Heat Loss Calculator

Heat loss (BTU/h) = U-value × Area (sq ft) × Temperature Difference (°F). U-value is 1/R-value (so R-19 insulation has U = 0.053). Temperature difference is your inside setpoint (68–70°F) minus your outdoor design temperature. Sum this calculation for all surfaces (walls, ceiling, floor, windows) and add 15–30% for infiltration.

The DOE recommends: Attics in most US climates R-38 to R-60. Walls (2×4 framing): R-13 to R-15. Walls (2×6 framing): R-20 to R-21. Floors over unheated spaces: R-25 to R-30. These recommendations vary by climate zone — colder climates require higher R-values. Check your state energy code for specific requirements.

Use your indoor design temperature (typically 68–70°F) minus your local 99% design temperature (coldest 1% of winter hours). Common design temperatures: Minneapolis: -12°F (ΔT ≈ 82°F), Chicago: -4°F (ΔT ≈ 74°F), New York: 11°F (ΔT ≈ 59°F), Atlanta: 19°F (ΔT ≈ 51°F), Dallas: 22°F (ΔT ≈ 48°F). ASHRAE Fundamentals Handbook lists design temperatures for all locations.

Online calculators provide useful estimates for initial planning and sizing, but a professional Manual J calculation is required for HVAC permit applications in most jurisdictions and is far more accurate. Manual J accounts for local climate data, exact construction details, window orientation, internal gains, and infiltration testing (blower door). Use this calculator for budgeting and understanding your home's energy profile.