PA-0103: Moisture, Building Enclosures, and Mold

Effective Date

How water gets into a structure, why it doesn't leave, and how these architectural flaws become HVAC headaches. This two-part article was first published in HPAC Engineering, December 2001 and January 2002.


Buildings leak. Conditioned air escapes, and outside air is sucked in. The relatively easy science of keeping unwanted water out of a structure is nearly a lost art. And many of the modern materials we use to build actually hinder the drying process once water enters. A typical new building has dozens of holes in the shell that don't show up in the blueprints because they were never intended. You may say to yourself, none of these problems technically fall within the purview of the mechanical engineer, and you're right. But keep two points in mind: Many of these problems translate into indoor-air quality (IAQ) complaints, which are your problem; and eve ryone involved in construction needs to develop a whole-building philosophy in order to improve how we build.

If such lofty goals don't appeal to you, think of your career. I speak from experience when I say that when you are called on the (wet) carpet for a water problem, it helps to be able to show that the problem was from poor architectural design, materials, or installation, rather than an HVAC " condensation" problem .

Learning to spot where breaches commonly occur in buildings is essential for mechanical engineers. With this knowledge, you may be able to prevent a catastrophe during the plan review. With this knowledge, you can ask to see the necessary architectural detail for determining if a water problem might exist while still early in a project — especially if you are re presenting the building owner. Simply tell the architect you need to see how much dehumidification you’ll need for that unplanned natatorium they’ll be building in the basement.

In this two-part series, I’ll first discuss why these are pervasive problems in the construction industry and how they are inherited by HVAC technicians and engineers once the structure is occupied. I’ll also discuss why keeping ground and rain water out of a building should be easy, but isn’t, and what to do once water gets in. In the Janaury issue of HPAC Engineering, part 2 of this series will review air control and pressurization, ventilation, and humidity control and will include some resources to help expand your knowledge of design and construction errors that lead to HVAC headaches.

The Dysfunctional Construction Family

Controlling water problems in buildings should be easy. Keep the rain and ground water out. Design and construct the building enclosure to be able to dry when it gets wet—and make no mistake about it, it will get wet. Control the airflow across the building enclosure because air carries water, so build the building enclosure without holes—at least without big ones. Remember that as part of this air-control system, you have to control the air pressure across the building enclosure. Should buildings be pressurized or depressurized, or should you attempt the impossible to maintain a neutral pressure? I think you should pressurize building enclosures everywhere, but we’ll get into that later. Lastly, ventilate and condition the building enclosure in a controlled manner to control humidity.

All easy stuff, right? Unfortunately, it’s not. Out of necessity, we divide up the responsibility of doing all of this among various professionals and tradesmen. The people involved don’t speak the same language, or just don’t speak at all. They don’t know the same things, and too often, no one seems to know the entire picture. It’s like working on a puzzle without having a picture on the box to go by, so you don’t know until the end of a very inefficient process that some pieces are missing, while others don’t belong at all. In other words, the construction team is a big dysfunctional family.

Often, those making the financial decisions know even less. They let salesmen talk them into the next great thing: “Sure this works, we have 5,000 systems in Lower Elbonia.” Finally, we “value engineer” the project—a term that to me can be best defined as: “A process by which someone who has no clue about sound construction takes out the stuff we really need to make things work.”

Keep the Rain and Ground Water Out

Controlling rain and ground water are the most important factors in the design and construction of durable buildings and for the control of mold. Let me make something painfully clear: Air-conditioning and dehumidification systems cannot be used to control rain- and ground-water problems. It is not the job of the HVAC engineer to design rain and ground-wtaer-control strategies. There are no guidelines available to size systems to handle rain and ground water — except to make these systems really big, which is a bad use of energy and poor engineering. Do n’t even try it. Fix the rain problem and fix the ground - water problem with drainage. There are situations where installing a dehumidifier in a basement works well, but that is not what I define as a ground-water problem. You don’t install a dehumidifier when there is running water or standing water in a basement.

The fundamental principle of rain and ground-water control is to shed water by layering materials in such a way that water is directed downward and out of the building or away from the building.

Drain Everything

Gravity is the driving force behind water management and drainage. The “down” direction harnesses the force of gravity, and the “out” direction gets the water away from the building-enclosure assemblies, openings, components, and materials. In general, the sooner water is directed out, the better.

The most elegant expression of this concept is a flashing. Flashings are the most underrated building-enclosure component and arguably the most important.

Drainage also applies to materials. Water that is absorbed in a material cannot be drained away. We paint and stain wood siding so that water is not absorbed by it and can be drained from the siding surfaces. We damp-proof concrete foundations for the same reason.

Drainage applies to assemblies such as walls, roofs, and foundations, as well as to the components that can be found in walls, roofs, and foundations, such as windows, doors, and skylights. It also applies to the openings for the windows, doors, and skylights and to the assemblies that connect to walls, roofs, and foundations, such as balconies, decks, railings, and dormers. Finally, it also applies to the building as a whole. Overhangs can be used to drain water away from walls. Canopies can be used to drain water away from windows, and site grading can be used to drain water away from foundation perimeters.

Drainage is the key to rain- and ground-water control :

  • Drain the site.
  • Drain the building.
  • Drain the assembly.
  • Drain the opening.
  • Drain the component.
  • Drain the material.

In other words, drain everything .


My advice to engineers is to thoroughly review architectural drawings, particularly the window-opening details and flashings. These openings cry for “pan flashings.” Windows leak. I’m tempted to say always, but it’s sufficient to say that windows leak frequently enough that you need to treat them as if they are going to leak. An under-window “gutterr” (or “pan flashing”) is essential to redirect this leaking water to the exterior.


Review wall assemblies to make sure they have drainage planes: a membrane covering the wall behind the exterior cladding. This is real important when we get to brick veneers and stucco—especially synthetic stucco. Remember that there are only two kinds of stucco: stucco that has cracked and stucco that will crack. There are now some magnificent synthetic stucco systems available : “Drainable EIFS” (external insulation and finish systems). These work because they drain. Watch out for the non-drainable systems especially when they are used with windows.

It’s not uncommon to find wall sandwiches made from OSB or paper-covered gypsum, steel studs, cellulose or fiberglass in the cavity (to keep water from draining away), paper-cove red gypsum, and vinyl - covered wall paper to keep the wall structure from drying. Does vinyl repel mold? Not at all. As shown in “How Mold Eats Walls,” mold will digest vinyl paper adhesive and grow into the paper as well.

Let It Dry

In 25 years of being in the construction industry, I’ve concluded that we approach things backward. We focus entirely on preventing things from getting wet, which is a good idea, but we also need to provide a contingency plan by engineering the structure to let things dry after they get wet. The problem is that many techniques that pre vent wetting also pre vent drying. Vapor barriers are a prime example of this foolish thinking. Yes, vapor barriers stop vapor flow. But what if there is vapor in the assembly already? And remember that there are two sides to a wall: the outside and the inside. Walls can get wet from both the inside and the outside. Walls also can dry to both the inside and the outside. We seem to think that walls only get wet from the inside and that they can only dry to the outside—what I call “cold-climate chauvinism.” We do the calculations for the winter, but the walls rot in the summer.

I love code recommendations. One of my favorites directs engineers to “put a vapor barrier on the warm side.” Warm side when? In January or in July? We may need vapor barriers in Canada, but we don’t need them in the Lower 48. In Canada, there are only two seasons: this winter and last winter. In the rest of the world, especially the air-conditioned world, moisture flow is from the outside in. This means that a vapor barrier installed on the interior of a wall assembly is on the wrong side.

If you don’t believe me, . . .

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