Wall Assembly Dew Point & Vapor Pressure Calculator

Steady-state dew point + Glaser vapor check, with a simplified air-leakage approximation · Imperial units · Abeazley Architecture

Preset / wall type

Saved wall types live in this browser only. Use Export to back them up or move them to another computer.

Conditions

Climate zone
Design condition
Values are approximate; verify against ASHRAE Fundamentals climatic design tables for the project site.

Air leakage (simplified approximation)

Envelope tightness

Layers (outside → inside)

NameMatThk inRR isR totPermAB
"R is": total = R for the whole layer, per in = R per inch × thickness ("R tot" shows what the calculation uses). Switching mode converts the number so total R stays the same. In total mode, editing thickness scales R for insulation (batt/foam/IMP); for other materials it only changes the drawn section. Perm 0 is treated as 0.001. AB = continuous air barrier (blocks the air-leakage blend).
About the method

Temperature profile: steady-state 1-D conduction; temperature drops across each layer in proportion to its share of total R (ASHRAE Handbook – Fundamentals, ch. 25–27, heat/air/moisture in building assemblies).

Vapor profile (Glaser method): steady-state vapor diffusion; vapor pressure drops across each layer in proportion to its vapor resistance (1/perm). Where actual vapor pressure reaches the saturation pressure at that face's temperature, condensation is predicted.

Saturation pressure: Magnus formula, Ps = 6.112·exp(17.62·T/(243.12+T)) hPa (T in °C), converted to inHg; dew point by inverting it.

Air leakage: a transparent blending approximation (see Air leakage card). It is not an airflow or CFD model.

Limitations: steady state only (no moisture storage, drying time, solar-driven vapor, rain, night sky cooling or seasonal cycling); 1-D; constant material properties; ignores girts and thermal bridges; ignores latent heat; air leakage is a rough blend. Results show tendencies, not quantities of water.

Detailed analysis: WUFI (transient hygrothermal simulation, per ASHRAE 160) is the industry tool for real assemblies with hourly climate data and moisture storage.

Results

    Wall cross section (to scale)

    Vapor pressure profile

    Interface table

    Materials library

    Insert new layer:
    MaterialCategoryThk inRPermABNotes / source
    Add / edit custom material
    Thk R Perm
    Click a layer row in the Layers table to select it (for "above selected" and "Replace"). Perm values are for the listed thickness; changing thickness does not rescale perm. Values are typical published figures (ASHRAE Fundamentals ch. 26 tables, manufacturer data); verify against the specified product. Custom materials are saved in this browser and included in Export/Import.