| Insulation
and Ventilation of Wood-Frame Roof Assemblies
Moisture Control and Climate Type Ventilation of insulated roof assemblies has become standard practice in the design and construction of wood-frame buildings throughout North America. These practices have developed as much from myths and misunderstanding as they have from documented and validated research. Regulations and codes concerning ventilation have been based on general assumptions regarding insulation type and climate that often do not account for the specifics of a building’s design or location. Historically, wood roof assemblies were open to heat and moisture flow so durability problems related to moisture were rare. With increased levels of insulation, the wetting potential from condensation is greater, while drying potential has been reduced from loss of heat flow and increased use of vapor impermeable materials. Current Code Requirements Building codes generally require that the minimum net free ventilating area for attic vents be a 1/150 ratio of the area of the attic space being ventilated. The codes also generally allow the ratio to be reduced to 1/300 if the venting is “balanced,” and a vapor retarder of 1 perm or less is installed on the warm “ceiling” side of the insulation. Balanced ventilation means that 50% of the ventilation is provided low on the roof, while the other 50% is placed high. These requirements, however, have been developed and enforced largely without regard to: climatic differences, roof configuration, cathedral ceilings vs. open attics, and differences in the properties of materials used in roof assemblies. Outside of code requirements, many feel that ventilation can help control roof temperatures to 1) extend the service life of asphalt shingles and 2) reduce cooling loads in warm seasons. TenWolde and Rose [1999] conclude that shingle color, not venting, likely has greater effect on shingle durability. They also found that the quantity of attic insulation and location of ducts are most important to lowering attic temperatures. Ventilation is also seen as a strategy to prevent or minimize the occurrence of ice dams. Moisture Control for Roof Assemblies The key to moisture control is establishing a balance between wetting and drying. Design and construction must minimize the potential for wetting and maximize the potential for drying. Climate is another important factor because moisture control strategies that work in one climate do not necessarily work in another. In fact, ventilation is not necessary in all climates, and in some climates may actually lead to moisture and durability problems. For example, venting a roof in warm, humid climates tends to increase rather than decrease the moisture level in a roof space. From a performance standpoint, moisture control is important for preventing mold growth as well as physical deterioration of framing components within roof assemblies. In roofs with cavity insulation, mold growth can occur at the underside of roof sheathing in cold, heating climates, or on a ceiling-side vapor retarder in warm, humid, cooling climates. If moisture levels are excessive, deterioration of the sheathing can occur, particularly in cold climates. Moisture control for roof assemblies in all climates should begin at the “supply” side, using the primary strategies of: 1) interior humidity control, 2) air leakage control and 3) vapor diffusion control. Interior humidity control involves mechanical exhaust ventilation of bathrooms, kitchens, and laundries, dehumidifiers, and whole house ventilation that includes fresh air intake to dilute moisture levels of the interior air. Other control measures include installing vapor barriers at crawlspaces and basements to prevent the introduction of ground moisture, and terminating ventilation exhaust outlets to the building exterior. Air leakage control involves sealing the roof, ceiling, and walls at all penetrations and perimeters to ensure moist interior air or, in some cases, moist exterior air, does not leak into the roof assemblies. Minimizing the amount of leakage is critical to moisture control as air leakage has the potential to transport much greater quantities of moisture into assemblies than diffusion. In addition, mechanical pressurization or depressurization of the building can be used to control the dominant direction of airflow to reduce air leakage. Vapor diffusion control generally means installing a vapor retardant material at the warm side of the insulation. Vapor diffusion retarders, such as polyethylene sheets, kraft paper-faced batts, and paint coatings on drywall, have a critical impact on both the wetting and drying potentials of roof assemblies and must be carefully considered. In many climates, the vapor drive reverses daily and/or seasonally. Use of materials that are highly impermeable can lead to problems with interstitial condensation and mold growth in some climates. Ventilation drying is really a second line of defense for moisture control. It is not appropriate in some climates, nor effective in some roof configurations. Climate-specific Strategies Over the past decade, researchers have undertaken a broad reassessment of the need for roof ventilation. Many of their findings and recommendations are found in recent editions of the “ASHRAE Fundamentals Handbook” [ASHRAE, 2001]. ASHRAE indicates that the advantages and disadvantages of roof ventilation be evaluated on a case-by-case basis, with climate the main consideration. Discussion in the Handbook of Fundamentals is organized by the three climate zones developed by Lstiburek and Carmody in 1994: heating, cooling, and mixed. In heating climates, researchers have emphasized the need for interior humidity control, airtight ceiling construction, and vapor diffusion control at the warm [ceiling] side of the construction. In mixed and cooling climates, condensation may develop on top of polyethylene vapor retarders in the ceilings of vented roofs during interior cooling cycles and lead to mold growth. Current research suggests that roof ventilation should not be required in cooling climates, and vapor retarders should not be placed in ceilings, so as to allow drying to the interior. In mixed climates, roof ventilation is recommended for back-up protection against moisture build-up in attics and cathedral ceilings, however, vapor retarders should not be required at ceilings of vented roofs, again to allow drying to the interior. Roof Configurations In addition to climate factors, roof configuration and insulation type can affect the need for roof ventilation. Prescriptive code requirements for ventilation have been based on assumptions about simple roof forms that use low-density insulation materials such as blown fiberglass, blown cellulose, or fiberglass batts. However, many of today’s roof designs involve complex forms that are difficult, if not impossible, to ventilate according to codes. Cathedral ceiling assemblies are often insulated with rigid foam [extruded polystyrene, expanded polystyrene, or polyisocyanurate] or spray foam [polyurethane or polyicynene]. The air tightness and vapor resistance properties of these materials are much different than those of low-density insulations. Given these properties, there may be roof designs, even in heating climates, where ventilation is not required. Again, the primary moisture control strategies should be incorporated into the design, along with a high quality of construction. Ventilation may serve as a back-up strategy where it contributes to the drying potential of the assembly. Contributor:
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