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Understanding Today's Vapor Barriers

Today’s vapor barriers are ensuring buildings’ airtightness

Mcn Bonus Feature Oct17 5 Low Rez
Vapor barriers are used in buildings to reduce the rate at which vapor can move through a material. When installed properly, vapor barriers reduce condensation problems and reduce air leakage on fiberglass-insulated walls. Without this barrier, water or moisture can be trapped in the wall producing moisture and other related problems such as mold, sick building syndrome, rot and thermal performance issues.

Vapor barriers are measured in terms of how much water or moisture will pass through the material. This moisture vapor transmission rate is established by standard test methods. Permeability can be reported in perms, a measure of the rate of transfer of water vapor through a material. A vapor barrier is usually defined as a layer with a permeance rating of 0.1 perm or less. Vapor retarders are more permeable and allow some movement of moisture; they’re usually defined as a layer with permeance greater than 0.1 perm but less than or equal to 1 perm.

“The term vapor barrier usually refers to a product that functions as both an air barrier and vapor retarder,” says John Pierson, PE, engineering services manager for The Garland Co. Inc., Cleveland. “Far more moisture issues occur in metal buildings due to air leakage than vapor diffusion through materials. Further, vapor barrier implies a product with no vapor permeability. Very few products have no vapor permeability, so it is more correct to use the term vapor retarder. Vapor retarders are available in a range of vapor permeance, which may be desirable depending on climate and building use. So, we have made a practice of referring to these products as air barriers with a given vapor control performance.”

Vapor and Metal Buildings
Moisture is unavoidable and will get into wall cavities at some point. “Installing an escape route for moisture to pass through is crucial to the longevity of your wall structure,” says Allison VanVreede, product manager of building insulation, CertainTeed Corp., Malvern, Pa. “Air sealing a metal building can be difficult, which if not properly done can increase the risk for moisture to penetrate inward. By installing a vapor barrier that acts as both an air barrier and a vapor barrier, the metal building performance dynamic will increase and last longer.”
Vapor barriers in construction have a unique challenge. “Their primary function is to prevent moisture from dripping down onto machinery or products because of possible roof leaks when it rains, in periods of high humidity and/or to prevent water from seeping up through the foundation,” says Herman Torres, reflective insulation consultant, Innovative Insulation Inc., Arlington, Texas. “But their challenge now is also to let moisture out within an enclosed space like a wall so there’s no possibility of conditions developing that allow mold or mildew to grow. Metal building construction encompasses a multitude of vapor barrier uses depending on the application. For example, the requirements for a fulfillment center may be different from the requirements of an agricultural application. In flood-prone areas, on the other hand, a vapor barrier really needs to prevent moisture from seeping into a structure from a crawlspace under a building.”
Chris Roberts, technical director of Versaperm, Maidenhead, United Kingdom, believes there is little in principle that is different between vapor barriers used in metal constructions and those used in other constructions. He feels this is due to the fact that the barrier needs to be optimized against the requirements of the specific application rather than the overall construction method. “A vapor barrier used in a roof will depend on the roof type, rather than, for example, steel versus timber frame,” he says. “A roof vapor barrier needs to have different properties to a barrier in a wall or one used to prevent radon permeating through a floor. In this example, although roofs and walls both need similar water vapor barrier properties, the roof barrier often needs to be either inflexible or air permeable to prevent being blown away in an updraft caused in a storm. The geomembrane used in a floor, again, needs very different mechanical, and other, properties such as high puncture resistance.”
Bill Beals, district manager, Therm-All Inc., Lancaster, Pa., contends that, historically, metal building envelopes have been designed from the inside out. “In other words, moisture is kept out of the envelope by way of the vapor barrier,” he says. “The reality is, buildings withstand many temperature changes in addition to extreme wind from many directions and numerous types of mechanical systems. All of these factors play a part in how moisture enters the envelope. The unintentional introduction of moisture in the envelope happens in both directions. When moisture is present in a building envelope, and when the temperature within the envelope reaches the dew point temperature or below, it turns into a liquid. Liquid (water) is a conductor of heat and can make for a reduced performance of the entire envelope.”

Barrier Evolution
Throughout the 1960s, 1970s and 1980s, various versions of a vinyl vapor barrier were used in metal buildings. Other construction types used polyethylene films. “The perm ratings at the time were not very good; vapor barriers served as more of an air barrier than a vapor retarder,” Beals says. “Residential construction saw the introduction of kraft and foil-faced products, which began to replace the polyethylene approach. Metal buildings, however, saw polypropylene products combined with other layers of foils and kraft papers designed specifically for the lamination of metal building insulation. The perm ratings of these products went from 1.0 on vinyl to 0.09 and 0.02 perms with the new polypropylene versions. We also learned that vinyl as an exposed surface can deteriorate over time due to [ultraviolet (UV)] exposure.”

VanVreede says in mixed-climate regions of the country, buildings using traditional polyethylene vapor barriers may actually trap moisture in the cavity during the summer—raising the risk of costly moisture and mold issues, structural damage, health consequences and liability. “Tighter building infrastructures are exposing traditional vapor barriers’ Achilles’ heel: the inability to breathe and adapt to moisture,” she says.

In terms of mandated standards, Beals says the first mention of airtightness (i.e., air barriers) in the code literature was a paragraph in the IECC 2009 code. “Fast forward to the latest code cycle, and we see that IECC 2015 and ASHRAE 90.1 2013 have mandatory provisions for air barriers,” he adds.

Torres has seen vapor barriers evolve from polyethylene, rubber membranes, sheet metal and glass to plywood, asphalt-coated paper, fiberglass and cellulose. “Advancements of these basic vapor barriers were initially limited to ease of installation, and the development of grooves or channels that allow moisture on one side of the material to easily flow downward,” he says. “Vapor barriers have continued to evolve on the recognition that keeping moisture out of the building often results in trapping moisture inside the building. As a result, vapor barriers are now designed to be to breathable.”

Pierson contends early metal buildings did not use vapor barriers, instead, original metal construction installed asphalt-coated wood fiberboard below the metal panels. “It was understood that these would not stop condensation, but would minimize its formation to the extent that the asphalt-protected insulation board could manage the moisture,” he says. “Fiberglass insulation started to be used to increase R-value inexpensively, but it could not do the same job as the insulation board so foil and vinyl facers or sheets where used as an air barrier/vapor retarder below the insulation. The largest job these facers do is stop air leakage through the fiberglass, but, they also have a low vapor permeance, hence the term vapor barrier.”

Today’s Barriers
Today, more attention is now paid to today’s air and vapor barriers in modern building design. Not only are the envelopes more airtight, building materials are less-moisture tolerant that in traditional construction. “Modern metal buildings are mostly steel, fiberglass and gypsum board,” Pierson says. “These materials are cost effective and allow quick construction schedules, but leave very little room for error when considering moisture leaks and condensation challenges.”

Vapor barriers have advanced to meet these challenges. Most notably is the introduction of smart vapor barriers. “Smart vapor barriers exist where the product temporarily changes or adjusts to accommodate temperature and humidity levels,” VanVreede says. “This allows the product to be used in more climate zones than a traditional vapor barrier. Smart vapor barriers are equipped to sense and adapt to humidity changes within the walls. In low humidity, the smart vapor barrier remains tight in the winter to prevent moisture from entering. In high humidity, the permeability of the vapor barrier increases allowing moisture to escape, which helps keep the wall dry.”

Smart vapor barriers have custom-designed coatings with the specific properties individual products need to meet specific building requirements. A smart vapor barrier could be highly impermeable against water vapor, liquid water, radon and hydrocarbon. Others may have high liquid-water and water-vapor resistance, but low air-permeability resistance. These two examples could respond to geomembrane and roof vapor barriers. Walls require different properties.

These, “multi-layer or designer vapor barriers are laminates built up to meet an individual application’s specification,” Roberts says. “For example, one layer may be very strong, elastic and flexible to act as a base, but these are often very poor vapor barriers, so a far-less permeable layer is added to the laminate to match the specification. Modern, fast, instrumental permeability-measuring equipment—which can sometimes take a measurement in as little as 30 minutes as opposed to gravimetric measurement that takes weeks to take the same measurement—has the ability to custom-design the coatings to create new materials and barriers.”

Every material has different permeability to different gasses and vapors; some may be effective as a barrier to one gas, but poor to another. By building multi-layer laminates using different materials, a good result can be produced across a range of gasses. Smart vapor barriers can regulate more than just gas permeability. Even puncture resistance can be added to a geomembrane that also needs to be highly resistant to the ingress of water, water vapor and radon.

In addition to these improvements, today’s vapor barriers can come with a reflective material that not only prevents moisture from entering a structure, but also provides 95 to 97 percent radiant heat reflectivity. “This means an increase in energy efficiency,” Torres says. “Also, more and more metal building constructors are installing radiant heating systems under foundations. Reflective insulation like our Tempshield DBDF not only eliminates the use of a Styrene board insulation, but also acts as a vapor barrier and reflects 95 to 97 percent of the radiant heat upward toward the building.”

Pierson says with the rainscreen wall concept being adapted more in metal building construction recently, today’s vapor barriers have found a role in aiding rainscreen function. Rainscreens combine not only the use of an air barrier and vapor retarder, but also a water-control function. “So, in these assemblies, one product usually provides all three functions,” he says. “The benefit is that the exterior cladding does not need to be 100 percent watertight, only provide a rainscreen: hence the name. This has become very popular with designers since it allows more freedom in the design of the exterior of the building. Manufacturers are now providing the metal cladding and the air/vapor/water barrier systems together in a full-system rainscreen warranty.”

One thing that hasn’t changed with vapor barriers is the importance of their correct installation. Airtightness is greatly affected by installation techniques. “According to the Department of Energy, a poorly installed vapor barrier or air barrier can decrease the building envelope’s performance by up to 40 percent,” Beals says. “With the energy codes requiring much more insulation in the roof and walls, performance can be achieved if proper planning and installation of the various building envelope component is ensured.”