Wall & Roof Assembly R-Value, U-Factor & Dew Point

Layers entered inside to outside, each with its own sourced R-value: total R, U-factor, the steady-state temperature at every interface, the indoor dew point, and a condensation-risk screen.

Open the tool

Method

envelope-physics 1.1

Engine

1.1.0

Output status

Preliminary - review required

Review

Awaiting review

Formulas

R total = inside air film + sum of layer R-values + outside air film; U = 1 / R total
T(interface) = T indoor - (T indoor - T outdoor) x (R from the indoor air to that interface) / R total
Dew point (Magnus, Alduchov & Eskridge 1996): g = ln(RH / 100) + 17.625 T / (243.04 + T); Td = 243.04 g / (17.625 - g), T in degC
An interface at or below the indoor dew point is flagged as a condensation risk; when the dew point is below 0 degC the frost point (Magnus over ice, Alduchov & Eskridge 1996), which is warmer, is used instead
1 h·ft²·°F/Btu = 0.3048² x 3600 x (5/9) / 1055.05585262 = 0.1761102 m²·K/W (exact definitions)

Conventions

Layers are listed from the inside (conditioned side) to the outside.
Each R-value is for the layer as installed and carries its source. No insulation R-value table is built in.
Air films are an ASHRAE published value chosen by the user, a sourced value of their own, or none - never assumed.

Limits - what it will not do

One-dimensional clear-field section: framing, fasteners and other thermal bridges are not included.
The condensation flag is a steady-state screening, not a hygrothermal analysis such as WUFI: no vapour permeance, moisture storage, rain, solar gain or time.
Only moisture in indoor air is screened; inward vapour drive in warm, humid weather is not assessed.
Dew-point constants are fitted from -40 to 50 degC; indoor temperatures outside that range are refused.

Who must review it

Building-envelope consultant: layer values, air films and the condensation screen for the project climate

Worked example

Inputs Films 0.68 + 0.17, layers R-10 and R-5 (h·ft²·°F/Btu); 70 degF indoors at 40% RH, 0 degF outdoors

Expected R 15.85, U 0.0631; dew point 44.6 degF; 22.8 degF between the layers, flagged

Checked by an automated test on every change.

Sources

ASHRAE Handbook - Fundamentals: Heat, Air, and Moisture Control in Building Assemblies - Fundamentals (thermal resistance of layers in series) - ASHRAE, read 2026-10-16
One-dimensional steady-state conduction: resistances of layers in series add, U = 1 / R total, and the temperature drop across each layer is in proportion to its share of R total.
ASHRAE Handbook - Fundamentals: Heat, Air, and Moisture Control in Building Assemblies - Material Properties, surface film coefficients/resistances table - ASHRAE, read 2026-10-16
Surface film resistances for non-reflective surfaces (emittance 0.90), h·ft²·°F/Btu: still air, vertical surface, horizontal heat flow 0.68; horizontal surface, heat flow up 0.61; horizontal surface, heat flow down 0.92; moving air 15 mph (winter) 0.17; 7.5 mph (summer) 0.25. SI values are converted from these.
2021 Seattle Energy Code, Appendix A - Default Heat Loss Coefficients, Section A101.3 air films - City of Seattle (developed from the ASHRAE Fundamentals Handbook), read 2026-10-16
Publicly readable confirmation of the ASHRAE air film values: exterior 0.17, interior vertical 0.68, interior horizontal heat flow up 0.61, heat flow down 0.92 (h·ft²·°F/Btu).
Improved Magnus Form Approximation of Saturation Vapor Pressure (Alduchov and Eskridge, 1996), J. Applied Meteorology 35, 601-609 - American Meteorological Society, read 2026-10-16
Magnus form over water es = 6.1094 exp(17.625 T / (T + 243.04)) hPa, T in degC, recommended for -40 to 50 degC. Dew point Td = 243.04 g / (17.625 - g), g = ln(RH/100) + 17.625 T / (243.04 + T).
NIST Special Publication 811 - Guide for the Use of the International System of Units (SI), Appendix B conversion factors - National Institute of Standards and Technology, read 2026-10-16
Exact factors: 1 in = 0.0254 m; 1 ft = 0.3048 m; 1 yd = 0.9144 m; 1 mi = 1609.344 m; 1 lb = 0.45359237 kg; 1 U.S. gallon = 231 in3 = 3.785411784 L; 1 Btu (IT) = 1055.05585262 J; t(degC) = (t(degF) - 32) / 1.8; T(K) = t(degC) + 273.15.