air − T_cold) ÷ (T_air − dew point) − R_film
    0.238

How the surface temperature is found

Heat flows from the warm, humid air through a thin layer of still air on the surface (the air film), then through the insulation to the cold side. The temperature drop across each layer is proportional to its R-value, so:

  • Surface temperature = air temperature − (air − cold side) × film R ÷ (film R + insulation R)

The surface sweats when that temperature is at or below the dew point of the surrounding air. The tool also solves for the insulation R-value that would put the surface exactly at the dew point.

Air film values

The default still-air film resistances are the code default values for interior surfaces: R-0.68 for a vertical surface, R-0.61 for a horizontal surface with heat flowing up and R-0.92 with heat flowing down. You can enter your own value.

EPA’s Indoor airPLUS program asks for at least R-6 insulation on ducts outside the home’s thermal boundary; the default uses that value.

Insulation is not enough on its own

If humid air can reach the cold metal or pipe through gaps, seams or an unsealed vapour jacket, it will condense there no matter how thick the insulation is. Seal joints and use a continuous vapour-retarding jacket on cold ducts and pipes.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Air temperature around the surface
75 °F
Relative humidity of that air
60 %
Cold-side temperature
55 °F
R-value between the surface and the cold side
6 h·ft²·°F/BTU
Surface air film (warm side)
Vertical surface, still air (R-0.68)

Result

Surface temperature
73 °F
Dew point of the surrounding air
60.2 °F
Margin above dew point
12.8 °F
Minimum R-value to stay above the dew point
0.24 h·ft²·°F/BTU
Surface film R used
0.68 h·ft²·°F/BTU

The surface stays at 73 °F, 12.8 °F above the dew point.

Assumptions

  • Steady, one-dimensional heat flow through flat layers; the warm-side air film is the only resistance between the room air and the surface.
  • The cold-side film is ignored, which makes the surface slightly colder — a conservative assumption.

Limitations

  • Pipes and round ducts are treated as flat; for thick insulation on a small pipe the real surface is a little warmer.
  • Air or vapour reaching the cold surface through gaps in insulation will condense regardless of R-value.

Frequently asked questions

Why do attic ducts sweat in summer?

Attic air can be hot and humid, with a dew point above the 55 °F supply air. Bare or thinly insulated duct surfaces then fall below the dew point.

Does this work for pipes?

Yes, as an approximation. It treats the insulation as flat; on a small pipe with thick insulation the real surface is a little warmer, so the result errs on the safe side.

What margin should I leave?

The tool flags results within 3 °F of the dew point as marginal, because humidity and temperatures vary through the day.

Sources

  1. Canadian Building Digest CBD-36: Temperature Gradients Through Building Envelopes (Latta & Garden, 1962) — National Research Council Canada (archived copy at MIT). The temperature drop through each component is proportional to its thermal resistance; surfaces below the dew point of the air collect condensation; Table I example (70 °F in, 10 °F out): R 0.68, 0.11, 2.00, 0.05, 6.90, 0.97, 0.44, 0.17, total 11.32, interface temperatures 66, 54, 18, 13, 11 °F.
  2. 2021 Seattle Energy Code, Appendix A (A101.3 air films; Table A101.4 compressed batts) — City of Seattle, Department of Construction and Inspections. Default air-film R-values: 0.17 all exterior surfaces, 0.61 interior horizontal heat flow up, 0.92 interior horizontal heat flow down, 0.68 interior vertical; fibreglass batts R-30 at 9.5 in and R-49 at 15.5 in standard thickness; an R-19 batt in a 3.5 in cavity is rated R-13.
  3. PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
  4. Indoor airPLUS Version 2 Verification Requirements (July 2024) — U.S. Environmental Protection Agency. Item 4.4.2: HVAC supply and return ducts outside the thermal and air barrier boundary are insulated to at least R-6.

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