Mold: Enemy of sustainability

We all know efficiency can be defined in terms of getting the same benefit with less input, be it energy, labor, or time. An often-seen companion word, “sustainability,” is harder to pin down, but implies lower environmental impact from all phases of a building’s life, from construction through operation and maintenance, to eventual demolition. Durability is part of sustainability, because something that must be replaced or repaired less often, all else being equal, leads to lower environmental demands.

The main enemy of any building is water, and keeping water from more than occasional contact with building materials is key to durability. What follows persistent moisture in wood and gypsum drywall is mold, which leads to spores and toxins that in turn can harm occupants and result in lawsuits.

In recent years, many new building materials and techniques have appeared. Along with them have come more stringent requirements to tighten up building shells and save energy. At the same time, there’s been a disturbing rise in insurance claims for mold and water damage, accompanied by the inevitable lawsuits. In the midst of an outcry that reduced infiltration led to these problems, the California Energy Commission’s Public Interest Energy Research program began investigating the causes of mold and water damage. The answers that emerged were complex, surprising, and have relevance for most everyone, at least everyone who lives in a building.

Remarkably, problems arose most often in Southern California, the driest part of the state. The investigators, through interviews and field visits to problem sites, began to piece together common trends. Essentially, they discovered that all buildings leak to some degree. But the key to avoiding problems is to minimize the leakage, to keep the water away from components which it will damage, and to let wet components dry out.

Design, construction, and maintenance all play important roles in avoiding problems. Designing-in generous roof overhangs keeps water off the walls in the first place. Simpler designs with fewer valleys, dormers, and roof-wall intersections are less likely create opportunities for water entry into the building shell. Landscaping should slope to drain water away from the foundation, avoiding berms and edging which can pool water. Even the simple steps of making sure rain gutters are unblocked and that landscape irrigation sprinklers don’t spray against the side of the building are critical to minimize leakage into the building shell.

All cladding systems, especially stucco, leak a bit when water drives against them. This “feature” necessitates an effective water resistive barrier inside the cladding. Water that makes it through must have a clear path to carry it outward, avoiding blockages which can channel it deep into the wall. Properly overlapped tar paper, roofing felt, or house-wrap with proper flashings can provide this resistive barrier. But stucco needs a second, outer layer over the primary, to ensure an open drainage plane.

There are two types of windows, it’s been said: Those that will leak, and those that are leaking already. A leakproof sill-pan under the window should direct water out and keep it from penetrating the wall. Window assemblies should be air-sealed around, because strong winds often push rain into the wall around windows. Several trades often interact during installation of windows, so who does what and how matters at this phase. Installation “as per manufacturer instructions” is not adequate.

Drywall in kitchens and bathrooms can be spaced up from the floor by a centimeter or so, to prevent saturation from spills. These rooms are also sources of moisture from cooking and washing, and they need exhaust fans. Quiet fans are much more likely to be run long enough to do the job and are well worth the slight extra expense. Air conditioning systems can also help remove moisture, but only if they’re properly sized, with open condensate drains. An over-sized air conditioner, which doesn’t run at least a half-hour continuously, will not effectively dehumidify. Leaking and poorly insulated ducts and refrigerant lines can also cause unwanted condensation.

The final key to avoiding mold problems is to let components dry quickly when they’re wet. Paint, if used on stucco and the inner surface of exterior walls, should be permeable to moisture. Wood tolerates short-term contact with moisture, as long as moisture isn’t at levels which can support mold growth. Trying to seal water out with impermeable coatings often has the perverse effect of sealing the water in.