SettingsSettings for this calculationUS
The assumed indoor air temperature.
Use the winter interior design temperature for the space.
The outdoor winter design temperature for the site's climate zone.
Use a local winter design temperature (e.g. 99% or 97.5% design condition), not a record extreme low.
The full wall assembly's total thermal resistance, interior surface to exterior surface.
Every layer plus the surface films, end to end. The total matters because it fixes the temperature gradient, but the thing that decides whether the wall works is how that resistance is DISTRIBUTED through it — the dew point falls wherever the gradient crosses the dew-point temperature, and moving insulation from inside the structure to outside it moves that plane out of the framing. A correct total with the wrong distribution is a wall that condenses in the wrong place.
The portion of the total R-value between the interior face and where the vapor barrier sits.
A vapor barrier placed further toward the interior (warm) side has less R-value ahead of it, keeping it warmer; placed further toward the exterior (cold) side, it sits at a colder point in the gradient.
The dew point temperature of the interior air, based on its temperature and relative humidity.
If the assembly temperature at the vapor barrier location falls below this value, moisture in the air can condense there. The Dew Point and Relative Humidity Calculator works it out from the room's temperature and relative humidity.
Temperature at vapor barrier location
55.9 °F
The temperature at this location stays above the interior dew point shown below, so condensation is not predicted under the design conditions entered. No risk predicted under these conditions is not the same as none. The conditions are the ones you entered, and one surface is not the assembly.
- Interior air dew point
- 50 °F
They open the calculator with your figures already in it
Vapor Barrier Dew-Point Condensation Risk Calculator: 55.85 °F — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Simplified steady-state temperature gradient method: the temperature at any point within an insulated assembly = interior temp − (R-value from interior to that point ÷ total assembly R-value) × (interior temp − exterior temp); condensation risk exists if this temperature falls below the interior air's dew point
Inputs used
- Interior Design Temperature
- 69.8 °F
- Exterior Winter Design Temperature
- 14 °F
- Total Assembly R-Value
- 20
- R-Value from Interior Face to Vapor Barrier Location
- 5
- Interior Air Dew Point
- 50 °F
Intermediate steps
- Interior air dew point
- 50 °F
Confidence note: The temperature at this location stays above the interior dew point shown below, so condensation is not predicted under the design conditions entered. No risk predicted under these conditions is not the same as none. The conditions are the ones you entered, and one surface is not the assembly.
What this calculation does not cover
- Air leakage, not diffusion, is what usually wets a wall, and there is nothing about it here. The gradient gives the temperature at the plane; it cannot say how much moisture arrives there. A wall that passes this check and leaks warm interior air through a top plate, a service penetration or an unsealed electrical box deposits far more water at that plane than vapor diffusion through an intact assembly ever could.
- The R-values entered describe the clear field of the wall, between the framing. The path through every stud, plate and header is colder than this straight line says, and at a steel stud or an uninsulated slab edge much colder — so an assembly that passes in the middle of a bay can be sitting below dew point on the back of the sheathing at every framing member, which is exactly where mold turns up.
- It is one snapshot at one pair of temperatures, and it totals nothing. What damages an assembly is how many hours a year it spends below dew point and whether it dries out in between. A wall that dips below on a few cold nights and recovers is not the same wall as one that stays below for a month, and this returns the identical verdict for both.
- The obvious fix for a failing result can produce a wall that cannot dry. Moving the barrier inboard is right only if the outboard side is open to vapor. Where a low-perm layer already sits outside — exterior foam, a self-adhered membrane, an impermeable sheathing — a second one inside traps whatever gets past either of them, and the assembly then passes this temperature check with no drying path in either direction.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Simplified steady-state temperature gradient method: the temperature at any point within an insulated assembly = interior temp − (R-value from interior to that point ÷ total assembly R-value) × (interior temp − exterior temp); condensation risk exists if this temperature falls below the interior air's dew point
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