Moisture Control: Drain and Dry

“Air barriers can also provide a water-resistive barrier, which provides a structure with a durable, seamless moisture barrier resulting in superior drainage protection,” says Larsen. As an ally in protecting the building from degradation and reduced insulation effectiveness, among other challenges, the WRB is essential. But experts say so is a means for drainage and drying of the enclosure, so that moisture from rain, condensation or water vapor is drained out and away from the building, and moved down and outward by means of layered materials such as barrier materials, flashing and waterproofing.

Another concern is preventing the movement of water vapor from areas where it can cause moisture accumulation. “It really is a matter of designing and installing the product assemblies properly so that they are not exposed to water vapor where the temperature crosses the dew point,” says Zabcik. “There are excellent software tools available now that allow the designer to model the assembly using climatic data and ensuring the vapor barrier is in the proper place.”

Other means for water to enter the building enclosure include effects that occur with porous materials, such as inward vapor drive, which impels moisture into the enclosure, as well as capillary action, which can pull exterior moisture upward and inward. In these cases, an air space in the enclosure backed by a WRB across a continuous drainage plane will help prevent propagation of the moisture beyond porous claddings including wood, manufactured stone, concrete panels and the like. Manufacturers caution that traditional underlayments, such as commercial roll “wrap” materials and asphaltic papers, may not be sufficient to stem moisture migration; in addition, any WRB material will benefit from use of a ventilated gap of at least 3/8 inch, which can be maintained with a variety of materials or details. Modeling and testing by some manufacturers ensures that diffusion and drying in the ventilated gap will be sufficient to allow moisture to exit the envelope assembly too.

According to CENTRIA, windwardfacing walls are subject to more driving rain than leeward, protected walls. Wind parts as it flows around the building, so the center of the wall tends to be less vulnerable. At the sides and top edges of the wall, the wind accelerates, driving more rain into these façade areas; the taller and narrower the building, the more discrepancy in wetting intensity. An effectively designed enclosure will also be able to resist water penetration caused by air-pressure differential outside and inside the building. The movement of water behind the cladding caused by pressure differences -for example, during wind-driven rain -is tested for many prefabricated exterior systems using such standards as ASTM E 331, which is used for exterior windows, curtain walls, skylights and door assemblies. Insulated metal panel systems, EIFS claddings and insulated translucent façade systems also have been tested to E 331.

Boyer advises the use of nonporous cladding materials to help reduce overall moisture penetration and associated enclosure problems it can cause. Engineered metal systems, for example, offer advanced joinery technologies that control the entrance and formation of moisture and water vapor, he adds.

In summary, an effective, watermanaged enclosure system requires four components, says Dr. Joseph Lstiburek, P.Eng., principal, Building Science Corp., who investigates building failures. “Flashings and other layered materials, weep holes or other means for water to exit the envelope, a continuous WRB drainage plane, and an air space between somewhere between the cladding layer and the drainage plane.”

In spite of how important this is to highperformance building envelopes, however, some new energy codes and green building codes do not include provisions for moisture control. The 2009 IECC, for example, does not have language requiring vapor retarders and similar measures. Instead, the IBC and International Residential Code (IRC) have been modified to include the key requirements for moisture protection.

Summary: Controlling Heat, Air, and Moisture

IMAGES COURTESY OF CENTRIA

TOP: Maple Grove Hospital in Maple Grove, Minn., used manufactured panels and the Green Guide for Health Care to create a facility where patients and staff can embrace wellness.
BELOW: Many manufactured panels offer a variety of solutions in one product.

In all cases, the most important part of ensuring conformance to new energy codes is to control HAM -heat, air and moisture. Project designs for energy efficient walls require an understanding of the climate as well as specific computations for condensation, expected air leakage of typical assemblies and materials, and other psychrometric properties. Some façade systems should be studied for the impact of stack effect, in which pressures are generated by warm air rising within the enclosure and the building. Material properties for the enclosure and typical weather data help complete the picture.

Another part of controlling heat is thermal gain from sunlight, which is more pronounced in glazed assemblies. Yet while the new codes will focus on reducing heat gain, many of the sustainability standards will reward increased daylight and views for building occupants.

This will be a fundamental challenge of tomorrow’s building enclosures, say experts. New versions of energy codes and building codes will call for lower windowto- wall ratios: prescriptive requirements in the 2012 version of the IECC, for example, call for 30 percent window-to-wall ratio (WWR) and 3 percent skylight area, which is stricter than the 40 percent and 5 percent, respectively, allowed by the 2010 version of ASHRAE 90.1.

According to experts in the energy codes, the ratios can be used as prescriptive rules, but there are also performance-based criteria based on total wall U-factor, for example. As long as the whole-building energy consumption meets the requirements of IECC or the 90.1 standard -proven through calculations or modeling software -the enclosure design will pass muster. To improve the performance of walls with large portions of glazing, a number of low-E coatings are available to improve SHGC while keeping desired attributes such as visible light transmission (VLT) levels as high as possible. Some solar control products are available that can be built into the façade. Many of these glazing products, shades and light shelves are available with today’s manufactured and prefabricated enclosure systems.

With this year’s updates to energy codes and international building codes, it is more critical than ever to build high-performance exterior systems. Manufactured wall systems and pre-engineered, panelized wall products are more frequently used by project teams to reach their energy-efficiency goals and to achieve sustainability certifications, too. With the many new and improved enclosure designs on the market today, there is likely an ideal solution for every type of building and project need.

Parex USA is a leading manufacturer of EIFS, stucco, acrylic & elastomeric finishes, and air & water-resistive barriers designed to meet stringent code requirements. Parex USA brands include: Parex, LaHabra, El Rey Stucco, and Teifs http://www.parex.com/