Technical Updates

Hidden Risks of Green Buildings: Why Building Problems are Likely Part II

Examples of Technical Risks for Contractors & Designers

Moisture intrusion, whether bulk water intrusion through the building envelope or a relative humidity increase due to the heating, ventilating, and air conditioning (HVAC) system, results in a large percentage of construction claims in the U.S. Moisture intrusion not only results in building deterioration, but has been linked to occupant comfort and health issues, especially in those buildings that become contaminated with mold.3 Sustainable building practices, some of which are part of the LEED® accreditation process, can increase the potential for moisture intrusion if not carefully considered and implemented.

Examples include:

• Vegetative roofs which are more risky than conventional roofs (due to the constantly wet conditions) and must be carefully designed, constructed, and monitored after construction.
• Improved energy performance through increased insulation and the use of new materials which may change the dew point location in walls, resulting in damaging condensation and a reduced drying potential for wall assemblies. Lower risk buildings emphasize the drying potential of the envelope over increased insulation. While it is desirable to increase insulation for energy savings, the designer must also evaluate moisture impacts.

• Reuse of existing buildings or recycled components which may not provide optimum water-shedding performance in new configurations or may not be readily integrated to the adjacent new materials.

• Use of new green construction materials that have not been field-tested over time. The designer needs to assess new materials and their risks compared to traditional materials found in lower risk buildings.

• Increased ventilation to meet indoor air quality (IAQ) goals that may unintentionally result in increased interior humidity levels in hot, humid climates. The designer must consider the increased energy load (and cost) and HVAC equipment sizing required to properly dehumidify a building when exceeding the minimum outside air requirements recommended by the American Society of Heating, Refrigerating, and Air-conditioning, Engineers (ASHRAE).

• Building startup procedures, such as “building flush out,” which could result in increased humidity levels and mold growth. Lower risk buildings in rely almost exclusively on source control (which is also a green building goal) rather than relying on “flush-out” and increased building exhaust. Building “flush out” along with building “bake-out” were concepts developed in the late 1980′s by the indoor air quality industry which often caused more problems than they solved.

New green construction materials are entering the market at a staggering rate. Because many of these products help to achieve multiple LEED® credits, designers working on green buildings are eager to specify these products. The risk to contractors is that many of these new products are not time-tested, and designers often do not have the time to fully research the efficacy of these products. If the new product fails it may be difficult to determine if it is a design error, an installation error, or a product defect. Additionally, general contractors must rely on subcontractors to install new materials that they are inexperienced in installing.

Some of the expandable foam insulation products are examples of green materials that pose increased risks. The water absorption properties of these insulation materials can be quite different than what designers expect with traditional insulation. Additionally, some of the carbohydrate based foam insulation materials may retain more water than traditional hydrocarbon based foam insulation. Increased absorption of water into the insulation could negatively affect the wall performance. This is not to say that such materials should not be used; however, their properties need to be recognized and accommodated in the design.

The amount of ventilation (outdoor air) necessary for occupant health and comfort has been debated for decades. Although there are sound arguments on both sides of the debate, the emphasis on increasing ventilation to achieve LEED® environmental quality credits has increased the incentive to add more outdoor air to a building through its HVAC system (a minimum of 30% more outside air above ASHRAE recommended minimums is recommended by).

Increased ventilation is especially risky in the southeast U.S., where outdoor relative humidity levels are elevated for a good part of the year. Experience in the southeast, as well as other areas of the country with humid summers, has shown a direct correlation between the number of moisture problems and increased ventilation rates.

To effectively minimize the risk of moisture problems while increasing ventilation, designers may need to increase the complexity and capacity of the HVAC components and control systems to achieve proper dehumidification. This adds to contractor risk, since complex systems historically fail more often than simple systems. Additionally, the complexity of the system operation can result in unintended pressurization relationships where local depressurization causes humid outdoor air to be drawn into interstitial building cavities, causing condensation and mold growth.

Building owners, designers and contractors all assume more risk when they deal with complex, and possibly untried, technologies not generally found in traditional buildings. Pinpointing whether the problem is design- or construction-related may be very difficult after problem have already occurred.

Building startup procedures to meet LEED® credits include a credit flush-out of indoor containments using increased outdoor air either at the end of construction or during the initial occupancy period. The intent is to remove pollutants from off gassing of volatile organic compounds (VOCs) from new materials. The amount of air needed to meet the flush-out requirements places a building at increased risk because of the amount of moisture introduced with the increased outdoor air. LEED® requirements are that a minimum of 14,000 cubic feet per square foot of floor area is required for flush out. This presents multiple problems: most HVAC systems are not designed to dehumidify that amount of outdoor air which, in a 100,000 square foot building, is 1,400,000 cubic feet of outside air. Depending on outside conditions at the time of the flush-out as much as 240,000 gallons of water can be added to a 100,000 square foot building. This added moisture will get absorbed into building materials, finishes, and furnishings, increasing the risk of mold growth.

Most specifications put the general contractor in charge of the flush-out, including controlling the relative humidity levels during flush-out. If the system is not designed to handle the loads, the contractor is faced with a difficult challenge that may require the addition of a temporary, and extremely costly, dehumidification system. Lower risk buildings tend to avoid flush-out.

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With over 400 domestic and international projects, LBFG® has one of the most varied portfolios of successful project work in the building forensics community. These projects have included both new and existing buildings, cover both residential and commercial construction, and range in size up to $1 billion in cost.