I am building a garage in Mobile, Ala., for antique cars. The garage will be air conditioned during the summer and heated in the winter. It will be kept dry with dehumidifiers during the summer. The construction will be brick veneer with wood stud backup. The backup will be fiberboard sheathing with wood studs, filled with batt insulation, and gypsum wallboard on the interior.
Should I install Tyvek on the exterior face of the fiberboard and a polyethylene vapor barrier on the interior face of the studs? To match the design of the house, there will be very short roof overhangs. Will this construction detail present a problem?
The primary problem I envision with this construction is rainwater penetration through the masonry during warm weather. Because the garage has minimal roof overhangs, water will flow down the exterior face of the masonry during rains driven by light to heavy winds.
Depending on the quality of the construction and the extent of mortar bond to the brick, there may be considerable water penetration through the veneer. Even in the best circumstances, some water penetration should be expected.
For this reason, it is essential that a moisture barrier be installed on the exterior face of the sheathing. Also, a significant air space must be maintained between the exterior face of the moisture barrier and the back face of the brick veneer.
The air space between the back of the brick veneer and the face of the sheathing in residential construction is often maintained at 1 inch instead of 2 inches, which is the amount recommended by the Brick Industry Association (BIA) in Technical Note 28B for commercial masonry construction. If feasible, I would recommend increasing the air space to 2 inches to reduce the risk of mortar squeezing out of the bed joints during construction and bridging the gap.
In either case, it is essential that care be taken to prevent mortar bridges from occurring throughout the height of the wall, and from mortar droppings accumulating at the base of the cavity. I would further recommend replacing the fiberboard sheathing with a moisture-resistant version that does not support mold growth.
The air space between the veneer and sheathing generally has a high moisture content, especially following rains. Because the interior is air conditioned during the summer, moisture from the cavity may move through the wall system and condense on the cool surfaces that are below the dew point temperature of the cavity air.
Because of this condition, consider installing weep vents at the base of the brick veneer, and vents just below the soffit at the top of the wall. Venting helps the walls dry out following rain. Convection currents draw air into the lower vents, which then flow out of the upper vents. By reducing the length of time that the walls remain wet following rain and preventing water from bridging the cavity, and by maintaining a free-draining cavity during construction, the risk of developing moisture problems such as mold growth within the wall system is greatly reduced.
Because the interior is not humidified during the winter, and provided the moisture carried into the garage via rain on the cars or brought in by other means is limited, an interior vapor retarder is probably not necessary in this climate. If the moisture drive is primarily from the masonry cavity to the interior, installing a vapor retarder on the interior side of the wood studs may, in fact, increase the likelihood of condensation occurring within the wall system by preventing drying to the interior. Avoid using any coatings or wall coverings on the interior surface that inhibit the evaporation of water so that moisture is not trapped in the wall.