By: Gordon Shepperd, PE, LEED AP
Principal Consultant, Apollo BBC

It’s no secret: All buildings leak.

You probably already know that many factors can lead to leaks. Design error, improper construction, building modifications, lack of maintenance, exposure to ultraviolet light, age of materials and thermal movements are just a few of the common problems that can all wreak havoc with a building’s enclosure allowing water easy access to the interior. Finding the solution to water infiltration is often made difficult when several of these common factors are working together to cause the problem.

Some leaks may not be rain.

A major culprit to water-related issues lies within your air-conditioning system. We have all seen stained ceiling tiles next to air conditioning supply registers and associated them with some sort of condensation problem. Though frequently overlooked, this condensation problem can occur at many other locations, such as inside a wall assembly. What might appear to be rain entering the building through windows, walls or roofs may, in fact, be condensation from an air-conditioning unit.

These condensation problems are frequently built-in to our building assemblies. The enclosure assemblies being constructed today include advanced materials that contain an astonishing array of different performance parameters. In addition to insulating characteristics, some materials allow water vapor to migrate through, while others practically stop water vapor in its tracks. To make things more complex, some materials change properties, depending on how wet they get. The appropriate selection and assembly of materials is dependent on climate and require careful consideration to ensure that they perform adequately.

Houston, we have a problem. 

Air-conditioning systems have the ability to create problems in the building enclosure. In general, mechanical designers and contractors do not take these problems into consideration. Because of our cooling-dominated climate, the interior environment is generally colder and drier than the exterior. The building enclosure must be up to task of resisting this difference between the varying weather and controlled interior conditions.

While preventing rain and bulk water infiltration is key, often overlooked is the impact of humidity in the wall assembly and the vapor drive across the wall. Just like air conditioning registers, surfaces inside of walls can condense and lead to moisture damage in unseen places – and the air-conditioning systems can dramatically affect this condition.

The Heating, Ventilation and Air-Conditioning (HVAC) system sucks in outdoor air from the exterior and exhaust fans remove stale air from a building’s interior. Conditioned air is distributed to interior rooms to keep people comfortable. What’s not commonly understood is that this combination of system functions often results in cold air being distributed into all kinds of unanticipated places: walls, ceilings, floors and more. 

Another common issue is that air-conditioning systems affect pressures in small spaces such as exterior and partition walls, ceiling walls and chases. Leakage from ductwork, especially exhaust-air systems, installed in ceiling spaces and chases are unintentional, but can lead to unanticipated air pressures and movement. All the while, changing and gusting winds are pressurizing and depressurizing the building’s openings, gaps and imperfections with moisture rich outdoor air.

Microclimates Matter

Designing a building’s mechanical systems to manage humidity inside the building is important, but often neglected are the hygrothermal (temperature and humidity) conditions at the enclosure’s surfaces and inside the various components of the wall or roof assemblies. These localized hygrothermal conditions describe the microclimate, and the conditions seen there are vastly different than those found in the middle of a room.Microclimate evaluation helps determine whether condensation will form and whether wet building components will be able to dry.

Examples of microclimates include air conditioning diffusers blowing cold air on interior walls and cooling the surface to below 60°F. Another example includes radiant heating from the sun elevating a brick cavity space to over 120°F. These kinds of conditions can dramatically change the performance criteria of an assembly and may not have been considered during the design, many times resulting in unexpected condensation inside walls or roof systems.

What to consider when designing a building enclosure to resist water infiltration

The weather resisting components of a building’s enclosure are neither glamorous nor pretty, and don’t seem to add value to the bottom line of a project – but they absolutely have to work. If a building suffers from moisture problems, the finest Italian marble floors are not going to help make the project a success. Our environment is harsh and will punish imperfections in design, materials, and construction – you don’t want your building performance to rely on luck.

Even with the correct selection of materials, the construction of the assembly is key – and we all know that a perfect wall cannot be built. With this in mind, a designer needs to specify walls and roof systems that tolerate imperfection in materials and construction, and consider the impact of the air-conditioning systems. All transitions should be adequately flash-drained to the exterior. Punched openings should be constructed so that when a leak occurs it is drained out of the building and not into the inside of the walls. The building’s components should be integrated and designed such that even if a small amount of water has found its way into the building enclosure, it has a chance of drying out. 

Also, outside air should be prevented from mixing with the inside air. You wouldn’t construct a building with windows that don’t close, so why would you design an enclosure without an air barrier? Without a coherent air barrier, there is no way to control the temperature and humidity conditions inside the building enclosure components. With a properly designed air barrier, not only will you get better moisture-resisting performance out of your building enclosure, you can also achieve substantial reductions in energy use (just like when you close a window).

Bottom line, awareness of these “secret problems” can help alleviate leak problems. Listed below are some simplistic guidelines for mechanical systems. As a general reference, these rules hold true to reduce the risk of the air-conditioning system affecting the enclosure.

·        Air diffusers should not blow cold air on wall assemblies or partition walls because this can create a condition favorable to condensation forming inside the wall.

·        Ductwork – supply and exhaust - should be pressure tested to verify minimal leakage.

·        Common building air-return systems (using the space above the ceiling to pull air back to the mechanical room) are a bad idea because they affect the wall and roof systems. Resist this cost saving measure.

·        Thermostats in spaces should not be set below 75°F in the summer.

What about existing buildings?

To repair a wall or roof, you need to first understand the problem. What is the problem? How much, why and for how long? Does the original design intent hold water (so to speak)? We regularly find that moisture intrusion problems are the result of a combination of failures. One common and under-addressed issue is that air barriers are inadequate or missing altogether, greatly contributing to the possibility of condensation in interior and interstitial spaces.

Approaching these water problems in a methodical manner can assist you in getting the issue resolved the first time. Appropriate use of diagnostic tools, like thermographic imaging, destructive openings, water testing, moisture meters and psychrometers can provide valuable insight into the cause and extent of the problem. However, the best testing program is useless if they are not applied with an understanding of the underlying physics and the understanding of how building systems interact.

After all, the leak you find may not be a result of what you’ve come to expect.

Gordon Shepperd, P.E., LEED AP is a Principal Consultant with Apollo BBC, a Houston-based building consulting firm specializing in building science, forensic architecture, structural and mechanical systems, sustainability and building commissioning. He is a Director of the Greater Houston Area Chapter of the U. S. Green Building Council and President of the Texas Chapter of the Association for Preservation Technology International. He is a licensed professional engineer in Texas, Florida, Mississippi and Louisiana. Gordon can be reached at 713-869-0000 or at