Heat Loss Calculator — Wall and Insulation R-Value Estimator
Calculate heat loss (BTU/hr) through a wall assembly and estimate the total effective R-value from multiple insulation layers, including air films and sheathing.
Wall Assembly Layers (R-values per layer)
Enter 0 to omit a layer. R-values are in °F·ft²·hr/BTU.
Formulas Used
Total R-Value (series resistances):
Rtotal = Rint-air + Rdrywall + Rinsulation + Rsheathing + Rext-finish + Rext-air + Rextra
U-Value (overall heat transfer coefficient):
U = 1 / Rtotal [BTU / (hr · ft² · °F)]
Heat Loss Rate:
Q = U × A × ΔT [BTU/hr]
Where A = wall area (ft²), ΔT = Tindoor − Toutdoor (°F)
Equivalent forms:
Q = (A × ΔT) / Rtotal
Assumptions & References
- Heat transfer is modeled as one-dimensional steady-state conduction through a series of planar resistances (ASHRAE Fundamentals, Chapter 27).
- Default interior still-air film resistance: R-0.68 °F·ft²·hr/BTU (ASHRAE 2021 Fundamentals, Table 10).
- Default exterior air film resistance: R-0.17 (15 mph wind, ASHRAE 2021 Fundamentals, Table 10).
- Thermal bridging through studs is not accounted for; actual effective R-value may be 10–20% lower due to framing.
- Moisture effects, air infiltration, and radiation are not included in this simplified model.
- R-values are in U.S. customary units: °F·ft²·hr/BTU. To convert to SI (m²·K/W), multiply by 0.1761.
- Typical R-values: R-13 fiberglass batt (3.5″ cavity), R-19 (6″ cavity), R-5 rigid foam per inch of polyiso.
- IECC 2021 minimum wall R-values range from R-13 (Climate Zone 1) to R-20+5ci (Climate Zone 7–8).
- Annual heat loss estimate uses 180 heating days as a rough national average; use local HDD data for precision.