The Building Envelope: Excessive Negative Pressure

The HVAC system inside a building creates negative pressure when it draws in more air from the return ducts than it expels through the supply ducts. Excessive negative pressure can cause outward-opening doors to be difficult to open and prevent inward-swinging doors from fully closing. During hot weather, excessive negative pressure can cause outside air and the moisture it contains to be drawn through the components of the building envelope and can lead to mold growth or other excess moisture-related issues. In the extreme, excessive negative pressure can even draw rainwater through the building envelope into the interior.

Buildings can not (and should not) be absolutely airtight. However, the more locations in a building where air flows freely, the more locations incursions can occur. Combine multiple free airflow locations with the presence of negative pressure, and no manner of sealant applied to an area that exhibits leakage will prevent air and moisture from entering other free airflow building envelope locations.

Over Standard’s 70-plus years of providing commercial roofing and building envelope maintenance solutions for our customers, our experience has shown excessive negative pressure can be more burdensome in metal buildings than in buildings built using more conventional construction methods. For the most part, metal buildings can be constructed more cheaply and rapidly than conventional masonry, wood frame or buildings with insulated siding. Once site work is complete and the concrete slab is in-place, metal building construction moves quickly. However, if the building contractor is not fully familiar with all aspects of metal building construction and the designer does not have the time to fully oversee and vet any ventilation issues, serious problems can be “built-in” to metal buildings. Common conditions for leaks in metal buildings with excessive negative pressure are skylight curbs, roof penetrations and flashing locations. But leaks can also often be found in the roof eaves and even in metal wall panels.

No matter how a building is constructed, the key to minimizing excessive negative pressure (and mitigating leaks in the roof, walls and other components of the building envelope) is the proper design and control of a building’s HVAC system. Professional engineering of a building’s HVAC system may be more expensive and add some additional time to the construction process on the front end of a project, but the benefits of a properly designed and installed HVAC system many years after the building’s completion is well worth the time and money. However, if excessive negative pressure is causing problems in an existing building, manipulating the HVAC system to better balance exterior and interior pressure can greatly reduce excessive negative pressure, thereby reducing the potential of building envelope leaks.

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