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Kick condensation: Gaining control over moisture is key to solid metal roofing installationsAuthor: By John L. Pierson Jr., P.E. Date: Jan 1 2009 12:00AM URL: www.garlandco.com Understanding condensation, and how to prevent it, is critical to the lasting success of metal roofs. This article will help you identify the causes of condensation and offer some solutions. Condensation and Vapor Transmission Condensation may occur when there is a temperature difference between warm, humid air and a cold metal surface. If the humid air reaches a cooler metal surface at the right temperature, the air will condense. The right temperature for this condensation to occur is called the dew point temperature, or simply, the dew point. The movement of warm, humid air toward the dew point is known as vapor transmission. Vapor transmission, or vapor drive, can move in both directions, depending on climate. For example, during the winter months, the warm, humid air inside a high school swimming pool complex wants to move toward the cold exterior air. Therefore the vapor drive is from the interior to the exterior. Conversely, during the late summer, the outdoor temperature and humidity of a small office building located in a Southern climate will be very high, while the air conditioning will keep the offices at a comfortable 70 degrees Fahrenheit (21 degrees Celsius) and not so humid. In this case, the warm, humid air will drive toward the air-conditioned surfaces inside the building. Therefore, the vapor drive is from the exterior to the interior, exactly the opposite of our previous example.  Avoiding ProblemsCondensation can occur for a variety of reasons, so it is difficult to state exactly where condensation prevention should begin. However, it is generally recommended that some form of preventive measures be taken if any of the following conditions exist: • The January mean temperature is 40 degrees Fahrenheit (4 degrees Celsius) or less. • The structures are humidified or a facility’s operations generate humidity in excess of 45 percent relative humidity. • The structures are enclosed and heated with interior conditions that generate large quantities of humidity. • Any other situation in which a designer has stipulated that a vapor retarder is required. Some examples of high-humidity environments that typically require a moisture barrier are: • Pools • Freezers • Paper and textile mills • Laundries • Bakeries • Gymnasiums and locker rooms • Commercial kitchens The International Building Code directs how to design a building without condensation. The IBC’s 2006 International Energy Conservation Code requires the use of ASHRAE Standard 90.1 for minimum standards on controlling interior environments. These minimum standards lead to the use of three typical design elements to control condensation: • Insulation (with R-value determined from ASHRAE 90.1) • Vapor retarder • Ventilation These design elements can be used independently, but the most efficient designs combine all three.  Using Vapor RetardersTo clarify, the purpose of a vapor retarder is to stop condensation from occurring in roof or wall assemblies. Although sometimes referred to as vapor barriers, such descriptions are inaccurate. No material can completely stop vapor transmission, so the use of the word “barrier” is incorrect. A vapor retarder is used to separate the warm humid air from the surface at the dew point temperature, thereby preventing the air from condensing. Two of the most popular products used as vapor retarders in the roofing industry are polyethylene plastic sheets and self-adhering underlayments (usually 0.04 inch [40 mil] or thicker). But any material that has a permeability rating of 1.0 or less, in accordance with ASTM E 1745, is acceptable as a vapor retarder. Metal roofs by design require fasteners to attach to the building structure. Fasteners will puncture the vapor retarder. Therefore, ventilation must be used in conjunction with a vapor retarder to avoid vapor transmission through the assembly. As shown in the accompanying illustration (above), an air plenum can be used to ventilate the space between the interior finish and the roof deck where the screws are penetrating. In this way, vapor drive up the fasteners is eliminated. In higher-sloped metal roofs, this air plenum would be the unheated attic space above the ceiling and insulation. In lower-sloped metal roof constructions, an air plenum may be created with a drop ceiling. Using Ventilation Proper ventilation of interior air is the best way to eliminate the potential for condensation. Ventilation can be handled by removing the air with a roof- or wall-mounted fan. Economically, however, it is really not practical to use a vent that simply blows heated interior air out of the building. Typically, dehumidifying equipment is recommended, which combines a dehumidifier with a powered vent to remove the humidity without removing heat from the building. While dehumidification equipment can be expensive initially, the payoff is realized quickly in reduced heating costs. Best Defense Understanding how condensation works is your best defense against condensation-related failures of metal roof systems. Whether constructing a new building or renovating an existing one, your engineering professional or roofing material manufacturer can help you determine which ventilation option is right for you. John L. Pierson Jr., P.E., is the engineering services manager for The Garland Co., Cleveland. He frequently delivers seminars and AIA-accredited classes about installation techniques and roofing technology. | ||||||||||
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