Every room leaks. Doors, ceilings, pipe holes and window frames all pass air. Whether the room sits at positive or negative pressure decides which way that air goes.
So the question is never whether air leaks. It is whether it leaks in or out, and which of those you actually want.
Negative pressure means extracting more than you supply, so air leaks inward and contamination stays in the room. Positive pressure means supplying more than you extract, so air leaks outward and contamination stays out. Choose negative for anything you want contained, and positive for anything you want protected.
What each arrangement contains
| Arrangement | Air leaks | Protects |
|---|---|---|
| Negative pressure | Inward | The rest of the building |
| Positive pressure | Outward | The room itself |
| Balanced | Both, unpredictably | Neither reliably |
| Cascade | From clean toward dirty | A sequence of rooms |
Cascade layouts are worth knowing about. A run of rooms at falling pressure protects the cleanest space, and it still contains the dirtiest one at the far end.
Where negative pressure belongs
- Commercial kitchens, so cooking odour stays out of dining areas.
- Toilets and washrooms, for obvious reasons.
- Laboratories handling hazardous substances.
- Isolation rooms in healthcare.
- Workshops with welding fume, solvent vapour or dust.
- Finally, any space where an odour or contaminant must not escape.
Our kitchen exhaust guide covers the commercial kitchen case, which is the most common example.
Where positive pressure belongs
Positive pressure protects the room, whereas negative pressure protects everywhere else.
Cleanrooms, operating theatres, switchrooms, control rooms and paint prep areas all use it. Meanwhile electronics cabinets and roadside boxes do the same on a small scale, keeping dust out with a slight over-pressure.
Positive pressure needs filtered supply air. Pressurising a room with unfiltered outside air simply pushes dirt inward from a different direction.
Setting the difference
The pressure gap you need is small, although the airflow gap that makes it is not.
- Typical design differences run from 5 to 15 Pa between adjacent spaces.
- Cleanrooms often use 10 to 15 Pa between classified areas.
- Isolation rooms commonly specify around 2.5 Pa minimum, monitored.
- Kitchens usually run slightly negative to the dining area.
- The airflow offset is typically 10 to 20 percent of the total.
- Finally, leakier buildings need larger offsets to hold the same pressure.
Building tightness matters a great deal here. A leaky room needs far more airflow offset to hold the same negative pressure.
What goes wrong
Four failures cause most of the problems you meet on site.
Too much negative pressure makes doors hard to open, and it can upset a flue. Too much positive pressure pushes warm air straight out of the building. Meanwhile a loading supply filter slowly flips the balance you set, and one propped door undoes the lot in a second.
Our paint booth guide covers a case where balance is critical for both safety and finish quality.
Verifying and monitoring
A pressure difference is easy to measure, and just as easy to lose.
Use a differential pressure gauge across the boundary rather than assuming the design airflow is being achieved. A smoke pencil at a door gap gives an immediate visual check. Meanwhile critical spaces should be monitored continuously with an alarm, since balance drifts as filters load.
For cleanroom pressure rules, ISO 14644 covers classification and how to verify it.
One habit is worth building in from handover. Record the pressure across each boundary at commissioning, then check it whenever filters change. Balance drifts quietly, and nobody notices until an odour reaches a dining room or a door starts sticking.
Room pressure FAQ
How much pressure difference is needed?
Usually 5 to 15 Pa between adjacent spaces, and as little as 2.5 Pa in some healthcare applications. More than about 20 Pa makes doors difficult to open.
Can I have negative pressure without mechanical supply?
Yes, using extract only and letting air leak in through the building fabric. It works for small spaces, though it gives no control over where that air comes from.
Why do my doors slam or stick?
Excessive pressure difference. Recheck the airflow offset, and remember that filter loading on either side shifts the balance over time.
Does room pressure affect gas appliances?
It can, significantly. Strong negative pressure interferes with flue draught, which is a safety matter rather than a comfort one.
Negative pressure contains, and positive pressure protects. So decide which one the space actually needs, then prove it with a gauge rather than trusting the design airflow figures.
