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A slippery slope:

Solutions for the condensation challenges of metal roofing systems

Posted 10/1/2008

This white paper was developed by the Metal Construction Association, Glenview, Ill. More information can be found at www.metalconstruction.org.

Water is the most common cause of rot and corrosion in buildings, with moisture being a major contributor to the formation of mold. A 2004 Centers for Disease Control and Prevention report, commissioned in response to rising public concern about the problem, found that damp or moldy buildings are linked to episodes of sniffles, coughing and wheezing in otherwise healthy people.

Any evidence of liquid water within the building commonly is misunderstood by engineers and property owners as leakage or failure of roof covering materials-but condensation is frequently the cause.

The principal thermodynamic properties responsible for condensation in buildings are identical for all building envelope materials. Moisture condensation is not unique to metal roofing systems but frequently is the cause of rotting timbers or corrosion in metal decking, metal roof panels and metal fasteners.

The control of condensation in steep-slope metal roofing systems is important toward maintaining the effectiveness of the insulation and protecting the roof material from degradation.

Condensation in roof systems is based on the flow of water vapor through the various layers of building materials to a point where cooling forces moisture to condense. Each building component has some resistance to heat and to vapor transfer. The choice in composition and assembly techniques of building components is the key to successful control of condensation. A building's environmental conditions determine the moisture content of ambient air. Internal temperature, building use and air movement all impact the relative humidity of the interior air and dictate the design requirements necessary to prevent condensation.

The current construction practice of making buildings "tighter" to improve energy efficiency conversely traps moisture inside the building envelope. In many cases, the building is completely sealed off so moisture cannot escape. Moisture is generated inside a building when inhabitants breathe, cook, wash or shower; another less obvious source of moisture is liquid left behind within the construction materials.

The thermodynamic principle behind condensation is based on a phase change from a gaseous state to a liquid state when moisture-laden air contacts a cooler surface. Cold air has less capacity to hold water vapor than warm air. The temperature at which this phase change occurs is called the dew point. In a building, air that comes in contact with cooler surfaces at windows or the underside of the roofing plane will condense when it reaches the dew point. The result can be rotting wooden components, corrosion of metal components and mold formation.

Low-slope commercial buildings, such as warehouses and distribution facilities, typically are not vented and rely on different types of construction to control moisture inside the building. Steep slope buildings (> 3:12 slope), such as residential dwellings, institutional facilities or retail structures, usually have a higher human occupancy load, resulting in the need for greater moisture control. Building codes require passive ventilation for steep-slope roofing construction.

In a typical residential roofing assembly, several components are recommended to minimize condensation in the attic space:

• A vapor retarder located on the attic side of a ceiling board.
• Insulation located above the air barrier on the warm side of the surface.
• Vented air space above the ceiling.
• A moisture barrier or underlayment located between the roof covering material and the roof deck.
• A roof covering on top of the underlayment.

Vapor Retarders

Vapor retarders are an important roofing system component in areas where the mean daily temperature is 40 F (4 C) or lower, and where indoor relative humidity is greater than 45 percent. Vapor retarders physically block the fl ow of moisture laden air, which prevents it from contacting a cold surface. Vapor retarders are generally thin flexible membranes with a perm rating of 0.50 or less. Perm ratings are established by standard ASTM E96. The perm rating is a measure of a material' resistance to the passage of water vapor through it under specified duration, temperature and humidity conditions. A rating higher than 0.50 perms generally is considered too diffuse to serve as a vapor retarder. A 1.00 perm rating indicates the passage of 1 grain of water vapor through 1 square foot (0.09 m2) of material in one hour for each inch of mercury pressure differential that exists between the two sides of material (1 pound of water vapor = 7,000 grains; 1 inch of mercury = 0.491 pounds per square inch). ASTM C755 includes information about the selection of vapor retarders for thermal insulation.

Vapor retarders are effective when they are placed on the warm side of the building insulation, so for most roofing applications, the proper location would be above the building's drywall ceiling. Building owners are nevertheless advised to contact a design professional to determine the proper design for and placement of a vapor retarder. Calculations can be complex for more sophisticated designs, such as when placing vapor retarders within layers of insulation.

The vapor retarder can be placed anywhere in the building assembly as long as its temperature remains above the dew point temperature. This can be accomplished by installing a sufficient amount of insulation on the outer side of the vapor retarder-and again, consult a design professional for specific information on the need, type and location of vapor retarder materials within a building envelope.

The permeance rating of a vapor retarder material is an important consideration when designing the building assembly. Condensation can occur even with a properly chosen vapor retarder. Moisture-laden air can leak through the joints of vapor retarder membranes, through tears within the material or around penetrations, such as lights, flues or ventilation piping. As a result, joints in vapor retarder membranes must be sealed, especially in colder northern climates where the moisture drive is the strongest.

Other considerations when selecting vapor retarder materials include:

• The chemical compatibility with the building's environment and the other construction materials used.
• The effect of the vapor retarder materials on the fire performance of the total system. FM Global, Johnston, R.I., lists acceptable vapor retarder materials that do not affect the fire ratings of roof systems when used properly.
• Tear resistance, tensile strength and resistance to rot and mildew.
• The safety of the roofing installers. For example, a 6-mil (0.15-mm) polyethylene sheet material is economical, easy to work with and has an extremely low perm rating. However, installing this slippery material on a steep-slope roof unwisely could subject workers to hazardous conditions.

It is important to remember that vapor retarders can alleviate condensation concerns, but they do not remove heat or prevent the formation of ice dams. In some cases, moisture that is transported through air leaks is more of a problem than diffusion through nonvapor retardant membranes. In these cases, a properly installed air barrier can provide more protection against trapped moisture than vapor barriers.

Proper sealing of air barriers' side laps, end laps, window and door penetrations, and foundation sills is critical to their performance.

Insulation

In commercial roofing applications, the insulation material may be loose-laid over the purlin or truss structure with the panel attachment securing the insulation. A composite insulation board wood panel (wood panel laminated to a rigid insulation) is also a common substrate used under architectural metal panel roof systems. Care must be taken not to compress and deform the insulation. A metal or wood batten, counter-batten system also can be used to create ventilation space below the metal panels where needed.

For residential construction, insulation in the attic cavity minimizes the heat flow from the attic into the living space during the warm seasons. It also minimizes the escape of warm heat from the living space into the attic during the cold seasons. The heat flow into the attic space below a roof comes from a number of sources. Solar heating of the roof surface can raise the surface temperature and conduct heat into the space below the deck. Heat from chimneys, waste pipes and heating system duct work also raises the temperature of the attic cavity. Finally, warm air from the interior living space can escape through a ceiling into the attic and raise the air temperature, as well.

In some modern homes where cathedral style or vaulted ceilings are popular, insulation is placed between the roof rafters.

In construction where the insulation is placed above the substrate, a vapor retarder may be needed between the roof substrate and the insulation. In construction where the insulation is placed below the substrate, a vapor retarder may be needed below the insulation-on the warm side of the roof assembly.

Ventilation

Proper ventilation replaces the warm moist air with air that contains less water vapor. Ventilation is relatively easy to accomplish in conventional attic spaces; however, it is more complicated when used in cathedral ceilings and with structural insulated panels. Positive ventilation and air flow is necessary to maintain a consistent temperature between the vented area and the outside conditions.

Ventilation is generally expressed by the number of times per hour the building air is replaced with outside air. This is referred to as air changes per hour. The number of air changes required per hour varies depending on the application. For commercial building construction, general guidelines for whole building air exchanges, such as in building types that do not have an attic space, are 3 to 5 air changes per hour for warehouses, 5 to 10 air changes per hour for light manufacturing facilities and 10 to 20 air changes per hour for heavy manufacturing. Air flow requirements can be determined as shown in the graphic.

In residential construction, ventilation requirements are dependent on the size of the attic and the placement and rating of the vents. The two types of vents are intake and exhaust vents. The combination of these two types of venting provides free-flow ventilation, which is the most efficient way to move hot and moist air in enclosed areas. Most building codes require that a minimum free-flow ventilation area equal to 1 square foot (0.09 m2) per 150 square feet (14 m2) of attic floor area must be designed and properly installed to provide proper ventilation. Where a properly designed, 50/50 balanced eave and ridge ventilation system is used, or with the use of a vapor retarder (such as in new construction), a free-flow ventilation area equal to at least 1 square foot (0.09 m2) per 300 square feet (28 m2) of attic floor area is often sufficient. The best ventilation technique is to combine eave and ridge venting. This allows air in the entire attic to be pulled from the soffits in the low areas of the eave to the vents at the ridge in the high areas.

Homes with cathedral-style or vaulted ceilings must be designed with free-flow ventilation beneath the roof deck through the use of vent baffles or chutes, which create a space between the roof deck and insulation. Alternatively, a ventilated deck subassembly can be installed over or in place of the existing deck. Insulation should never block the eave intake vents or be in contact with the underside of a roof deck. Where residential construction does not include an attic space, the International Building Code calls for a minimum 1-inch (25-mm) vented airspace beneath the roof deck. In Canada, the ventilation space requirement for this type of construction is 1 1/2 inches (38 mm).

Venting air and water vapor from a building can be effective against condensation, but ventilation alone does not cure the problem.

Underlayments or Moisture Barriers

On roof applications over solid decking, an underlayment (with a slip sheet as needed) is recommended as a secondary barrier against water penetration through a roof system. The underlayment should "breathe" wherever possible, allowing air to pass through while shedding moisture.

The National Roofing Contractors Association, Rosemont, Ill., recommends a minimum of one #30 asphalt-saturated felt for use as underlayment with architectural metal roof systems. NRCA also recommends that one layer of rosin-sized sheathing paper or other slip sheet material be used over the asphalt-saturated felt prior to panel installation where adhesion or bonding of the metal panel to the felt underlayment may occur. In cold climates, an ice dam protection membrane is recommended around the perimeter, at valleys and around penetrations; this measure often is required by local building codes in addition to the felt underlayment. A minimum amount of coverage also may be required in areas on the roof surface where ice potentially could build up.

Moisture Balance

If a balance between wetting and drying is maintained, moisture will not accumulate and condensation related problems in roofing are unlikely. The extent and duration of wetting, storage and drying must be considered when assessing the risk of moisture damage. The concept of using materials with greater drying potential and storage capacity has begun to receive more attention. A quick reference guide using this Moisture Balance Approach is shown below.

Condensation Control Materials

A benefit of metal roofing is that it does not absorb condensed water like other roofing materials. Certain fleece materials are available to absorb liquid condensate into capillary pores and release it back by evaporation or drainage at a later point when conditions favor evaporation. The water vapor then gets transported out of the system by convection and diffusion. These self-adhesive fleece materials are typically laminated to the underside of metal roof panels during manufacturing but before forming. The adhesive layer acts as a barrier to prevent the moisture from contacting the surface of the metal roof underside. In many cases, these materials can replace anti-condensation blankets and vapor retarders.

Careful design of roof systems can prevent or at least minimize condensation in building assemblies. The proper use of vapor retarders, insulation, ventilation and moisture barriers can reduce the presence of moisture in a building and control the formation of condensation. Building codes and ASTM offer some guidance in the selection of these building components. However, it is always recommended that a moisture control or roof design professional be consulted for specific information about how to design metal roof systems to minimize condensation problems.

References

NRCA, "Metal Roofing and Waterproofing Manual."

MBMA, "Metal Roofing Systems Design Manual."

ARMA, "Technical Bulletin: Ventilation and Moisture Control for Residential Roofing."

Straube, J.F., "Moisture, Materials & Buildings," HPAC Engineering.

Straube, J.F., "Moisture in Buildings," ASHRAE Journal, January 2002.

Hens, H. and F. Vaes, "The Influence of Air Leakage on the Condensation Behavior of Lightweight Roofs," Air Information Review, Volume 6, No. 1, November 1984

www.metalconstruction.org

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