How Infiltration Changes Heating Load
Two houses can have similar floor area, insulation, and windows yet produce noticeably different heating loads. During a winter assessment, the difference often appears at rim joists, attic penetrations, fireplaces, attached garages, and leaky return chases: one enclosure controls outdoor air, while the other lets cold air enter whenever wind and stack effect increase.
Infiltration heating load is the sensible heat required to warm uncontrolled outdoor air that leaks into the building. It is separate from conduction through walls and from intentional mechanical ventilation, although all three appear in the total heating-load calculation.
Infiltration is airflow, not an insulation value
Insulation reduces heat conduction through an assembly. Air sealing reduces bulk air movement through cracks and openings. Fiberglass insulation can filter dust and still allow air to pass if an air barrier is missing or discontinuous.
A load calculation should not assume that a high nominal R-value means a tight house. Construction age, enclosure details, shielding, height, fireplaces, recessed fixtures, duct location, and measured leakage can change infiltration independently.
The sensible infiltration formula
A common U.S. approximation is:
Q = 1.08 × CFM × ΔT
Q is sensible heat in Btu/h, CFM is infiltration airflow, and ΔT is indoor minus outdoor temperature in °F. The 1.08 factor combines typical sea-level air density and specific heat. It changes with altitude and air conditions, so it must be treated as an assumption.
If estimated infiltration is 120 CFM and design ΔT is 60°F, the sensible load is about 7,776 Btu/h: 1.08 × 120 × 60. At high elevation, an appropriate air-density correction would reduce heat carried by each cubic foot.
Converting air changes to CFM
When an air-change rate is available, average airflow can be estimated as:
CFM = ACH × building volume ÷ 60
A 16,000-ft³ enclosure at 0.5 air changes per hour gives about 133 CFM: 0.5 × 16,000 ÷ 60. The critical question is whether the ACH value represents natural winter infiltration, a blower-door test condition, or an unsupported guess.
ACH50 from a blower-door test is measured at a 50-pascal pressure difference. It cannot be inserted directly as natural ACH. A conversion or infiltration model must account for climate, shielding, building height, leakage distribution, and method limitations.
Wind and stack effect drive leakage
Wind creates positive pressure on one side of a house and suction on others. Stack effect develops because warm indoor air is buoyant, tending to leave through high leaks and draw cold air through lower leaks. Taller buildings and greater temperature differences strengthen this effect.
Infiltration is therefore variable. The load calculation estimates a design condition rather than claiming the same CFM occurs every hour. A calm-day smoke observation cannot establish peak winter leakage.
Where leakage commonly occurs
- Attic hatches, plumbing and wiring penetrations, and dropped soffits.
- Rim joists, sill plates, and floor penetrations over crawlspaces.
- Fireplace dampers and unsealed chases.
- Weatherstripping around doors and operable windows.
- Attached-garage boundaries and kneewalls.
- Duct boots, return plenums, and cavities used as air pathways.
Leakage location matters for comfort and indoor air quality. Air entering from a garage, crawlspace, or attic can carry contaminants even when its energy impact matches outdoor air entering elsewhere.
Duct leakage can become infiltration
Leaky return ducts outside the conditioned enclosure can pull outdoor or buffer-space air into the HVAC system. Leaky supply ducts can depressurize the house and drive replacement air through the envelope. The building and duct systems interact.
The Manual D guide and return-air guide explain distribution design. A blower-door test measures enclosure leakage under its procedure; a duct-leakage test isolates a different system boundary.
Ventilation is not accidental infiltration
Mechanical ventilation is intentionally delivered and can be measured, filtered, distributed, and sometimes heat-recovered. Infiltration is uncontrolled. A tight home may need designed ventilation; “the house needs to breathe through cracks” is not a ventilation plan.
Load calculations must prevent double counting. If an approved method already combines infiltration and ventilation or credits heat recovery, adding the full airflow again inflates the load.
How design temperature changes infiltration load
For a fixed airflow, infiltration load rises in direct proportion to ΔT. The guide on Manual J heating design temperature explains why using a record low instead of the accepted local design condition can exaggerate this component.
Indoor design temperature also matters. Increasing the assumed setpoint raises ΔT for both infiltration and conduction. The calculation report should display both indoor and outdoor conditions.
Air sealing changes the load, but verify the result
Sealing major leaks can reduce peak heat loss, drafts, and pressure-driven contaminant entry. The best opportunities often occur at the enclosure’s high and low boundaries. Work near combustion appliances, chimneys, recessed fixtures, and attached garages requires safety and material awareness.
After substantial enclosure work, combustion-air and depressurization conditions should be reassessed for atmospheric appliances. Ventilation needs should also be evaluated rather than assuming lower leakage is the only objective.
From infiltration load to equipment size
Infiltration is one line item in the room-by-room and whole-building load. Equipment output is selected after all envelope, ventilation, and design-condition components are combined. The site’s Manual S guide explains the next step.
Do not add a second arbitrary infiltration allowance after the calculation. If uncertainty is material, improve the input with testing or scenario analysis and document the selected case.
A practical audit checklist
- Confirm conditioned floor area, average ceiling height, and enclosed volume.
- Identify whether leakage input is measured, modeled, or assumed.
- Do not use ACH50 directly as natural ACH.
- Check shielding, stories, fireplace, garage, crawlspace, and duct location.
- Separate mechanical ventilation from uncontrolled infiltration.
- Verify indoor and outdoor design temperatures and altitude assumptions.
- Recalculate after planned air sealing rather than carrying the old load forward.
Bottom line
Infiltration heating load converts uncontrolled winter air leakage into Btu/h. The formula is simple once CFM and ΔT are defensible; estimating natural airflow is the hard part. Use the accepted infiltration method, treat blower-door results correctly, separate ventilation and duct leakage, and document assumptions before selecting equipment.
Frequently Asked Questions
Is infiltration the same as ventilation?
No. Infiltration is uncontrolled leakage; ventilation is intentionally designed airflow.
Can ACH50 be used in the heating-load formula?
Not directly. ACH50 is measured under test pressure and requires an accepted conversion or model.
Does insulation stop infiltration?
Not necessarily. Insulation needs a continuous air-control layer to limit bulk air movement.
Does air sealing make a furnace oversized?
It can reduce the building load enough that existing equipment has more excess capacity, especially after major enclosure improvements.
Why are drafts worse on windy days?
Wind increases pressure differences across leaks, raising airflow through the enclosure.
Sources and verification
The technical statements in How Infiltration Changes Heating Load were cross-checked against the primary references below. Final sizing, airflow, electrical, combustion, and refrigerant decisions must also follow the exact equipment instructions and applicable local code.