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Heat Loss Calculation Calculator

Heat loss calculation calculator: estimate BTU/hr and watts from envelope area, indoor−outdoor ΔT, and R-value—size heating loads and compare insulation before you upgrade.

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Steady-state heat loss through one envelope segment is a straight line: area, temperature difference, and R-value. This guide walks through the heat loss calculation calculator before you sum walls, roof, windows, and infiltration for a full building model.

Benefits

  • Clear formula: BTU/hr ≈ (area × ΔT) ÷ R for a single surface assembly.
  • Outputs watts and kW for electric heat and heat-pump sizing cross-checks.
  • Useful baseline before insulation upgrades—rerun with higher R to see loss drop.

How it works

  1. Enter envelope area in square feet for the wall, ceiling, or segment you are modeling.
  2. Set indoor−outdoor ΔT in °F (design-day temperature difference).
  3. Input assembly R-value; review BTU/hr, watts, and kW steady heat loss.

FAQ

How do I calculate heat loss through a wall?

BTU/hr ≈ (area sq ft × ΔT °F) ÷ R. Example: 400 sq ft wall, ΔT 40 °F, R-20 → 400 × 40 ÷ 20 = 800 BTU/hr (~234 W). Double R to 40 and the same segment drops to 400 BTU/hr.

What ΔT should I use for heat loss calculations?

Use a design temperature difference for your climate—often 40–70 °F for winter heating depending on indoor setpoint and local outdoor design temp. Summer cooling uses outdoor−indoor when heat flows inward. Match ΔT to the season you are sizing.

Does this calculator cover the whole house?

This tool models one envelope segment at a time. Sum BTU/hr or watts across walls, roof, floor, windows (often with U-value), and add infiltration for a whole-building load. Use each surface’s area and R—or U = 1/R for conductance.

Technical specifications

  • BTU/hr = (area_sq_ft × ΔT_°F) ÷ R.
  • Watts ≈ BTU/hr ÷ 3.412.
  • kW = watts ÷ 1000.
  • Related: home-insulation-savings, heater-cost, heat-pump-vs-resistance.

R-value is resistance

Higher R means less heat flow for the same area and ΔT. Attic jumps from R-30 to R-60 can halve conductive loss through that plane—before air sealing stops bypass flow that R alone cannot fix. Run the calculator twice with before/after R to quantify an insulation quote.

One segment, then stack

Contractors think in whole-house BTU/h; diagnostics start per surface. Calculate heat loss for the north wall, gable, or kneewall separately, then add segments. Gaps in your model—uninsulated rim joists, single-pane bays—show up when the sum falls short of metered heating use.