IX. VAPOR RETARDERS AND INSULATED BASEMENTS
Below-grade basement walls differ from above-grade walls in that they are vulnerable to ground moisture wicking into the wall or basement floor. Because of this, it is important to maintain the drying potential of the wall since one never knows if the long-term moisture drive will be from the outside or the inside. A masonry wall is capable of absorbing large quantities of water due to the capillary action of concrete. If the masonry wall unit has hollow core, air movements within the wall also increase the thermal and moisture movement. For this reason, it is recommended that a vapor retarder not be used in a wall that is partially or fully below grade. If a wall is above grade, such as in a walk-out basement, then that wall may use a vapor retarder, if the climate dictates a vapor retarder in the above-grade walls.

If no stud wall is available, the insulation can be applied in blanket form with a perforated flame-resistant facing. Applied directly onto the wall, this is often used on the top half of the wall only, which may take it to the depth of the local frost line. If hollow core masonry units are used because of the air convection that takes place within the wall, the insulation should be applied on the entire wall.

While it is sometimes suggested that an airspace should be maintained between the masonry wall and the stud wall insulation in order to keep the wall dry, in actuality this may make matters worse. This vertical airspace can lead to a convective air loop, thereby increasing not only the thermal but also the moisture transfer within the wall. If a full height stud wall is used in addition to the masonry wall, this stud wall is often inset an inch or so, increasing the depth of the cavity to be insulated. The entire depth of this wall cavity should be insulated. This also insulates the back of the studs, reducing the thermal bridging of the wall.

If a stud wall is placed on a partially below-grade masonry wall, the stud wall should be insulated the same way as other above-grade walls in the house. When a vapor retarder is not desired, slashing a faced product's sheathing is not recommended, because narrow cuts are unlikely to signficantly increase vapor transmission.

X. VAPOR RETARDERS AND CRAWL SPACES
When the undersides of frame floors above crawl spaces are insulated with faced insulation, the vapor retarder facing, generally kraft facing, should be placed on the top side, and in substantial contact with the floor above. This prevents the kraft facing from being exposed in a concealed configuration and posing a fire hazard and reduces the opportunity for air to infiltrate between the floor and facing and bypass the insulation. In many localities it is standard practice to use unfaced insulation under floors, with the assumption that the flooring materials provide adequate vapor resistance to inside moisture.

When insulating the perimeter walls, they can be treated the same as a below grade masonry wall, and can use a perforated flame-resistant blanket that is attached to the top plate, extended down the wall and preferably extended two feet along the floor. Where the crawl space floor is bare earth, it is highly recommended that the entire area be covered with a polyethylene sheet ground cover to minimize the migration of underground moisture up into the structure.

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