Moisture Vapor Drive Calculator

Calculate vapor pressure difference across wall assembly to assess moisture risk and determine vapor retarder requirements.

Winter heating condition
°F
Winter indoor RH, typically 30-40%
%
Winter design temperature
°F
Winter outdoor RH, typically 60-80%
%

Results

Indoor Vapor Pressure-in Hg
Outdoor Vapor Pressure-in Hg
Vapor Pressure Difference-in Hg
Vapor Drive Direction-
References: ASHRAE Fundamentals Ch. 26 - Heat, Air, and Moisture Control, IRC 2021 Table N1102.5