VII. VAPOR RETARDERS AND ATTIC INSULATION
Insulation of any form should not be relied upon to prevent moisture movement within an insulated cavity. Whether batts or blown in fiberglass or cellulose, vapor retarders are required unless proper ventilation is provided. As with fiberglass batt insulation, materials used for vapor retarders for blown in insulations must have a perm rating of less than 1 perm. In a ceiling where the space above is adequately ventilated, a vapor retarder may not be required. The exception would be in cases where the cold side cannot be ventilated.

Attic vapor retarders are commonly omitted when blown in insulation is used. If sufficient attic ventilation exists, condensation problems do not occur in most U.S. climates. Sufficient attic ventilation is usually defined as having a net free ventilating area equal to 1/150 of the attic floor area. When an attic vapor retarder is used, ventilation requirements are halved; net free vent area can be 1/300 of the attic floor area.

It should be noted that all kitchen and bathroom exhaust fans must be vented to outside of the building.

VIII. VAPOR RETARDERS AND CATHEDRAL (SLOPED) CEILINGS
Since commonly used asphalt roof shingles have very low vapor permeance, cathedral ceilings can be likened to walls with very low permeance exterior skins.

If there is no vented airspace between the insulation and the wood roof deck, moisture problems may occur in the wood deck, and ice dams may occur in cold climates. Most asphalt shingle manufacturers require a ventilated ceiling below their shingles. Otherwise the shingle warranty is often reduced to ten years. An airspace of 1" or more should be provided between the insulation and the roof deck. This airspace, when coupled with eave and ridge vents, allows for the successful migration of moisture from the ceiling cavity. This airspace is usually maintained with a formed attic vent chute or baffle that is installed from eave to ridge. Since these baffles are sometimes made of a vapor retarder material, it is common to maintain an approximate 2" gap between the ends of adjacent baffles so that moisture may migrate into the vented airspace.

Airspaces without both eave and ridge vents will not add protection against moisture condensation in sloped ceilings; air won't move through a space unless it has a place to exit as well as place to enter.

Water vapor can move through many materials, including fibrous insulation, by diffusion. Therefore, limited amounts of water vapor that get around or through a vapor retarder can 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. However, without a vented airspace, one needs to be concerned if the moisture accumulation will exceed the ability of the ceiling to dissipate the moisture through diffusion alone.

The best strategy for cathedral ceilings in cold and mild climates would be to use a vapor retarder below the insulation and, if recessed lights are used, air/vapor tight fixtures. A kraft-faced batt is sufficient in those areas requiring a vapor retarder. If blown-in insulation is used, a continuous 4 mil polyethylene sheet can be used in heating climates and a vapor retarder paint in mild climates.

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