Basics of Building Air Leakage Rates and Pressure Differentials

But what about that "75 Pascals" or "50 Pascals" part? 

Eh. Close enough, right?

No. The pascal is critically important when it comes to infiltration—and ignoring it can lead professionals far astray.

Air Pressure and Pascals

The pascal (Pa) is a unit of measure for pressure and stress. 

Imagine standing in front of a table and pressing down on it with your hand. Your hand would be exerting pascals on the table. 

Named in honor of French philosopher and scientist Blaise Pascal (1623-1662), one pascal equals the pressure of one newton per square meter [one kilogram per meter per second squared]. Moreover, a newton is named after English physicist and mathematician Isaac Newton (1642-1727) in recognition of his work on mechanics, specifically his second law of motion.

In other words:

In I-P units: 1 Pa = 1 N/m²

In SI units: 1 Pa = 1 kg/(m × s²)

Pascals are very small and therefore the kilopascal (kPa) of 1,000 N/m² is more commonly used.

For context, standard atmospheric pressure (or 1 atm) is defined as 101.325 kPa. This pressure decreases exponentially with height as there is less air pressing down from above.

To bring it back to Earth, one pound per square inch of pressure equals 6.895 kPa.

Pascals and Room Pressurization in Buildings

What does this mean for buildings?

Room pressurization uses pressure differentials, measured in Pascals, to control airflow and prevent contaminants from entering or escaping a room. A room is positively pressurized by supplying air with an HVAC system to create a slight pressure difference, typically ranging form +5 to +20 Pa, which forces air outwards through controlled openings like door gaps, establishing a barrier to pollutants.

Due to fire safety considerations, stairwells and refuge areas are usually kept at a slightly higher pressure than the typical rooms across various floors. This higher pressure (e.g., +25 to +50 Pa) helps to prevent smoke from seeping into escape routes, keeping them safe for evacuation and for firefighters to enter.

As a matter of course, if a room is negatively pressurized, likewise typically ranging form -5  to -20 Pa, it will "pull" air into it.

If operating close to neutral pressurization, differentials of ±1 to 2 Pa, sometimes higher at around 4 or 5 Pa, may be exhibited between rooms or HVAC zones.

Pascal readings taken with a micromanometer can verify the degree to which a room is operating compared to adjacent spaces.

Building Envelope and Air Leakage Testing

This gets us back around to those codified maximum air leakage rates using field testing.

Field testing can be conducted to quantify the air leakage of a building envelope. The testing can be conducted using some sort of fan-based depressurization. In such an approach, air is extracted from the building's conditioned spaces using a portable exhaust fan. This controlled air exhaust results in a negative air pressure within the building relative to the exterior (this is known as depressurization). 

The amount of air exhausted from the building's interior is increased until a standard reference negative air pressure is achieved. Conventionally, this standard reference negative air pressure is 50 pascals—though, 75 pascals is also common. 

The amount of air necessary to be exhausted from the building's interior in order to depressurize the unit to the reference negative pressure can be measured. The leakier the building envelope, the greater the amount of air required to depressurize the building to the reference pressure. This amount can be averaged across the entire thermal enclosure on a per unit area basis. 

For instance, if a building's tested average air leakage rate at a pressure differential of 75 pascals is 0.35 CFM/ft² [1.8 L/s m²] or lower, then it complies with Standard 90.1-2022 and 2024 IECC. If the rate is greater, then corrective measures should be taken to tighten up the building's air barrier.

How much pressure is 50 or 75 pascals?

50 Pa ≈ 0.2 inches of water

75 Pa ≈ 0.3 inches of water

This is important to note. It might not seem like much, but 75 pascals is about 50% more pressure than 50 pascals.

Assessing Infiltration in Energy Modeling Tools

Building performance simulation (BPS) tools will consider some degree of infiltration as part of a whole-building energy analysis. Often, users will be prompted to approximate their project's air leakage as a flow rate per area of the thermal envelope such as CFM/ft² [m³/h per m²].

It is critical to understand what air pressure differential is assumed for the air leakage input.

While some BPS platforms may utilize methodologies that assume air leakage rates at 50 Pa or 75 Pa, others may not. For instance, some BPS platforms use EN 15242:2007 Ventilation for Buildings - Calculation Methods for the Determination of Air Flow Rates in Buildings Including Infiltration. This European Standard describes the method to calculate the ventilation air flow rates for buildings to be used for applications such as energy calculations, heat and cooling load calculation, summer comfort and indoor air quality evaluation—and it assumes a neutral pressurization of 4 Pa.

Since the same building will exhibit different infiltration rates at different pressures, this is critical.

When a building's air leakage rate is assessed during, say, a blower door test, the rate is measured at 50 Pa or 75 Pa. An energy analysis tool assuming "normal" operating conditions will assume a pressure of 2 Pa or 4 Pa—under such conditions, the air leakage rate will be far different.

Converting Air Leakage Rates to Account for Different Pascals

I want to tip my hat to Patrick Chopson at cove, who brought to my attention that one can convert between different pressures the power law equation can be utilized.

For instance, if one needed to convert between an infiltration rate (CFM/ft²) at 75 Pa to 4 Pa, then apply:

(infiltration@4 Pa) = (infiltration@75 Pa) × (4/75)^0.65

The pressure exponent of 0.65 is a typical value for all buildings; however, if fan pressurization test data is available, the building's specific value may be used. 

Let us apply 0.35 CFM/ft² at 75 Pa:

infiltration@4 Pa =  0.35 × (0.053)^0.65

infiltration@4 Pa =  0.05 CFM/ft² 

For reference, below is a tabulation of typical infiltration rates from EN 15242:2007 Annex B, converted from 4 Pa to 50 Pa and 75 Pa.

Default air leakage rates at various pressure differentials. Figure by Daniel Overbey.