Air change rate n50
Air changes per hour at 50 Pa pressure difference. Measures building envelope leakiness via blower-door test. Passive House standard requires n50 ≤ 0.6/h.
What is the n50 value and what does it measure?
The n50 air change rate is a single number that describes how much the building envelope leaks. It is the number of times the entire interior air volume is replaced per hour when the building is held at a constant 50 pascal pressure difference from the outside.
The unit is written as n50 in 1/h (air changes per hour). The value comes from a blower-door test, where a calibrated fan is sealed into a door opening and measures exactly how much air flows through all the gaps in the envelope at that 50 Pa pressure. Dividing that airflow by the building's interior volume gives the n50 value.
The 50 pascal threshold is arbitrary but practical: it is roughly equivalent to the air pressure that a steady wind exerts on a building facade. It is high enough to reveal every gap, yet low enough to be achieved reliably in a test. Real conditions in use involve much lower pressures, so the actual air leakage in daily life is substantially smaller than n50 would suggest.
Why is n50 normalized by volume instead of envelope area?
This is the fundamental source of confusion and misunderstanding about n50. The metric divides airflow by the interior volume of the building, not by its external surface area. This means that two buildings with identical construction quality: the same sealing details, the same materials, the same workmanship, will score very differently if they have different shapes.
A compact cubic house with a volume of 100 cubic meters leaks through a certain number of gaps. Scale that up to a long, low rambling house with 500 cubic meters and identical gaps in the same density, and the rambling house reports a much better (lower) n50, even though the seams and joints are no tighter. The difference is that the same leak count is now divided by five times the volume.
This is why some architects and building scientists prefer q50, normalized by the envelope surface area instead of volume. The q50 value, measured in m3/(h.m2), represents the leakage flow per unit of facade area. It is a fairer indicator of actual sealing quality because it removes the penalty for building a large or geometrically complex house. However, q50 is less commonly reported, so airtightness is still judged almost always by n50.
| Building profile | Interior volume | Physical leaks (same count) | Resulting n50 |
|---|---|---|---|
| Compact house | 100 m3 | 10 gaps | 1.0/h |
| Medium house | 250 m3 | 10 gaps | 0.4/h |
| Sprawling house | 500 m3 | 10 gaps | 0.2/h |
| Large mansion | 1000 m3 | 10 gaps | 0.1/h |
This table illustrates why reference volume disputes matter. If the parties to a Passive House certification disagree on what counts as heated volume, the n50 result shifts. A building declared to have 400 m3 might be redefined as 430 m3 if basement storage is included, and suddenly a claimed 0.6/h becomes 0.56/h. This small shift that makes all the difference to certification.
What do different n50 values mean in practice?
The n50 value places a building on a clear scale of envelope performance. Each step down represents a large reduction in the heat carried away by uncontrolled airflow.
| Building type and n50 range | Air change rate | Heat loss context | Building envelope typical condition |
|---|---|---|---|
| Existing older house | 5 to 15/h | Very high uncontrolled losses | Multiple draughts, obvious gaps |
| Standard new build (code minimum) | 2 to 4/h | Significant uncontrolled losses | Meets building regs, still visibly leaky |
| Good energy-efficient house | 1 to 1.5/h | Small uncontrolled losses | Careful sealing, professional work |
| Low-energy certified | 0.6 to 1.0/h | Minimal uncontrolled losses | Deliberate airtight layer, documented sealing |
| Passive House certified | 0.6 or lower/h | Virtually none | Continuous, protected airtight layer |
Moving from 3/h to 1/h represents a three-fold reduction in air leakage. Moving from 1.5/h to 0.6/h represents 2.5 times less leakage. The compounding effect shows why passive house targets are not just incremental: they represent a fundamentally different approach to how the building envelope is designed, detailed, and built.
How is n50 measured, and what does pressurization mean?
During a blower-door test, the building is first pressurized: air is forced in until a stable 50 Pa difference is reached, and airflow is recorded. Then the building is depressurized: air is extracted until the same 50 Pa difference is reached in the opposite direction, and airflow is again recorded.
These two results are often slightly different because wind, thermal effects, and gravity create asymmetry. The reported n50 value is the average of pressurization and depressurization flows. The mean is used because it best represents the long-term average leakage under real conditions, where pressure differences come from both wind and temperature gradients, acting in both directions randomly.
The pressurization half of the test is often more useful for locating leaks: air is forced inward through every gap, so smoke flow and thermal imaging can trace the path of each leak from inside. Depressurization also reveals leaks but in the opposite direction, and sometimes picks up different ones (for example, cracks that seal under negative pressure might open under positive pressure). A competent test team will use both to locate and document every weak point.
Once leaks are located, they can be sealed with tape or sealant appropriate to the construction stage. A mid-construction test allows repairs while the building envelope is still open and accessible. After the building is finished, a formal certification test is performed to verify that the repairs held and that no new damage occurred during later trades.
What are the common traps and misconceptions about n50?
The first misconception is that n50 tells you where the leaks are. It does not. It is only a number that confirms the building leaks. The detective work of identifying the location and cause of each gap happens during the test through smoke flow and thermal imaging.
The second trap is comparing n50 values across buildings without knowing their reference volume. A reported n50 of 0.8/h from a 300 m3 house is not comparable to 0.8/h from a 600 m3 house without understanding their physical leakage in absolute terms. This is why specifications sometimes include both n50 and q50, or specify a reference volume range.
The third misconception is that airtightness alone is sufficient. Sealing the envelope stops uncontrolled air leakage, but the building still needs fresh, filtered air for occupant health. An airtight house without mechanical ventilation becomes stuffy, humidity rises, odors accumulate, and occupants become uncomfortable. Mechanical ventilation with heat recovery is the necessary complement: it supplies controlled, filtered air and recovers most of the heat from exhaust air.
A fourth mistake is assuming that air-sealed means the sealing happened early. Some builders wait until the building is nearly finished to attempt airtight sealing, but by then plasterboard, electrical conduit, and plumbing have already punctured the layer many times. The most cost-effective approach is to seal progressively during construction when establishing and protecting the airtight layer as soon as possible, then sealing each new penetration as trades arrive, and finally testing mid-construction to catch and repair failures while work is still open.
Frequently asked questions
- What exactly does n50 = 0.6/h mean?
- At a 50 pascal pressure difference, the entire volume of air inside the building is replaced 0.6 times per hour. That is, the building would lose and regain its total interior air 0.6 times if that pressure were held constant. In real life (much lower pressures), actual air loss is far smaller. The Passive House standard requires n50 to be 0.6 or lower.
- Why do two identical houses sometimes get different n50 results?
- The same number of physical air leaks produces different n50 values depending on the building's interior volume. A compact 100 m3 house and a sprawling 500 m3 house with identical craftsmanship will score very differently. The large house gets a better (lower) n50 because its leaks are spread over more volume. This is why q50, normalized by envelope surface area, is sometimes used for fair comparison of construction quality.
- Does n50 tell me where the leaks are?
- No. n50 is just a number. It tells you the building leaks, but not where. During the blower-door test, the team uses smoke and thermal imaging to locate each gap: around window frames, at service penetrations, along membrane joints. That detective work is what makes testing valuable for sealing during construction, not the n50 figure alone.
- When is the best time to do a blower-door test?
- The most valuable test happens when the airtight layer is complete but still accessible: before plasterboard, screeds, and finishes are installed. Leaks found at this stage can be sealed quickly and cheaply. A second formal test on the finished building certifies the final result, but without the mid-stage test you lose the chance to fix problems while work is still open.
- Does low n50 mean I don't need ventilation?
- The opposite. A very low n50 makes mechanical ventilation mandatory, not optional. Airtight means uncontrolled air leakage is stopped, but occupants still need fresh air. Heat-recovery ventilation supplies that air deliberately and recovers most of the heat from exhaust air. Without it, an airtight house becomes stuffy and unhealthy.
- Why do buildings with the same air leakage sometimes report different reference volumes?
- Reference volume is the key variable. It includes all heated space, sometimes disputed: does it include unconditioned spaces like attics or basements? Is there a minimum thickness threshold? Small disputes over what counts as interior volume can shift n50 up or down by 10-15%, especially in buildings with complex geometry. Always verify the reference-volume definition when comparing results.