In the floodplain, as we are here, FEMA and the International Residential (building) Code require that HVAC ductwork below the base flood elevation (in this case, 13' above mean sea level) be waterproof and strong enough to withstand hydrostatic forces. Standard galvanized steel ductwork is not allowed below the finished first floor. We opted to use a product called BlueDuct, a direct-burial underground duct system made from HDPE (high-density polyethylene) and engineered to protect against corrosion, mold and mildew, radon, rust, and air leaks. Laying out the rigid duct and securing it in place will take several weeks. When finished, it will be covered with fill and then the crawlspace slab will be poured on top, leaving visible only the vertical risers, connecting the trunks to the floor registers.
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