Every building brings outdoor air inside. Some of it comes in on purpose through a ventilation system that a designer sized and specified. Some of it comes in whether anyone wanted it to, through cracks around doors, gaps in the envelope, holes made by trades, and negative pressures created by exhaust fans. The distinction between those two paths is one of the most misunderstood ideas in building operation, and it drives a huge share of the comfort, energy, and humidity problems we get called to investigate.
The two paths in plain terms
Ventilation is outdoor air a designer put there on purpose. It comes in through a rooftop unit or an energy recovery ventilator, gets filtered, gets tempered to a reasonable condition, and gets distributed to the spaces that need it. The airflow is specified in the drawings. The load is included in the equipment sizing. The point is to keep occupants healthy and comfortable and to meet code requirements like ASHRAE 62.1.
Infiltration is outdoor air that shows up on its own. It leaks through cracks, gets pulled in when bathroom exhaust fans run without makeup air, sneaks under overhead doors, and comes down elevator shafts. The airflow is not specified anywhere. The load is not in the equipment sizing. Nobody planned for it. In a leaky building, infiltration can easily exceed the designed ventilation rate, especially in winter when stack effect drives cold air in through low openings.
Why the difference matters
Load. Both ventilation and infiltration have to be cooled in summer and heated in winter. Both carry moisture. A ventilation load is planned and the equipment can handle it. An infiltration load is a surprise and the equipment often cannot.
Comfort. Ventilation air is filtered, tempered, and distributed. Infiltration air is dirty, cold or hot, and shows up in the wrong places. You feel it as drafts along walls, cold floors near doors, and rooms that never quite match the thermostat.
Humidity. A ventilation system passes outdoor air over a dehumidification coil before it enters the space. Infiltration bypasses the coil entirely. In humid climates, infiltration is the single biggest reason existing buildings run wet in shoulder seasons.
Pressure. A properly ventilated building runs slightly positive, which pushes air out through the envelope and keeps outdoor air out. A leaky building running short on ventilation runs negative, which pulls infiltration in. See the note on building pressure.
A common failure pattern
A building has a design ventilation rate of, say, 500 cfm through the rooftop unit. Over years of use, the outside air damper drifts closed, or a facility manager blocks it off to reduce energy cost, or the linkage breaks. The rooftop unit now brings in almost no outdoor air. But the toilet exhaust fans, kitchen hoods, and lab exhausts still run. They pull 1,200 cfm out of the building. That air has to come from somewhere. It comes in through infiltration, through every crack in the envelope, past the storefront glass, under the overhead door.
In summer, that 1,200 cfm of hot humid outdoor air lands in the building without ever meeting a dehumidification coil. The space feels muggy at 72. Utility bills go up. Nobody understands why. The rooftop unit ran fine yesterday and it runs fine today.
The fix is not the rooftop unit. It is restoring the outdoor air damper and, sometimes, tightening the envelope so infiltration cannot overwhelm the ventilation system.
Order-of-magnitude example
| Load component | Summer (97°F / 74°F WB) | Winter (13°F) |
|---|---|---|
| Sensible load (BTU/h) | 27,000 | 62,000 |
| Latent load (BTU/h) | 44,000 | 0 |
| Total (BTU/h) | 71,000 | 62,000 |
| Equivalent tonnage | ~6 tons of cooling | ~5 tons of heat |
What actually fixes it
Verify the ventilation is real. On existing buildings, the first step is measuring how much outdoor air the equipment is actually delivering. Design intent and installed reality often disagree. The measurement takes an afternoon with an air balancer and closes half the arguments before they start.
Balance the exhaust. Total exhaust airflow (toilets, kitchen, labs, dryers) should be less than or equal to total outdoor air supply. If exhaust exceeds supply, the deficit is made up by infiltration. This is the number one cause of "why is my building always negative" complaints.
Tighten the envelope. Air sealing at wall penetrations, door weatherstripping, and overhead door thresholds reduces infiltration without touching the mechanical system. Blower-door testing quantifies the leakage before and after.
Right-size the ventilation for the space's actual use. ASHRAE 62.1 has different rates for different occupancies. Office defaults are low. Animal spaces, kitchens, medical, and industrial applications need much higher rates and often mandatory exhaust. Using the office default on a veterinary clinic or restaurant guarantees a comfort complaint.
What to ask your engineer
- What outdoor air rate did the design assume, and what standard did it come from?
- How much of the building's total exhaust is intentional, and is the makeup air specified?
- Has the outdoor air quantity been measured in the field, or is it a paper number?
- What happens to building pressure when all exhaust fans run at once?
- How much of the ventilation air passes through a dehumidification coil before it reaches the space?
The one-line version
Ventilation is outdoor air a designer specified. Infiltration is outdoor air a building took anyway. Both carry load. The one that surprises the equipment is the one that causes problems.