Infiltration

Infiltration, also known as air leakage, occurs in every building and is the unintentional flow of outdoor air into the building through cracks and other inadvertent openings, as well as air flow into the building due to door openings. Intentionally introducing outdoor air into a building, when providing forced or natural ventilation, is usually not classified as infiltration. Exfiltration is the corresponding removal of air from a building to the outside through the same cracks, inadvertent openings, and open doorways. Air pressure from stack effect, wind, and other sources drives infiltration and exfiltration in buildings. Infiltration often increases the thermal load on HVAC systems serving a building by introducing hotter, often more humid air from outside during cooling equipment operation or colder air during heating operation. These effects can be quite significant, especially in older buildings, which include no mitigation strategies to reduce infiltration.  

Ventilation is the intentional introduction of outdoor air into a building by use of intakes, fans, ducting, and distribution throughout the building. Ventilation quantities are prescribed by standards such as ASHRAE Standard 62.1 and 62.2 based on different occupancy types in order to provide outdoor air to the occupants for respiration, dilute odors and pollutants, and improve indoor air quality. Ventilation air is almost always conditioned prior to distribution throughout the space so that it is comfortable for the occupants. Ventilation systems are designed and often assume no air from infiltration since infiltration is not designed nor controlled, so it cannot be counted on to provide the required air for ventilation. When computing the load on a space, the infiltration should be factored in since it can impact both heating and cooling loads. Ventilation that does not use fans but instead uses the intentional opening of windows, grills, and other openings is called natural ventilation and is similar to infiltration in that it is driven by wind and air pressure differences, but it is planned and designed to provide outdoor air that is at comfortable conditions to the occupants.

Whether the designer intentionally wants the air from outside coming into the building is ultimately the difference between ventilation and infiltration. This line becomes blurred in residential buildings, especially older ones, which may have no specific outdoor air intakes and instead rely on infiltration to provide all the outdoor air that occupants need.

How It Works

Mechanisms that cause infiltration. (Source: Illustrated Guide—Achieving Airtightness. Copyright BC Housing. Reprinted with permission.)

Infiltration is driven by several different mechanisms:

• Wind pressure
• Stack effect
• Indirectly by mechanical systems

Each of these drivers is described below, and while treated individually, it is very common for the impacts of each of these to be combined for the overall impact of infiltration. Each of these drivers causes air pressure differences, which move air between outside and inside.

Wind Pressure

As wind flows around a building, a distribution of air pressures is imposed on the surface of the building depending on the wind direction and speed and, to a lesser extent, on air density. In general, air pressure is positive on the windward side of the building, facing into the wind, and negative on the leeward side, facing the direction the wind is heading. The distribution of pressure can be complicated to determine due to the shape of the building and its surroundings. Approximations may be used for simple shaped buildings, but computational fluid dynamics <link> are sometimes used for more complicated shaped buildings. Since the wind speed is so important, the wind speed from the weather file may need to be adjusted for local conditions, often depending on the height and surroundings at the building site. For annual modeling, it is rare to use computational fluid dynamics <link> to determine infiltration rates, but it may be used for modeling natural ventilation, especially where the shape of the building or nearby topography is complicated.

Stack Effects

Air buoyancy in buildings causes the flow of air into and out of buildings through intentional and unintentional openings. This stack effect of buoyancy occurs due to differences in the air density between indoors and outdoors resulting from temperature and moisture differences. The larger the air density differences and the taller the building height, the greater the stack effect and the greater the potential air infiltration. During the heating season, warm air in the building rises and exits through unintentional openings near the top of the building. This induces colder outdoor air to enter the building through openings near the base of the building.  During the cooling season, a similar process can occur in reverse, with warmer outdoor air being induced into the building near the top. Large vertical shafts in buildings, such as stairwells, elevators, and electrical and mechanical passages, can contribute to the stack effect. For computations, there is a height at which the pressure from the stack effect is zero, and this is called the neutral pressure level.  In reality, the neutral pressure level height varies, but it is often conceptualized as a specific height to identify the impact of air leakage being infiltration or exfiltration. The stack effect is especially noticeable on tall buildings during the summer, where poorly designed entrance doors can be held open by air flowing through them.

Indirect Impact of Mechanical Systems

The operation of mechanical systems, including exhaust systems, can impact the pressure in the building and change the amount of infiltration that occurs. Many buildings are intentionally kept at a slight positive pressure compared to the outdoor pressure in order to reduce infiltration. This is typically done by slightly decreasing the volume of the exhaust air compared to the fresh air intake volume entering the building.  This imbalance is intended to cause exfiltration throughout the building, although it rarely eliminates infiltration completely. The positive or negative pressurization of air is rarely uniform within a building and different areas experience different air pressures and this induces airflow. Infiltration may be especially important in humid climates where infiltration could cause portions of the building envelope to absorb water, prompting the formation of mold or rot. In buildings, especially residences, where the supply and return registers may be separated by interior doors, it is important to provide sufficient airflow between them via door undercuts in order not to induce pressure differences that may cause additional infiltration and exfiltration for the pressurized and depressurized rooms.  In addition, exhaust air fans for bathrooms and kitchen hoods cause lower air pressure in those spaces that need to be balanced; otherwise, they will induce additional infiltration to those or connecting spaces.

Information Needed for the Model

Determining air leakage rates using blower door testing. (Source: Illustrated Guide—Achieving Airtightness. Copyright BC Housing. Reprinted with permission.)

Most building energy modeling software can model infiltration by using a constant flow rate or that flow rate adjusted by wind speed based on the weather file, stack effects, and temperature differences. Other options for calculating infiltration in software include:

• single-zone models
• multi-zone models (also called pressure network models)
• computational fluid dynamics

The level of complexity of the inputs for these progresses in roughly the same order. For most modelers, infiltration flow rates adjusted by wind and stack effects are probably the most common choice since they are relatively simple to input, and often, more detailed assumptions are not even available. When additional data is available and when the building is sometimes operated using natural ventilation, a multi-zone model is probably the next most likely to be used. These two options are the ones described below.

Flow Rate Adjustment Model Inputs

There are multiple ways of specifying the inputs related to flow rate adjustment models. One of the most common is simply a design flow rate, which is then adjusted by wind and stack effects. The flow rate is usually specified per zone and is input as one of the following:

• Nominal flow
• Flow rate per floor area
• Flow rate per exterior wall area
• Air changes per hour

The adjustments are usually in the form of a formula that, depending on the simulation program, is either exposed or hidden from the user. The formula generally involves a quadratic equation or a quadratic equation within a square root with terms based on:

• The absolute temperature difference between inside and outside
• The windspeed (often squared)

If these terms are exposed to the user, it is often best to let them default unless a more detailed study (usually based on a multi-zone model) has been performed to calibrate them. In order to compensate for the impact of the system operation, it is common to use a schedule multiplier of 1 when the system is not operating and 0.25 when it is. This is a rule of thumb (see Gowri ) and not based on experimental data, so if project-specific information on the impact of HVAC operation is available, it should be used instead. NIST has created correlations for the prototype buildings, and they are available here . These correlations are better than just using the default and may apply to similarly configured buildings.  

Another common model uses the effective leakage area as an input. This effective leakage area is often found when blower door tests have been performed, so this form of input is especially useful for actual buildings that have been tested. There is often a stack coefficient as well as a wind coefficient. These terms can be found for a few select buildings, mostly residential, in the ASHRAE Handbook of Fundamentals chapter 16 on ventilation and infiltration. This approach usually uses a different form for the equation that modifies flow. Again, schedule multipliers are used to differentiate system operating and non-operating times.

The use of flow-coefficients is another form of infiltration input based on adjusting flow rates, mostly targeting residential buildings but may be applicable to other building types. It is also described in the  ASHRAE Handbook of Fundamentals chapter 16 on ventilation and infiltration and differentiates by the Shelter Class, which relates to how much local obstructions and buildings may reduce the effect of wind. Schedules are again typically used to factor in building operations.

Multi-Zone Model Inputs

By modeling the connections between each zone in a pressure network, multi-zone models can be significantly more accurate and are especially useful in modeling buildings that rely on natural ventilation for cooling during part of their annual operation. For typical buildings that never use natural ventilation, the added effort and assumptions may not make sense to use this method solely for infiltration modeling unless it is expected to be large, such as when modeling a large older existing building where the occupants have noticed significant infiltration issues. Mutli-zone model software is available directly in some building energy simulation software, and if it is not, separate stand-alone software that does just the pressure network is available. Using non-integrated multi-zone model software often means redundant input specification since zones will need to be described in both the BEM and the multi-zone pressure network software.

The multi-zone models look at the mass balance for each zone in the building and the pressures required to move a given amount of airflow between zones as well as through the exterior envelope. This software accounts for wind pressure on the building and can factor in the stack effects, but the user must enter quite a bit of information. Unlike the adjustment models, the multi-zone models that are integrated with BEM software are usually closely coupled with the operation of the HVAC system so no additional adjustments for scheduling are usually required.

Like some of the flow rate adjustment models, the multi-zone models still require a specification of effective leakage areas or cracks for infiltration. In addition, openings that can be controlled are usually described, including their orientation and location on the building and the flow rate through the opening when in the open and closed states. Describing the flow through these openings can be complicated, especially if the opening can also have partially opened states. Also, points, often called nodes, are described as either inside the occupied portion of the building, inside the HVAC equipment or distribution system, or exterior of the building. Each node is usually connected to others to provide the network of interconnections needed to describe the pressure network.

Common Measures

Maintaining air barrier continuity at various interfaces of the building structure. (Source: Illustrated Guide—Achieving Airtightness. Copyright BC Housing. Reprinted with permission.)

To control infiltration in an existing building cracks around doors and windows and other penetrations of the building envelope are often sealed using caulking, weather stripping, and sealing, which is often described as weatherization. It is common for smoke tracers or infrared imaging to be used to detect air leakage paths. During the construction of a new building, a continuous air barrier, sometimes called an air infiltration retarder, can be very effective. It is important to make sure all penetrations of the barrier are carefully sealed. The barrier also acts as a vapor barrier, so it is important that they are installed appropriately for the climate. The 90.1 section on installing continuous air barriers includes the following requirements that identify the many places where sealing is needed:

“The following areas of the continuous air barrier in the building envelope shall be wrapped, sealed, caulked, gasketed, or taped in an approved manner to minimize air leakage:

1. Joints around fenestration and door frames
2. Junctions between walls and floors; between walls at building corners; between walls and roofs, including parapets and copings; and walls at foundations
3. Penetrations through the continuous air barrier in building envelope roofs, walls, and floors
4. Building assemblies used as ducts or plenums
5. Joints, seams, connections between planes, and other changes in continuous air barrier materials
6. Building and service components projecting through or attached through the continuous air barrier
7. Junctions of the continuous air barrier that separate conditioned spaces from unconditioned spaces, semiheated spaces, and areas that are not enclosed spaces”

Model Output Checks

If the building has been tested either with blower-door tests at a rating point, building pressurization tests, or tracer-gas tests at specific conditions, it is possible to confirm the infiltration for an hourly step closest to those conditions. For most buildings that are still in the design phase, they, of course, have not been tested, and in those cases, comparisons to other similarly tested existing buildings may be the only way to confirm infiltration.

Confirming that the infiltration varies as expected is actually usually easier by comparing results for individual timesteps under different conditions. The impact of windspeed at nearly the same temperature should confirm the infiltration impact, as would the impact of temperature at two timesteps with a similar wind speed. For models with stack effects that are sensitive to height, looking at the amount of infiltration at zones at different heights in the building should be performed.

Related Energy Code Requirements

Both the ICC’s International Energy Conservation Code (IECC) and ASHRAE Standard 90.1 include requirements related to infiltration, as do the ICC’s International Green Construction Code (IgCC) and ASHRAE Standard 189.1 . ASHRAE Standard 90.1, since 2010, has included a requirement for continuous air barriers in some buildings. ASHRAE Standard 90.1-2022 includes some details on how the air leakage and infiltration need to be modeled when using the performance rating method. From Table G3.1 Proposed:

"To simulate air leakage, infiltration shall be modeled using the same methodology and  adjustments for weather and building operation in both the proposed design and the baseline building design. These adjustments shall be made for each simulation time step and must account for but not be limited to weather conditions and HVAC system operation, including strategies that are intended to positively pressurize the building. The air leakage rate of the building envelope shall be converted to appropriate units for the simulation program using one of the methods in Section G3.2.1.7.

1. When whole-building pressurization testing is required or specified during design, and completed in accordance with Section 5.4.3.1.4, the measured air leakage rate of the building Envelope (I75Pa) at a fixed building pressure differential of 75 Pa (0.30 in. of water) shall be modeled for purposes of demonstrating compliance with this standard.

Informative Note: Before the start of pressurization testing, the maximum air leakage rate of the building envelope (I75Pa) specified in Section 5.4.3.1.4 or as specified in design documents may be simulated to estimate the energy impact of building envelope air leakage. The final measured value is used for compliance; therefore, care should be taken when using estimated air leakage as a trade-off for performance based code compliance.

2. For buildings providing verification in accordance with Section 5.9.1.2, the air leakage rate of the building envelope (I75Pa) at a fixed building pressure differential of 75 Pa (0.30 in. of water) shall be 0.45 cfm/ft2."

A similar requirement appears in the corresponding table 12.5.1 for the Energy Cost Budget Method.

These requirements are based on Section 5.4.3 Air Leakage, which contains specific requirements for measuring air leakage in new small buildings and the steps that need to be taken if the target leakage rate of 0.35 cfm/sqft is exceeded. The testing requirements reference the following:

• ASTM E3158 Standard Test Method for Measuring the Air Leakage Rate of a Large or Multizone Building <link>
• ASTM E779 Standard Test Method for Determining Air Leakage Rate by Fan Pressurization <link>
• ASTM E1827 Standard Test Methods for Determining Airtightness of Buildings Using an Orifice Blower Door <link>

In addition, Section 5.4.3 requires the installation of continuous air barriers, including specific requirements for sealing shown above.

For modelers, Section G3.2.1.7 Modeling Building Envelope Air Leakage provides formulas that need to be used to convert air leakage rates that are measured to the inputs required by simulation tools since the basis of the area is different. The simulation tools often use just the area of the above-ground walls or the floor area, but the measured leakage rate is based on the total building envelope area, including above or below-grade walls, floors, plus the roof. There is an exception when multiple zone airflow models are used.

Ratings

Buildings are rated for airtightness based on metrics such as equivalent leakage area or effective air leakage area based on blower door tests or building pressurization tests. The metrics assume infiltration is equivalent to a hole in the building of a specific area. These ratings are usually performed at a specific pressure that is part of the rating procedure. Originally, blower door testing was limited to residential and very small commercial buildings, but now it can be used on much larger buildings. Some codes and standards require ratings on residential and commercial buildings. Building pressurization tests utilize the HVAC fans to overpressurize the building to test leakage. An alternative to blower door tests is the use of tracer gas measurements. These are special non-toxic gases that are added to a building during a test, and then the concentration of the gases is measured over time to determine the leakage rate. Standards and protocols to determine the air leakage rating include:

• ASTM Standard E779 Standard Test Method for Determining Air Leakage Rate by Fan Pressurization
• The United States Army Corps of Engineers Air Leakage Test Protocol for Building Envelopes
• ISO 9972:2006 – Thermal performance of buildings — Determination of air permeability of buildings — Fan pressurization method
• ASTM E1827 – Standard Test Methods for Determining Airtightness of Buildings Using an Orifice Blower Door
• ASTM E3158 – Standard Test Method for Measuring the Air Leakage Rate of a Large or Multizone Building
• ATTMA – Measuring Air Permeance of Building Envelopes (Dwellings)
• ATTMA – Measuring Air Permeance of Building Envelopes (Non-Dwellings)
• ABAA – Standard Method for Building Enclosure Airtightness Compliance Testing
• ASTM E741-11 Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution
• CGSB Standard 149.10 Determination of the airtightness of building envelopes by the fan depressurization method
• CGSB Standard 149.15 Determination of the Overall Envelope Airtightness of Buildings by the Fan Pressurization Method Using the Building's Air Handling Systems
• CIBSE Standard TM23 Testing buildings for air leakage

Additional Resources

https://www.nist.gov/el/energy-and-environment-division-73200/nist-multizone-modeling/chimp

Standard 90.1-2019 Performance Rating Method Reference Manual, December 2023. S Goel, M Rosenberg, E Mets, C Eley

Illustrated Guide Achieving Airtight Buildings, September 2017. BC Housing.

AIVC Guide 5 Ventilation Modelling Data Guide. Malcolm Orme, Nurul Leksmono. 2002. International Energy Agency.

Airflow and Indoor Air Quality Analyses Capabilities of Energy Simulation Software. Lisa C. Ng and Andrew K. Persily.  Indoor Air 2011.

Evaluation of Existing Infiltration Models Used in Building Energy Simulation. Yeonjin Bae, Jaewan Joe, Seungjae Lee, Piljae Im, Lisa C. Ng. BS2021. https://doi.org/10.26868/25222708.2021.30610

Building Airflow Physics Video - NIST

https://www.nist.gov/el/energy-and-environment-division-73200/nist-multizone-modeling/nist-infiltration-correlations

References

https://en.wikipedia.org/wiki/Ventilation_(architecture)

https://en.wikipedia.org/wiki/Infiltration_(HVAC)

https://en.wikipedia.org/wiki/Stack_effect

ASHRAE Standard 62.1 and Standard 62.2

https://en.wikipedia.org/wiki/Computational_fluid_dynamics

2021 ASHRAE Handbook – Fundamentals – Chapter 16 Ventilation and Infiltration

2021 ASHRAE Handbook – Fundamentals – Chapter 13 Indoor Environmental Modeling

Infiltration Modeling Guidelines for Commercial Building Energy Analysis. K Gowri, D Winiarski, R Jarnagin. PNNL 18898. September 2009.