For estimation and sales-phase use. A preliminary psychrometric check for condensation risk. Moisture and vapor-control design needs its own analysis. Not a substitute for a licensed engineer's review.

Calculator · Psychrometrics

Dew point calculator - will this duct sweat?

Enter the dry-bulb temperature and relative humidity of the air around a duct or grille, or next to a crawlspace wall. The calculator returns the dew point, or the frost point below 32°F. Any surface at or below that temperature collects water. The number comes from the same tool the BuildSolver assistant runs in chat.

Two inputs, sea-level pressure. Water surface at or above 32°F dry-bulb, ice surface below.

Air temperature where the surface sits: attic, crawlspace, room.

Percent, not a decimal. Enter 55 for 55% RH.

Example result for the default inputs

80 °F dry-bulb · 60% RH · sea-level pressure

Dew point

64.9°F

Dew point

18.3°C

Calculation details
Saturation surfaceover water (Sonntag 1990 water coefficients)

For estimation purposes only. Consult a licensed engineer for final designs.

Check the whole job in chat

Free without signup, 3 questions a day

How the dew point is calculated.

The calculator runs the Magnus-Tetens approximation with the Sonntag 1990 coefficients, the same code the BuildSolver assistant uses when you ask for a dew point in chat. Math runs in °C, then converts back to °F. Coefficient pairs switch on the dry-bulb temperature: at or above 32°F the saturation surface is water, below 32°F it is ice, and the result is a frost point.

Dew point
T_dp = (b × γ) / (a − γ)The formula works in °C, and the result is converted back with °F = °C × 9/5 + 32.
Gamma
γ = ln(RH / 100) + (a × T) / (b + T)T is the dry-bulb you enter in °F, converted to °C for the formula. RH is relative humidity in percent.
Over water (T ≥ 32°F)
a = 17.62, b = 243.12 °CSonntag 1990 water coefficients. The result is a dew point.
Over ice (T < 32°F)
a = 22.46, b = 272.62 °CSonntag 1990 ice coefficients. The result is a frost point: the temperature where frost forms on a surface.
Pressure
p = 14.696 psia (sea level)No altitude or barometric correction. Magnus-Tetens uses RH and temperature only.

A worked example with a supply duct in an 80°F crawlspace.

Vented crawlspace in July · 80°F dry-bulb, 60% RH measured next to the supply trunk · duct wrap torn at two fittings. The question: will the bare metal sweat?

80°F dry-bulb · 60% RH · sea-level pressure · over water

64.9°F dew point · 18.3 °C

For estimation only · Magnus-Tetens, Sonntag 1990 · computed by the calculator above

Any surface in that crawlspace colder than 64.9°F collects water. A metal fitting carrying cooled supply air runs well below that, so the exposed spots will drip. Rewrap the fittings with an intact vapor retarder on the outside of the insulation, and look at the crawlspace moisture source.

Standards and data sources.

  • ASHRAE Handbook - Fundamentals (2021), Ch. 1 Psychrometrics - the reference the tool cites for dew point from dry-bulb and relative humidity.
  • Sonntag (1990), adopted in the WMO/CIMO Guide - the water and ice coefficient pairs used in the Magnus-Tetens formula.

Limitations of this calculator.

  • Sea-level pressure (14.696 psia) is assumed. Elevation is not an input and does not change the result.
  • Magnus-Tetens is an approximation. Use it for field checks and quotes, and run permit-grade work through a full psychrometric property model.
  • Water and ice curves do not meet smoothly at 32°F. Within about 1°F of freezing the tool warns that the Sonntag (1990) water and ice coefficient pairs can give dew points up to 1.9°F apart.
  • The tool also flags RH under 10% (accuracy drops), RH under 1.5% (likely a decimal such as 0.5 typed for a percent such as 50), dry-bulb below −20°F (the ice branch is less certain) and 99.5% RH and above (condensation is imminent, with the dew point within a fraction of a degree of the dry-bulb).
  • The result describes the air. It does not model surface temperatures, insulation, vapor drive or how long a surface stays below the dew point.

Common dew point mistakes.

  1. Using indoor conditions for a duct in the attic or crawlspace. Condensation on a supply duct depends on the air around the duct. Take dry-bulb and RH in the attic or crawlspace where the duct runs.
  2. Entering RH as a decimal. 0.6 means 0.6% RH, not 60%. The dew point drops far below any real condition. The tool flags values under 1.5%.
  3. Checking the dew point against the discharge air reading. Sweating starts where a surface is at or below the dew point. Put a surface thermometer on the grille face, the duct jacket and any uninsulated fitting, and compare those readings to the dew point.
  4. Reading a frost point as a dew point. Below 32°F dry-bulb the tool uses ice coefficients. The number is where frost forms on a surface. Label it as a frost point in your notes and quotes.
  5. Trusting a single reading near saturation. Near 100% RH, a small sensor error moves the dew point right up to the dry-bulb. When a reading is that high, confirm it with a second instrument before blaming the duct insulation.

Common questions about dew point.

+Why do supply grilles and ducts sweat?

A supply duct or grille carrying cooled air can run colder than the dew point of the air around it. In a humid attic or crawlspace at 80°F and 60% RH the dew point is near 65°F, so any duct jacket, fitting or grille face under 65°F in that space will sweat. Enter your own readings above to get the number for the job.

+What dry-bulb and humidity should I enter?

Enter the conditions of the air that touches the surface you care about. For a duct in a vented crawlspace, measure in the crawlspace. At a supply grille, use the room air. For the outside of a vapor retarder, use the air on the humid side.

+What is a frost point and when does the calculator show it?

When the dry-bulb is below 32°F, the tool uses the Sonntag 1990 coefficients for ice instead of water. The result card then changes its label to Frost point.

+Does the calculator correct for altitude?

No. The tool assumes sea-level pressure of 14.696 psia. The Magnus-Tetens formula uses only temperature and relative humidity, so elevation does not change this result. Humidity ratio, enthalpy and density do depend on pressure; BuildSolver's psychrometric calculator also assumes sea level, so treat those values as sea-level figures on high-elevation jobs.

+What does the condensation imminent warning mean?

At 99.5% RH and above, the air is at or very near saturation. The dew point equals or nearly equals the dry-bulb, so any surface slightly cooler than the air will collect water.

+Why is there a warning near 32°F?

The water and ice formulas give different answers at the freezing point. Within about 1°F of 32°F dry-bulb, the tool warns that the dew point may differ by up to 1.9°F depending on which surface is assumed.

+How does dew point relate to coil latent capacity?

A cooling coil removes moisture only where its surface is below the dew point of the entering air, so a higher entering dew point means more water to pull. This page stops at the dew point. Run the latent load through a whole-house Manual J.

+How many calculations can I run?

The on-page calculators share a limit of 10 calculations per day per visitor. The BuildSolver chat runs the same dew point tool and can carry on into a whole-house Manual J or a quick duct size. Manual S equipment selection, room-by-room loads and full Manual D need a Pro or Team plan.

Take the condensation call into chat.

Describe the duct run and whether it sits in an attic or a crawlspace. The assistant runs the dew point tool, asks for any missing readings, and can run a whole-house Manual J or a quick duct size on the same job. Room-by-room loads, full Manual D and Manual S are on the Pro and Team plans.

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