Heat Loss Calculator

The Heat Loss Formula

Heat flows from warm to cold through every surface of your building, and the rate depends on three things: the area of the surface, how well it resists heat flow (its U-value or R-value), and the temperature difference across it. The formula is Q = U × A × ΔT, where Q is heat loss in BTU/hr, U is in BTU/(h·ft²·°F), A is area in sq ft, and ΔT is the temperature difference in °F. U and R are reciprocals: U = 1/R. Higher R-value means better insulation means lower U means less heat loss.

Typical R-Values

Walls: 2x4 with fiberglass batts = R-13; 2x6 with dense-pack cellulose = R-20; 2x6 with closed-cell foam = R-23; double-stud with 9" cellulose = R-35. Roofs: R-19 is outdated; modern code is R-38 to R-60 in cold climates. Floors: over a vented crawl R-19 to R-30; over a heated basement use 0 (don't count). Windows: single-pane U-1.2; double-pane air U-0.50; double-pane argon low-E U-0.30; triple-pane argon low-E U-0.20.

Indoor and Outdoor Design Temperatures

Use 68-72 °F indoors (70 °F is standard). Outdoor design is the 99% winter design temperature from ASHRAE — the temperature that the outside air is colder than only 1% of the time. Examples: Anchorage −15 °F, Minneapolis −11 °F, Chicago −2 °F, New York 14 °F, Atlanta 22 °F, Houston 32 °F. Metric: −26 °C, −24 °C, −19 °C, −10 °C, −6 °C, 0 °C.

Air Infiltration

Even well-built houses leak. The calculator uses the rule Q = 0.018 × ACH × V × ΔT, where ACH is air changes per hour and V is the house volume. Typical values: 1950s house = 1.0 ACH, 1980s code = 0.5 ACH, modern energy-efficient = 0.25 ACH, passive house = 0.05 ACH (must be measured by blower door test). Infiltration is 10-40% of total heat loss and is often the biggest single item you can improve.

Typical Whole-House Heat Loss Benchmarks

Approximate design heat loss ranges for single-family houses at a 70 °F ΔT (0 °F outdoor, 70 °F indoor). Use these to sanity-check your result — they are empirical numbers from hundreds of Manual J calculations, not hard limits.

• 1,500 sq ft passive-house / deep-retrofit (R-40 walls, R-60 roof, triple-pane, 0.6 ACH50): 8,000-14,000 BTU/hr (2.3-4.1 kW)
• 1,500 sq ft modern code-built (R-21 walls, R-49 roof, double-pane Low-E argon, 3 ACH50): 18,000-26,000 BTU/hr (5.3-7.6 kW)
• 2,000 sq ft average 1980s house (R-13 walls, R-30 roof, double-pane clear, 0.5 ACH natural): 40,000-55,000 BTU/hr (12-16 kW)
• 2,000 sq ft uninsulated 1950s home (R-7 walls, R-11 attic, single-pane, 1.0 ACH natural): 80,000-110,000 BTU/hr (23-32 kW)
• 3,000 sq ft modern large home (R-21 walls, R-49 roof, Low-E argon, 2 ACH50): 36,000-48,000 BTU/hr (10.5-14 kW)

Design temperature adjustment: these numbers are at ΔT = 70 °F. Scale linearly for your local design temperature. A Boston home with 60 °F ΔT (10 °F outdoor) uses 60/70 ≈ 0.86 × the above. A Minneapolis home with 81 °F ΔT (−11 °F outdoor) uses 81/70 ≈ 1.16×. A Miami home barely needs heat at all.

Rule of thumb for modern homes: 10-20 BTU/hr per sq ft at design conditions. Older homes run 30-50 BTU/hr per sq ft. Passive houses are under 10. If your result falls way outside these ranges, recheck your inputs — especially window U-value (easy to get wrong by 2×) and infiltration ACH.

Frequently Asked Questions

Is this a Manual J calculation?

No — it is a simplified whole-house estimate. A full ACCA Manual J is room-by-room, accounts for orientation, internal gains, and more. Use this for planning; hire a pro for code compliance.

What size furnace should I buy?

Match output BTU/hr to the calculated design heat loss. A little oversizing (10-20%) is OK for cold snaps; more than that causes short cycling.

Should I include the basement?

Yes if it is heated. Use the basement wall area below grade and an R-value that accounts for both the foundation wall and surrounding soil.

Why doesn't solar gain help?

It does, but only during the day. Heat loss calcs target the worst-case nighttime scenario at the design temp.

How do I convert between U-metric and U-imperial?

U (metric, W/m²·K) × 5.678 = U (imperial, BTU/h·ft²·°F). The calculator does this automatically.