Often a WHF will not easily install in the traditional horizontal position. In some instances trusses or other obstructions will interfere with a normal installation. More often there is not enough clearance between the attic floor and the roof. One solution is to create a plenum box which the WHF will attach to. The plenum box can easily be constructed out of plywood and 2×4’s. The red line in the photos represents the plenum box constructed for an AirScape 4.4e installation. The flexible ductwork and fan can then be mounted straight off of the damper box (the ductwork may have to angle slightly upward so that the fan does not touch the attic floor). A 90 degree bend in the ductwork is not necessary for sound attenuation in this application.
– Line the inside of the box with a layer of sheetrock to maintain any required fire ratings
– Use a sound attenuating duct liner for maximum sound reduction
– Seal all framing joints with latex caulk to make the box air tight
– Insulate the outside of the box to minimize heat loss
– Support any sags in the ductwork with additional chain