Condensation from HVAC units has to go somewhere, but if it discharges on the roof, it can create localized ponding, which increases the risk of damage. Constant moisture attracts airborne debris, leading to mold, vegetative growth and premature membrane deterioration. It creates unsightly stains and discoloration as well. The solution is to direct HVAC condensation discharge away from the surface of the roof, either through interior discharge lines, when feasible, or by installing condensation lines that direct the discharge to the nearest roof drain.
Adding HVAC Units to an Existing RoofThe ideal time to install HVAC units is during new construction, when the HVAC engineer and the designer can coordinate to ensure that curbs, drainage lines, tubing and electrical access are properly planned for. Adding an HVAC unit to an existing roof is trickier. An improperly installed unit can lead to wind uplift at the separation site and subsequent membrane tears. In regions where the roof is subject to snow loads, adding heavy HVAC units can reduce the roof’s ability to hold as much snow.
Common HVAC-related contributors to moisture problems include:
• Inadequate dehumidification by the HVAC system during humid weather. The resulting high indoor air dew point can lead to condensation, near condensation and mold growth. Comfort problems also are common when the indoor dew point is high because the relative humidity is also high. Occupants often demand lower thermostat settings in an attempt to be more comfortable. This is counterproductive. Lowering the thermostat overcools the building, which increases the risks of condensation, excess moisture absorption and subsequent mold growth. Cold temperatures also further reduce comfort and increase energy costs.
• Leaking return and exhaust air duct connections, leaking indoor air handler compartments operating under suction, and leaking return air plenums. These leaks create suction in building cavities, pulling humid outdoor air into the building where it can condense moisture onto cool surfaces and support mold growth.
• Leaking supply air duct connections and leaking indoor air handler compartments operating under positive pressure. When the weather is hot or humid, these cold air leaks chill surfaces behind walls and above ceilings, creating condensation and supporting mold growth. During cold weather these same leaks can force warm, humid indoor air into cold cavities where it can condense and support mold growth and corrode structural fasteners.