Vapor retarder locations in the home
As mentioned earlier, water vapor that is trapped in a wall, ceiling, or floor assembly can lead to a number of problems. There are some locations in the home, under certain conditions, where a vapor retarder is not required. Attic vapor retarders are commonly omitted when blown – in insulations are used. If sufficient attic ventilation exists, condensation problems do not occur in most U. S. climates. СІМА does provide general guidelines for attic ventilation. In vented attics without vapor retarders, standard practice is to provide 1 ft2 (0.093 m2) of net vent area for each 150 ft2 (13.94 m2) of ceiling area. In vented attics with vapor retarders, standard practice is to provide 1 ft2 (.0903 m2) of net vent area for each 300 ft2 (27.87 m2) of attic floor area. When using a combination of roof and eave vents and no ceiling vapor barrier, there should be 1 ft2 (0.093 m2) of net vent area for each 300 ft2 (27.87 m2) of ceiling area. Vents should be installed with 50 percent of the total area in the eaves and 50 percent of the total area in the roof near the peak. The design professional or builder should verify these required mini – mums with local building codes.
In homes with cathedral ceilings, a continuous vapor diffusion retarder with sound, reliable airsealing is very important. Moisture vapor can move through many materials, including fibrous insulation, by diffusion. Therefore, moisture vapor that gets around or through a vapor retarder must be allowed to exit a cathedral ceiling rafter bay through a vent opening even when an airspace does not exist. Moving air can carry lots of moisture, but air movement is not necessary for moisture to escape from buildings. For example, since commonly used asphalt roof shingles have very low vapor permeance, cathedral ceilings can be likened to walls with very-low-permeance exterior skins.
Continuous vapor retarders can prevent condensation problems, but if the vapor retarder is penetrated by recessed lights that are not air/vapor-tight, some means must be provided to allow moisture to escape. (When ventilated airspaces are provided in milder climate areas, kraft vapor retarders may be adequate.) This can be accomplished with eave, ridge, or other vents. Note that airspaces alone, without both eave and ridge vents, will not add protection against moisture condensation in sloped ceilings. Air will not move through a space unless it has a place to exit as well as a place to enter. When both eave and ridge vents are provided, a V2" or thicker airspace between the top of the insulation and the roof sheathing is desirable. As stated before, the design professional or builder should verify these required minimums with local building codes. Not only does this arrangement remove any unwanted moisture vapor, an airspace also helps remove heat in hot weather, and many professionals believe that it extends the life of roofing shingles as well.
Even when not required to prevent condensation problems, attic vapor retarders may be worthwhile; their presence may help maintain more comfortable humidity levels. When a vapor retarder is desired and blown-in ceiling insulation is used, a combination of faced batts and blown-in insulation, followed by a vapor retarder ceiling paint, can be used. Homes with irregular ventilation or high moisture levels should have a continuous vapor retarder. Vapor retarders are not necessary or recommended on interior walls where unfaced sound batts are installed.
As with wall assemblies, vapor retarders in floor assemblies should be installed on the warm side of the structure. In a heating climate, install it either in the space between the subfloor and finish floor or between the floor joists and the subfloor. An exception to placing the vapor diffusion retarder on the warm side of the floor is in houses constructed over a concrete slab. To protect the slab from soil moisture, place the vapor diffusion retarder above the gravel subbase.
In any climate, a crawl space vapor retarder may be necessary to reduce condensation resulting from ground moisture. In most traditionally constructed homes, the ground is a significant source of moisture. Moisture condenses on the colder sections of the foundation and framing, including nail penetrations. Placing 6-mil polyethylene sheeting directly over the ground cover reduces this likelihood. Local building codes require vapor retarders unless minimum crawl space vent requirements are adhered to. Additional strategies to help eliminate unwanted crawl-space moisture should include making sure the minimal constructed clearance meets all regulatory requirements, installing small, area, or through-the-wall drains, sloping the grade toward the drain intakes, verifying that all polyethylene seams are overlapped at least 6 in (seal to drain), and extending it up the foundation wall on the interior (avoid covering vents). Regardless of the number of foundation vents or climate, it is always good practice to install a vapor retarder over the soil.
For extensive renovations where wall cavities are opened and filled with blanket or rigid board insulation, it is simple to add a vapor retarder before the wall is refinished. Walls that have been retrofitted with blown-in or loose-fill insulation typically do not have vapor retarders. In cases where indoor moisture levels are not extreme, researchers have found that moisture that enters the wall eventually will evaporate and not damage the building materials. Many design professionals guard against this strategy because a homeowner’s activities and lifestyles vary with time and technology, leading to unpredictable and possibly undesirable consequences.