Controlling Air Leakage
in Metal Buildings

Modern metal buildings typically have a lifespan of 50 years or more, which means their envelopes have to perform for the long haul, not just on day one. Two common challenges that metal buildings face are air leakage through the envelope and insulation systems that fail to remain sealed or intact over time. To prevent these issues, builders turn to continuous air barrier systems that control air movement, maintain performance and support long-term durability.

In line with this, energy performance requirements for metal buildings continue to increase with each new code cycle. Currently, the International Energy Conservation Code (IECC) and ASHRAE 90.1, Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings, require an air barrier in the thermal envelope. These barriers must be clearly identified and detailed in the construction drawings.

Where Metal Buildings Commonly Leak Air

Typically, metal buildings do not leak through the middle of a wall or roof panel. Instead, they are more likely to leak at transition and penetration points. The most common problem areas include roof-to-wall transitions, wall-to-foundation transitions, corners, and penetrations for vents, louvers and fenestrations. Each of these locations requires careful detailing to prevent air leakage and thermal performance failure.

These details can be particularly challenging because constructing a building’s envelope often involves multiple trades and systems, including roof and wall panels, fiberglass insulation or liner systems, doors, windows, louvers and mechanical penetrations. If the air barrier is not continuous across all of these interfaces, the building will leak air, even when the components are well-made.

Perforated vs. Non-Perforated Membranes

There are two main types of building air barrier membranes, perforated and non-perforated, and understanding how each one behaves under capillary action is essential. For example, when wind-driven rain wets the outer surface of a building, water naturally moves toward drier areas, including the interior.

In most residential construction, and even in some commercial projects, perforated housewraps are often used to wrap the building’s exterior. The small holes, or perforations, in the membrane lower the permeance rating and allow the assembly to “breathe,” but they also make it easier for air and moisture vapor to move through the material. As a result, capillary movement can draw moisture into the wall or roof assembly. When moisture moves through porous materials such as wood sheathing, drywall and other products, it could lead to failures down the road.

In metal buildings, non-perforated flexible membranes with higher permeance ratings provide a better balance for many assemblies. Permeance ratings that are usually greater than 10 perms when tested in accordance with ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials, generally work better, since their lack of holes helps maintain airtightness and reduces the chance of moisture buildup within the envelope.

What Qualifies as an Air Barrier?

When builders ask how to find an air barrier system for energy compliance, the answer starts with meeting specific code criteria. Air barrier systems essentially “wrap” the building shell and help protect it against air leakage. In doing so, they create a barrier against water vapor and pollutants such as suspended particulates, dust, insects and odors. 

IECC and ASHRAE 90.1 both list materials that can serve as air barriers. To qualify, a material must meet the maximum allowable air permeance limits of 0.004 cfm/ft² at 1.57 lb/ft² or 0.02 L/(s∙m²) at 75 Pa, when tested in accordance with ASTM E2178, Standard Test Method for Air Permeance of Building Materials. Today, most air barriers meet these criteria. 

In some metal buildings, the vapor retarder may also act as the air barrier. Additionally, board insulation systems and insulated metal panels (IMPs) can function as part of an air barrier assembly when designed and tested for that purpose.

Air Barrier Placement in the Assembly

In metal buildings, there are three basic placement options for the air barrier. Install it:

• On the interior side of the envelope (often serving as the vapor retarder).
• On the exterior side of the envelope (behind the metal siding).
• Within the assembly itself (as seen in Therm-All’s MaxTight™ Air Barrier System). 

The best location depends on the building type, climate and the overall wall or roof design. 

The Air Barrier Association of America (ABAA) is a great resource for information on air barrier materials, tested assemblies, and best practices across multiple construction types.

Achieving a code-compliant system, however, requires more than just selecting the right material. Manufacturers’ instructions are crucial because field installation errors, such as unsealed transitions, untaped laps or incompatible tapes, can undermine the entire envelope, even when the products themselves are engineered to perform.

The Next Generation of Air Barriers for Metal Buildings

New air barrier systems are being designed specifically for metal buildings. Systems currently available for metal buildings may incorporate a dedicated air barrier layer in addition to the traditional vapor retarder. In such assemblies, a membrane wraps the exterior of the building, much like housewrap, but is selected and detailed to meet air-barrier performance criteria.

In these metal building air barrier systems, the air barrier material is installed within the thermal envelope or on the exterior side behind the sheeting, protecting it from damage by trade workers and providing a fully sealed, airtight envelope.

Wind, mechanical systems and exhaust fans can also create positive and negative pressures that drive air through the envelope. When that air carries water vapor and encounters cold surfaces at or below the dew point, condensation can occur inside the assembly. Next-generation air barrier systems are designed to interrupt this chain of events, keeping uncontrolled air movement in check, supporting consistent insulation performance and reducing the risk of condensation-related problems.

Air Barrier Compliance Changes Under IECC 2021 and 2024

Air barrier requirements have changed in recent IECC cycles. In previous code cycles, this compliance path was relatively simple. Builders had three compliance methods to choose from: the materials approach, the assembly approach, or whole-building blower door testing. Most chose the materials approach. In other words, they selected the material that would serve as the air barrier, detailed it accordingly, and moved forward.

Now, under IECC 2021 and IECC 2024, builders have two compliance methods. Both require third-party testing or verification.

Option 1: Visual Verification

The visual verification method is almost always the easier choice for a large non-residential commercial building. It requires a review of the important components before close-in, allowing corrections to be carried out and documented. The process works as follows:

• Design review: A registered design professional or approved agency reviews the construction documents to ensure the barrier details are clear and compliant. They will also check that the air barrier and sealing materials meet requirements.
• Site observation: An approved party inspects the installation of the continuous air barrier and its components during construction.
• Documentation: Any deficiencies found during either the design review or site observation must be documented. It’s also important to provide the details of any corrective measures. In most cases, a final building envelope commissioning report must be delivered to the building owner and code official and include the equivalent of an issues log and corrective actions.

Option 2: Whole-Building Testing and Blower Door Testing

For large commercial buildings, this method may pose a risk because the project team won’t know whether they comply with requirements until they’re tested. Passing a whole–building blower door test requires careful consideration of air-barrier details, including verification of installation.

Blower door tests may sound intimidating, but metal buildings have already demonstrated that they can perform very well. Building envelopes with proper detailing of all fenestrations and proper installation should not have any problems meeting the whole-building air leakage rate of 0.35 cfm/ft² specified in IECC 2024.

In most cases, metal buildings perform very well, and the results fall well within the allowable leakage rate. However, if your building fails the blower door test, you must make all necessary repairs until it passes. The Metal Building Manufacturers Association (MBMA) has conducted numerous blower door tests on metal buildings, and its guide to complying with air leakage requirements for metal buildings is a great resource.

Sealing the Future of Metal Buildings

Success starts in the design phase with an air barrier system properly designed and detailed for all areas of concern. On-site communication during construction is also critical to ensure all trades understand the sequencing of the air barrier, including all fenestrations and transitions. 

As energy codes tighten, continuous air barrier systems, like MaxTight™, are becoming a core part of metal building design. Looking ahead, the metal buildings that perform best over their full service life will be the ones where air control is planned from the start. Good planning ensures that the air barrier location is clearly defined, materials are selected and tested to meet code and insulation systems and envelope details all work together to limit air leakage, manage moisture and protect the building for the long haul.

About the Author 

Bill “The Code Man” Beals, District Manager of Therm-All, is a 40-year veteran of the metal building industry and a contributing member of several committees, including the Metal Building Manufacturers Association’s Energy Committee and the National Insulation Association’s Laminators Committee. Bill also belongs to ASHRAE and the International Code Council (ICC).

He has contributed to over 100 articles and reference guides on commercial energy codes and has taught AIA-accredited courses for over eight years. Bill was inducted into MCN’s Hall of Fame in 2024.