Infiltration, Ventilation, and Internal Gains

Why Infiltration and Internal Gains Matter

A load calculation that ignores how air leaks into a building—or forgets the heat that people and appliances release—will produce an oversized or undersized system. Both outcomes hurt comfort, efficiency, and equipment life. This lesson covers the two envelope-related loads most commonly underestimated: infiltration and internal gains, plus the minimum mechanical ventilation requirement that every residential designer must acknowledge.

Infiltration: What It Is and Where It Comes From

Infiltration is the uncontrolled movement of outdoor air into a conditioned space through gaps, cracks, and penetrations in the building envelope—around windows and doors, at sill plates, through electrical outlets on exterior walls, and wherever pipes or wires pass through the shell. It is distinct from ventilation, which is controlled, intentional fresh-air delivery.

In summer, infiltration brings in hot, humid outdoor air, adding to the cooling load. In winter, it brings in cold, dry air, adding to the heating load. Either way, the HVAC system must condition that air—it is not free.

Manual J Infiltration Method

ACCA Manual J 8th Edition is the industry-standard residential load calculation procedure. For infiltration, Manual J uses a simplified ACH (Air Changes per Hour) method tied to construction quality categories: loose, average, and tight. Each category assigns an infiltration ACH value appropriate for that level of envelope tightness.

• Loose construction – older homes, poor caulking, single-pane windows, visible daylight around doors. Manual J assigns a higher ACH value.
• Average construction – typical code-built homes from the past few decades, some caulking and weatherstripping.
• Tight construction – energy-efficient homes with foam sealants, double-pane windows, continuous air barriers. Manual J assigns the lowest ACH value.

Consult the Manual J infiltration tables for the exact ACH values assigned to each category; the tables also account for the number of stories and the local wind exposure.

Basic Infiltration Calculation Steps

1. Determine the conditioned volume of the space (floor area × ceiling height, in cubic feet).

2. Select the ACH value from the Manual J table based on construction quality.

3. Calculate CFM of infiltration: (Volume × ACH) ÷ 60

4. Calculate sensible heat load (BTU/h): CFM × 1.1 × ΔT (heating season: design indoor temp minus design outdoor temp; cooling season: reverse)

5. For latent load in cooling, a separate calculation uses the humidity ratio difference, but for this lesson we focus on sensible only.

Blower-Door Test Confirmation

A blower-door test measures actual envelope leakage. The result is reported as CFM50—the airflow (in CFM) required to depressurize the house to 50 Pascals. To convert CFM50 to a natural infiltration rate in CFM, divide by a factor (commonly 20 for single-story homes in average terrain; Manual J and ASHRAE provide specific divisors). This converted CFM can then replace the ACH-derived CFM in step 3 above. A blower-door result is more accurate than a table estimate and is required by some energy codes.

> Safety note: Blower-door tests are performed by others (energy auditors, building scientists), but as a technician you may receive the report. Read it carefully; never confuse CFM50 with natural infiltration CFM. Using the raw CFM50 figure in a load calculation inflates the infiltration load by roughly 20×.

Mechanical Ventilation: ASHRAE 62.2

Because tight homes limit natural infiltration, they need intentional fresh air. ASHRAE Standard 62.2 sets minimum whole-building ventilation rates for low-rise residential buildings. At a concept level: the required airflow depends on conditioned floor area and the number of bedrooms. The designer adds this mechanical ventilation CFM as a separate load—it is 100% outdoor air that must be heated or cooled just like infiltration air. Do not double-count: infiltration and mechanical ventilation are separate loads added together.

Internal Gains

Internal gains are heat released inside the conditioned space by occupants, lighting, and appliances. In cooling load calculations, internal gains add to the load. In heating load calculations, Manual J allows a modest credit (reduction) for internal gains because they help warm the space—but engineers apply this credit conservatively.

Occupant Gains

The human body releases both sensible heat (warming the air) and latent heat (moisture/humidity). Manual J uses standard values per occupant. A typical occupied residence is assumed to have occupants during peak cooling hours.

Lighting Gains

Incandescent and halogen fixtures convert most of their wattage to heat. LED fixtures are much more efficient but still release some heat. Convert watts to BTU/h using: 1 watt = 3.412 BTU/h.

Appliance Gains

Kitchen appliances, computers, and entertainment equipment all contribute heat. Manual J provides default diversity values for residential kitchens. In commercial applications (not covered here), appliance schedules become much more detailed.

> Safety note: When inspecting existing systems, confirm that combustion appliances (furnaces, water heaters) are properly vented to the outdoors. Unvented combustion is a carbon-monoxide hazard—always verify CO detector placement and function before working in a mechanical room.

Putting It Together

The total sensible cooling or heating load for a space is the sum of:

• Envelope conduction losses/gains (walls, roof, windows, floor)
• Infiltration load
• Mechanical ventilation load
• Internal gains (cooling: added; heating: may be credited)

Accurate infiltration and internal gain values are the difference between a system that runs efficiently at design conditions and one that short-cycles or can't meet setpoint on the hottest or coldest day of the year.