Heat loss is the conductive piece of a heating load calculation — how many BTUs the building gives up through the walls, windows, ceiling, and floor when the indoor and outdoor temperatures don't match. It's the foundation of Manual J, the basis of every weatherization decision, and the math behind whether sealing the rim joist is actually worth the weekend.

The formula — Q = U × A × ΔT

Per surface:

Q = U × A × ΔT

  • Q = heat flow in BTU/hr
  • U = U-value, the rate of heat flow per ft² per °F. U = 1/R where R is the insulation rating
  • A = surface area in ft²
  • ΔT = temperature difference between inside and outside in °F

Run the math once for each surface — wall, window, ceiling, floor — and add them up. That's the whole-house conductive heat loss at design conditions.

R-value vs U-value

Same physics, inverted scale. R is the resistance to heat flow; U is the rate of heat flow:

  • R-19 wall → U = 1/19 = 0.053
  • R-30 ceiling → U = 1/30 = 0.033
  • Single-pane window → U = 1.0 (R = 1)
  • Double low-E window → U = 0.30 (R ≈ 3.3)
  • Triple-pane window → U = 0.18 (R ≈ 5.6)

Insulation is sold by R-value because that's the quick mental model — R-19 is more than R-13. Windows are sold by U-value because the assembly (glass + frame + spacer) doesn't have a uniform R-value, and the U number better describes the performance.

The 99% design temperature

ΔT in the formula uses the 99% outdoor design temp — the temperature your area is colder than only 1% of the heating season hours. Sizing equipment to record-cold means oversizing for 99.9% of the year. ACCA Manual J Table 1 lists the 99% design temp for every U.S. city; the calculator's tooltip has typical values:

  • Phoenix: 31°F
  • Atlanta: 23°F
  • St. Louis: 11°F
  • NYC: 11°F
  • Boston: 6°F
  • Chicago: -3°F
  • Minneapolis: -12°F
  • Duluth: -19°F
  • Fairbanks: -47°F

What the calculator doesn't include

Real Manual J adds three pieces beyond conductive loss:

Infiltration. Air leaking through cracks around windows, doors, electrical penetrations, and the rim joist carries heat with it. Typical loss = 0.018 × volume × ACH × ΔT. A 20,000 ft³ house at 0.35 natural ACH and ΔT 65 = 8,200 BTU/hr. Substantial.

Duct losses. Ducts in unconditioned attics or crawlspaces lose 15-25% of system output to that space. Unconditioned-space ducts are the single most expensive design decision in residential HVAC.

Internal gains. Bodies (250 BTU/person sensible), lights (3.4 BTU per W of lighting), appliances (1,000-1,500 BTU/hr from cooking, dishwasher, dryer). On the heating side these reduce the equipment-sized load.

A reasonable rule: take the conductive loss from the heat loss calculator, multiply by 1.25-1.40 for infiltration, then subtract typical internal gains (~1,500-2,500 BTU/hr for a small house) to get the heating equipment size.

Where the heat goes — typical breakdown

Whole-home loss for a typical 2,000 ft² 2-story house in Chicago (ΔT = 73°F):

  • Walls (R-19, 1,600 ft²): 6,150 BTU/hr
  • Windows (U-0.32, 240 ft²): 5,600 BTU/hr
  • Ceiling (R-38, 1,000 ft²): 1,920 BTU/hr
  • Floor (R-19, 1,000 ft²): 3,840 BTU/hr
  • Total conductive: ~17,500 BTU/hr
  • + infiltration (×1.30): ~22,800 BTU/hr
  • − internal gains (~2,000): ~20,800 BTU/hr design load

That's a 21,000 BTU/hr (1.75 ton) heat-pump heating load. A contractor selling you a 4-ton unit "to be safe" is oversizing 2× — Manual J catches that.

Common heat-loss mistakes

Using nominal R-value, not effective. R-19 batt installed badly performs at R-13. Thermal bridging through 2x4 studs drops a "R-13 wall" to a whole-wall R-value of about 9. Effective R, not nominal R, is what goes in the calc.

Forgetting basement walls. A heated basement loses heat through below-grade walls at a slower rate than above-grade (soil temp moderates), but it's not zero. Manual J includes basement loss; the calculator doesn't — add it manually for a heated below-grade level.

Ignoring the floor over the garage. The garage is unconditioned. Floor over it gets the full ΔT, not the indoor-to-indoor ΔT. Insulate the floor or condition the garage; either way, count it.

Estimate only. The heat loss calculator gives conductive loss only. Infiltration, duct losses, and internal gains must be added for a true Manual J load. Use this for envelope-improvement decisions and as a sanity check; equipment sizing requires a credentialed HVAC contractor running full Manual J.