# Heat Loss Calculator

Calculate building heat loss through walls, windows, ceiling, and floor. Estimate BTU/hr loss and required furnace size based on insulation.

## What this calculates

Understanding where your home loses heat helps prioritize insulation upgrades and correctly size heating equipment. This calculator estimates heat loss through walls, windows, ceiling, and floor using the standard formula Q = U x A x ΔT, then recommends a furnace size with a safety margin for air infiltration.

## Inputs

- **Wall Area** (sq ft) — min 0 — Total exterior wall area (excluding windows)
- **Window Area** (sq ft) — min 0 — Total window area
- **Ceiling/Roof Area** (sq ft) — min 0 — Top-floor ceiling area exposed to unconditioned space or outdoors
- **Floor Area** (sq ft) — min 0 — Floor area over unconditioned space (crawlspace, garage, etc.)
- **Temperature Difference** (°F) — min 0, max 120 — Difference between indoor target (e.g., 68°F) and outdoor design temperature
- **Window Type** — options: Single-pane (U=1.0), Double-pane (U=0.5), Triple-pane Low-E (U=0.25) — Type of windows installed
- **Wall Insulation** — options: None / Minimal (U=0.25), R-13 Standard (U=0.077), R-19 Enhanced (U=0.053), R-21+ High-Performance (U=0.048) — Wall insulation R-value

## Outputs

- **Total Heat Loss** (BTU/hr) — Combined heat loss through all surfaces
- **Wall Heat Loss** (BTU/hr) — Heat lost through exterior walls
- **Window Heat Loss** (BTU/hr) — Heat lost through windows
- **Ceiling Heat Loss** (BTU/hr) — Heat lost through ceiling/roof
- **Floor Heat Loss** (BTU/hr) — Heat lost through floor
- **Required Furnace Size** — formatted as text — Minimum furnace capacity with 20% safety margin

## Details

Heat flows from warm areas to cold areas through every building surface. The rate of heat loss depends on three factors: the U-value (thermal transmittance of the material), the area of the surface, and the temperature difference (ΔT) between inside and outside.

Windows are typically the weakest point in a building envelope. Single-pane windows have a U-value of 1.0, meaning they lose 1 BTU per hour per square foot per degree F, roughly 13 times more than R-13 insulated walls. Upgrading from single to double-pane windows cuts window heat loss in half. Triple-pane low-E windows reduce it by 75%.

The calculator adds a 20% safety margin to the calculated heat loss to account for air infiltration, the warm air that escapes through gaps around doors, windows, electrical outlets, and other penetrations. In drafty older homes, infiltration can account for 25-40% of total heat loss. A professional blower door test can measure your actual infiltration rate.

## Frequently Asked Questions

**Q: What temperature difference should I use?**

A: Subtract the outdoor design temperature from your indoor target (usually 68°F). The outdoor design temperature is the coldest temperature expected in your area. For example, if your area's design temperature is 5°F, use 68 - 5 = 63°F. You can find design temperatures from ASHRAE or your local building code office.

**Q: What is a U-value vs. R-value?**

A: R-value measures insulation's resistance to heat flow (higher is better). U-value measures heat transmission rate (lower is better). They're reciprocals: U = 1/R. R-13 insulation has a U-value of 0.077. U-values are used in heat loss calculations because they can be directly multiplied by area and temperature difference.

**Q: Where do homes lose the most heat?**

A: The biggest heat loss areas, in typical order: (1) Air leaks and infiltration: 25-40%. (2) Windows: 25-30%. (3) Walls: 15-20%. (4) Ceiling/attic: 10-15%. (5) Floor/foundation: 10-15%. (6) Doors: 5-10%. Prioritize sealing air leaks first; it's the cheapest improvement with the biggest impact.

**Q: What about heat loss through the floor?**

A: Floor heat loss depends on what's below. Slab-on-grade loses heat primarily around the perimeter (about 0.75 BTU/hr per linear foot per degree F). Floors over unconditioned crawlspaces or garages lose heat through the floor area (U-value depends on insulation). Basements lose heat mainly through the walls above grade.

**Q: Why is a 20% safety margin added?**

A: The 20% margin covers air infiltration: warm air escaping through cracks, gaps, and penetrations. It also accounts for extreme cold snaps that exceed design temperature and duct losses in unconditioned spaces. For very tight, well-sealed homes, 10% may be sufficient. For older, drafty homes, 30% is safer.

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Source: https://vastcalc.com/calculators/construction/heat-loss
Category: Construction
Last updated: 2026-04-21