Building Heat Loss & Infiltration Calculator
Estimate whole-building peak heat loss combining envelope conductive transmission (U * A * Delta T), slab perimeter F-factors, and 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 | Preliminary heating power required to maintain indoor temperature under winter design conditions | BTU/hr |
U_i | Assembly U-Factor | Overall thermal transmittance of building envelope surface (1 / R_effective) | BTU/hrยทftยฒยทยฐF |
A_i | Surface Area | Net surface area of above-grade walls, ceiling/roof, windows, and exterior doors | sq ft |
F_slab | Foundation F-Factor | Linear heat loss coefficient per linear foot of exposed slab perimeter (e.g. F-0.50 for uninsulated slab) | BTU/hrยทftยทยฐF |
Delta T | Design Temperature Difference | Indoor setpoint minus the local 99% ASHRAE winter design outdoor temperature | ยฐF |
CFM_inf | Infiltration Airflow | Natural outdoor air leakage volume entering through envelope cracks (Volume * ACHnat / 60) | CFM |
Conductive Transmission, Foundation F-Factors & Infiltration
A home loses heat through three primary physical pathways:
- Envelope Conduction (Q = U ร A ร ฮT): Heat transferring through solid above-grade walls, roof/ceiling, windows, and exterior doors. To account for framing thermal bridging across wood or steel studs, evaluate assembly U-factors with the Effective R-Value Calculator.
- Foundation Perimeter Conduction (Q = F ร P ร ฮT): Slab-on-grade heat loss occurs predominantly at the exposed slab edge perimeter rather than uniformly through the slab floor. ACCA Manual J and ASHRAE characterize slab loss using linear F-factors (e.g., F-0.50 for uninsulated edge slabs).
- Air Infiltration Leakage (Q = 1.08 ร CFM ร ฮT): Cold outdoor air entering through unsealed envelope penetrations. Natural air changes per hour (ACHnat) are derived from blower-door tightness tiers (ACH50 / N-factor) multiplied by building volume.
Screening Estimates vs. ACCA Manual J & Manual S Sizing
This calculator provides a preliminary building-level heat loss estimate based on simplified geometric baselines. It is not a substitute for a full room-by-room ACCA Manual J load calculation, which accounts for exact window orientations, duct heat losses, internal heat gains, and room boundary surfaces. Furthermore, equipment selection requires applying ACCA Manual S sizing limits against manufacturer-verified heating output tables at local design temperatures.
Standard Engineering Reference Matrix
| Building Envelope Vintage | 2,000 Sq Ft Peak Heat Loss | Thermal Intensity | Infiltration Share | Preliminary Demand |
|---|---|---|---|---|
| 2020s High-Efficiency Tight Home | 15,950 BTU/hr (4.7 kW) | 8.0 BTU/sq ftยทhr | 16% | 16.0 kBTU/hr Load Reference |
| 1990s Standard Code Suburban | 30,120 BTU/hr (8.8 kW) | 15.1 BTU/sq ftยทhr | 25% | 30.1 kBTU/hr Load Reference |
| 1960s Semi-Insulated Ranch | 46,840 BTU/hr (13.7 kW) | 23.4 BTU/sq ftยทhr | 32% | 46.8 kBTU/hr Load Reference |
| Pre-1940 Historic Leaky (Uninsulated) | 72,650 BTU/hr (21.3 kW) | 36.3 BTU/sq ftยทhr | 44% | 72.7 kBTU/hr Load Reference |
Worked Engineering Sizing Example
Scenario: Calculating preliminary peak heat loss for a 2,000 sq ft home in Denver, CO where 99% winter outdoor design temperature is 10.0ยฐF and indoor heating setpoint is 70.0ยฐF (ฮT = 60.0ยฐF).
Calculation Steps:
- Design Temperature Difference: ฮT = 70.0 - 10.0 = 60.0ยฐF.
- Building Dimensions: Perimeter P = 4 ร โ2000 = 178.9 ft. Gross wall area = 178.9 ร 9 ft = 1,610 sq ft. Glazing (15%) = 300 sq ft. Doors = 40 sq ft. Net wall area = 1,270 sq ft. Volume = 18,000 cu ft.
- Above-Grade Wall Conduction: 1,270 sq ft net wall @ nominal R-19 (estimated layer R-20.5, U-0.04878) ร 60.0ยฐF = 3,717 BTU/hr.
- Ceiling & Attic Conduction: 2,000 sq ft ceiling @ nominal R-38 (estimated layer R-39.0, U-0.02564) ร 60.0ยฐF = 3,077 BTU/hr.
- Window Conduction: 300 sq ft Low-E glass @ U-0.28 ร 60.0ยฐF = 5,040 BTU/hr.
- Exterior Doors Conduction: 40 sq ft insulated doors @ U-0.35 ร 60.0ยฐF = 840 BTU/hr.
- Slab Perimeter Conduction: 178.9 ft perimeter @ F-0.50 ร 60.0ยฐF = 5,367 BTU/hr.
- Air Infiltration Leakage: 18,000 cu ft volume @ 0.38 ACHnat = 114 CFM. Infiltration loss = 1.08 ร 114 CFM ร 60.0ยฐF = 7,387 BTU/hr.
- Total Preliminary Peak Heat Loss: 3,717 + 3,077 + 5,040 + 840 + 5,367 + 7,387 = 25,428 BTU/hr (7.5 kW).
- Equipment Sizing Note: This preliminary building load (25.4 kBTU/hr) represents the baseline screening rate. Equipment selection requires full room-by-room ACCA Manual J load calculations and manufacturer-verified heating output tables per ACCA Manual S.
Frequently Asked Questions
How is conductive envelope heat loss calculated?
How does air infiltration affect building heat loss?
What is the difference between preliminary heat loss and ACCA Manual J load calculation?
How is slab-on-grade foundation heat loss modeled?
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.
Calculation engines undergo software-level verification against documented equations, reference values, boundary conditions, and automated tests referencing ASHRAE, ACCA, SMACNA, and NIST publications.
โ๏ธ Engineering Reference & Regulatory Disclaimers
Engineering Reference Notice: HVACLogic.org is an independent computational reference and engineering design aid authored by Miad S. Calculations are based on consensus engineering formulations (including ASHRAE, ACCA, and SMACNA publications) and are intended solely for preliminary estimation, parametric analysis, and educational use. HVACLogic does not provide licensed professional engineering services, structural evaluations, or legally binding code determinations.
Professional Review & Permitting Notice: Where the applicable jurisdiction, project type, occupancy classification, permit process, or professional-practice law requires licensed professional review, certification, or a sealed/stamped calculation, the user must obtain that review from an appropriately licensed Professional Engineer (PE) or qualified mechanical contractor. Where a jurisdiction or Authority Having Jurisdiction (AHJ) requires specific calculation software, documentation, or permit submittal forms, users must follow the applicable local requirements.
Manufacturer Data Notice: Generic engineering formulas provide baseline theoretical approximations. Actual equipment performance, expanded cooling/heating capacities at specific outdoor temperatures, sensible-to-total heat ratios, fan airflow curves, and electrical characteristics (MCA/MOP) must be verified against manufacturer technical product data specifications.