Building Heat Loss & Infiltration Calculator
Calculate whole-building peak heat loss combining envelope conductive transmission (U * A * Delta T) and blower door air infiltration leakage.
Interactive Calculator & Visualizer
Engineering Methodology & Governing Equations
Hydronic & Forced-Air Thermal Generation Flow
Fuel combustion and reverse-cycle heat pumping to offset building thermal envelope transmission losses.
Whole-Building Peak Heat Loss & Infiltration Equations
| Symbol | Variable | Description | Standard Units |
|---|---|---|---|
Q_total | Peak Building Heat Loss | Total heating power required to maintain setpoint at design temperature | BTU/hr |
U_i | Assembly U-Factor | Thermal transmittance of each surface (1 / R-value) | BTU/hrยทftยฒยทยฐF |
A_i | Surface Area | Net surface area of walls, roof, glazing, and doors | sq ft |
Delta T | Design Temperature Difference | Indoor setpoint minus the 99% ASHRAE winter design outdoor temperature | ยฐF |
CFM_inf | Infiltration Airflow | Natural air leakage volume entering through envelope cracks | CFM |
Conductive Transmission vs. Air Infiltration
A home loses heat through two primary thermodynamic pathways:
- Conductive Transmission (Q = U ร A ร ฮT): Heat traveling directly through solid materials (glass, drywall, wood/steel framing, siding). Determine exact whole-wall assembly U-factors accounting for stud thermal bridging with the Effective R-Value & Thermal Bridging Calculator.
- Air Infiltration (Q = 1.08 ร CFM ร ฮT): Cold outside air rushing through unsealed gaps around windows, rim joists, can lights, and electrical outlets. Air sealing with closed-cell spray foam or acoustic caulking is often the most cost-effective way to slash heating bills.
Accounting for Framing Thermal Bridging in Heat Loss
Standard residential cavity insulation (such as R-13 or R-19 batts) is interrupted every 16 or 24 inches by wood or light-gauge cold-formed steel framing. In steel stud framing, thermal bridging derates cavity insulation performance by 40% to 60% per ASHRAE Standard 90.1 Appendix A. To prevent undersizing heating equipment in cold climates, switch the calculator to ASHRAE 90.1 Assembly U mode or compute your exact assembly transmittance using the Effective R-Value Calculator and Framing Thermal Bridging Guide.
Standard Engineering Reference Matrix
| Building Envelope Vintage | 2,000 Sq Ft Heat Loss | Intensity | Infiltration % | Recommended Furnace |
|---|---|---|---|---|
| 2020s High-Efficiency Tight Home | 19,500 BTU/hr (5.7 kW) | 9.8 BTU/sq ft | 12% | 30,000 BTU / 2.0 Ton Heat Pump |
| 1990s Standard Code Suburban | 32,450 BTU/hr (9.5 kW) | 16.2 BTU/sq ft | 22% | 40,000 BTU / 3.0 Ton Heat Pump |
| 1970s Semi-Insulated Ranch | 48,900 BTU/hr (14.3 kW) | 24.5 BTU/sq ft | 31% | 60,000 BTU / 4.0 Ton Heat Pump |
| Pre-1950 Historic Leaky (Uninsulated) | 74,200 BTU/hr (21.7 kW) | 37.1 BTU/sq ft | 42% | 90,000 BTU / Dual-Fuel System |
Worked Engineering Sizing Example
Scenario: Calculating peak heat loss for a 2,000 sq ft home in Denver, Colorado where outdoor 99% design temperature is 10.0ยฐF and indoor setpoint is 70.0ยฐF (ฮT = 60.0ยฐF).
Calculation Steps:
- Calculate Temperature Difference: ฮT = 70.0 - 10.0 = 60.0ยฐF.
- Above-Grade Wall Conduction: 1,270 sq ft net wall @ R-19 (U-0.049) ร 60.0ยฐF = 3,733 BTU/hr.
- Ceiling & Attic Conduction: 2,000 sq ft ceiling @ R-38 (U-0.026) ร 60.0ยฐF = 3,120 BTU/hr.
- Window Conduction: 300 sq ft Low-E glass @ U-0.28 ร 60.0ยฐF = 5,040 BTU/hr.
- Slab Perimeter Conduction: 179 ft perimeter @ F-0.50 ร 60.0ยฐF = 5,370 BTU/hr.
- Air Infiltration Leakage: 18,000 cu ft volume @ 0.38 ACHnat = 114 CFM. Infiltration loss = 1.08 ร 114 ร 60.0 = 7,387 BTU/hr.
- Total Peak Heat Loss: 3,733 + 3,120 + 5,040 + 840 + 5,370 + 7,387 = 25,490 BTU/hr (7.5 kW).
- Equipment Recommendation: Adding a 15% ACCA Manual S safety factor yields 29,313 BTU/hr, matching a 30,000 to 40,000 BTU 96% AFUE furnace or a 2.5 Ton cold-climate heat pump.
Frequently Asked Questions
How is conductive envelope heat loss calculated?
How does air infiltration affect building heat loss?
What is the difference between whole-building heat loss and heating equipment capacity?
Governing Research Monograph: Deterministic Building Science & Dynamic Enclosure Infiltration Modeling for Residential Space Heating and Decarbonization Sizing
Report: HL-TR-2026-ENV03 โข Authors: HVACLogic Research Group, Miad S.
Calculations implemented and unit-tested against published engineering equations from ASHRAE Fundamentals, ACCA Manuals, SMACNA, and NIST thermodynamic references.