Mold & Moisture: Double Trouble for Schools
When it comes to keeping your school healthy, a tight building envelope is the best defense. Water penetration through the building envelope can lead to mold, which causes poor indoor environmental quality and contributes to health problems such as allergies and asthma. Moisture intrusion is also a leading cause of premature structural failure, and preventive maintenance plays a huge role in the sustainability and lifespan of your building. Read on to learn how to solve moisture problems before they become mold problems or shorten the life expectancy of your school facility.
Importance of the Building Envelope
By definition, an “envelope” is an encapsulating covering such as an outer shell or membrane. In simple terms, the building envelope consists of the roof, above grade wall systems and below-grade wall systems.
Successful building envelopes include an air barrier to control the movement of air, a vapor barrier to prevent diffusion of moisture, a thermal barrier to insulate and a drainage plan to effectively manage water infiltration. While many building components and systems may have a single required function, the building envelope fulfills many roles, including:
- Primary line of defense against water/air intrusion (rain, snow, hail, ice and vapor).
- Captures and diverts water to storm drainage systems.
- Establishes the building’s thermal envelope (resistance to the movement of heat and cold)
- May serve as an important work platform for the building (supporting and protecting critical equipment).
When we think of efficiency, we often focus on internal systems like lighting, HVAC and plumbing. However, the building envelope plays an equally significant role, as uncontrolled air leakage can account for as much as 30 to 50 percent of a building’s heating and cooling costs.
Forces at Work on the Building Envelope
Whether in vapor, liquid or solid form, water penetration poses the single greatest threat to the building envelope. Between 80 and 90 percent of all building problems are associated with water, and water penetration continues to be the No. 1 source of complaints and lawsuits from building owners.
All exterior wall components are subject to water infiltration. Walls leak when three conditions exist simultaneously:
- Presence of water (rain, soil moisture, air vapor).
- Forces are present to drive or draw the water inward (pressure differentials, capillary, gravity, wind).
- Openings are present in a wall (gaps, cracks, windows, doors).
Eliminating any one of these factors ensures that water will not penetrate the enclosure.
Common Defects & Failure Modes
Rarely is leakage through the envelope a result of material or system failure, but rather installation errors, incorrect system or materials being specified, incompatible materials being used together, or individual components not functioning as a cohesive unit.
It is impossible to eliminate rain, snow and groundwater, and it is also impossible to stop Mother Nature and the forces of gravity, air currents, capillary suction and so on. Thus, school builders and maintenance personnel should work to eliminate as many of the building envelope’s intentional and inadvertent openings as possible.
This can be a tough task, as water can enter through numerous openings found in every exterior wall assembly – as small as 0.005 inches. These openings may include:
- Cracked/porous brick and plaster (causes absorption).
- Improperly installed or cracked mortar joints.
- Poorly bonded dissimilar materials and “movement type” joints.
- At top of walls, coping caps, through-wall flashing.
- Various standard types of wall penetrations, like doors, windows, pipes/ conduits, ductwork and signage.
Managing Your Building Envelope
In designing your building envelope, limit water penetration with barriers such as membranes and sealants; diversions like sloping surfaces and gutters; and screens such as projections, overhangs and baffles. Next, prevent water accumulation by providing drainage, drying/evaporation and ventilation.
Finally, neutralize the physical forces that transport water. Stop capillary movement by installing a capillary break made of a hydrophobic or impermeable material (such as metal, glass or plastic). Flashing with a drip lip guides runoff away from the structure. Rain screens – an exterior cladding infrastructure that sits away from the outside barrier – create an air cavity behind the cladding that helps to protect the building’s important weather-resistant barrier.
Where applicable, install back-up or secondary systems for an additional layer of protection. Through-wall flashing can be used to divert moisture, which has entered the wall, to the outside, before it can cause damage. Damp-proofing involves treating a surface or structure to resist the passage of water in the absence of hydrostatic pressure. Weeps are small openings in a wall, through which accumulated moisture may drain to the exterior. Other successful building envelope design guidelines to consider are:
- Minimize the number of protrusions and penetrations.
- Minimize number of different cladding and waterproofing systems and materials being used, to limit termination and transition detailing.
- Minimize reliance on sealant systems for termination and transition detailing.
- Design joints to shed water.
- Make proper allowance for thermal expansion, contraction and weathering cycles.
- Eliminate level or horizontal envelope areas that would allow ponding water.
- Drain water away from an envelope as quickly as possible.
- Recess windows, curtain walls and doors.
Maintenance for Life
Sustainability often brings to mind energy efficiency, recycled materials and alternative power sources. When we consider that approximately one-fourth of our facilities are new construction, and the remaining three-quarters are existing facilities, sustainability takes on a whole new meaning – the capacity to endure.
The single most important thing you can do to cut energy costs and minimize capital expenditures it to proactively maintain your facilities, extending their useful lives while improving their performance. Maintenance is often inadequately budgeted or not properly funded, and is postponed until it is too late. Reactive maintenance often involves limited options and increased costs.
To maximize the life expectancy of your school, develop a roofing and building envelope maintenance program that becomes part of your routine. Establish standard inspections to identify active and potential leak areas, including annual roof inspections and walkthroughs after every significant storm event. For an outside perspective, consider hiring a professional roofing and building envelope consultant or establishing an annual contract with a contractor.
Todd Spore is President of PBK Facility Consulting Group, and specializes in building envelope planning and management. Visit www.pbk.com to learn more.