Building Science
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
Representative Building Vintages:
Client-Side Math โข No Sign-Up or Database Required โข Instant Local Execution
๐ASHRAE Climatic Design Conditions50 States + CA
View Full Climatic Database โWinter 99%
-2ยฐF
Summer 0.4%
90ยฐF
Coincident WB
74ยฐF
IECC Zone
5A
Wall Conduction ModelThermal Bridging Tool โ
Preliminary Peak Building Heat Loss
25,428 BTU/hr
Power Demand: 7.5 kW โข ฮT = 60ยฐF (70ยฐF in / 10ยฐF out)
12.7 BTU/sq ftยทhr Specific Thermal Intensity
Technical References:
Air Infiltration Loss
7,387 BTU/hr (29%)
Envelope Conduction
18,041 BTU/hr (71%)
Estimated Infiltration Airflow
114 CFM (0.38 ACHnat)
Foundation / Slab Conduction
5,367 BTU/hr (F-0.5)
Next Steps: Detailed Load Calculation & Equipment Selection
Find Cold-Climate Heat Pump Balance Point & Heating DeficitโSize Auxiliary Electric Heat Strip for 25,428 BTU Heat LossโDerive Exact Wall Assembly U-Factor with Stud Thermal BridgingโSize Replacement Gas Furnace for 25,428 BTU Heat LossโWhole-House Room-by-Room Heating & Cooling Load ScreeningโEngineering Methodology & Governing Equations
โ๏ธ
Hydronic & Forced-Air Thermal Generation Flow
Fuel combustion and reverse-cycle heat pumping to offset building thermal envelope transmission losses.
๐ฅHeat InputThermal InputBurner / Heat Pump
๐ก๏ธTransferHeat Exchanger80%โ98% AFUE rating
๐CirculationBlower DeliveryDelta-T rise (35ยฐFโ65ยฐF)
๐ ComfortConditioned ZoneEnvelope loss offset
๐ก Engineering Note: Heat pump Coefficient of Performance (COP) decreases as outdoor ambient temperatures drop; balance point calculations dictate auxiliary strip heat engagement.
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?
Conductive heat loss is calculated using Fourier's heat transfer equation: Q = U * A * Delta T, where U is the surface assembly U-factor (1 / R_effective), A is net surface area in square feet, and Delta T is the difference between indoor setpoint and outdoor 99% design temperature.
How does air infiltration affect building heat loss?
Air infiltration heat loss is calculated using the sensible air equation: Q = 1.08 * CFM * Delta T. Airflow entering through envelope leakage is derived from building volume and natural air changes per hour (ACHnat = ACH50 / N-factor).
What is the difference between preliminary heat loss and ACCA Manual J load calculation?
This calculator provides a preliminary whole-building screening estimate based on simplified geometric baselines. A formal ACCA Manual J calculation evaluates room-by-room boundary areas, directional solar heat gains, internal loads, and duct transmission to size equipment per ACCA Manual S.
How is slab-on-grade foundation heat loss modeled?
Slab foundation heat loss occurs primarily through the exposed slab edge perimeter. Per ASHRAE Fundamentals and ACCA Manual J Table 4A, slab loss is modeled as Q = F * Perimeter * Delta T, where F is the linear edge heat-loss factor (e.g., F-0.50 BTU/hrยทftยทยฐF for uninsulated slab edges).
๐Scientific Methodology & Academic Courseware
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.
๐ก๏ธ
Engineering VerificationFormula Tested
Calculation engines undergo software-level verification against documented equations, reference values, boundary conditions, and automated tests referencing ASHRAE, ACCA, SMACNA, and NIST publications.