8.15   Sealing Duct Leaks

Ducts located outside the thermal boundary or in an intermedi­ate zone like a ventilated attic or crawl space should be sealed. The following is a list of duct leak locations in order of their rel­ative importance. Leaks nearer to the air handler are exposed to higher pressure and are more important than leaks further away.

8.15.1   General Duct-Sealing Methods

Duct sealers install duct mastic and fiberglass mesh to seal duct leaks. When they need reinforcement or temporary closure, the duct sealers use tape or sheet metal. Observe these three stan­dards.

1.      Seal seams, cracks, joints, and holes, less than ¼ inch using mastic and fiberglass mesh.

2.      Bridge seams, cracks, joints, holes, and penetrations, between ¼ and ¾ inch, with sheet metal or tape and then cover the metal or tape completely with mastic reinforced by mesh at seams in the sheet metal or tape.

3.      Overlap the mastic and mesh at least 0ne inch beyond the seams, repairs, and reinforced areas of the ducts.

8.15.2   Sealing Return Ducts

Return leaks are important for combustion safety and for effi­ciency. Use the following techniques to seal return ducts.

ü       First, seal all return leaks within the combustion zone to prevent this leakage from depressurizing the combustion zone and causing backdrafting.

ü       Seal panned return ducts using mastic to seal all cracks and gaps within the return duct and register.

ü       Seal leaky joints between building materials composing cavity return ducts, like panned floor cavities and furnace return platforms. Remove the panning to seal cavities con­taining joints in building materials.

ü       Carefully examine and seal leaks at transitions between panned floor joists and metal trunks that change the direc­tion of the return ducts. You may need a mirror to find some of the biggest return duct leaks in these areas.

ü       Seal filter slots with duct tape to allow removal for easy changing of filters.

ü       Seal the joint between the furnace and return plenum with silicone caulking or foil tape.

8.15.3   Sealing Supply Ducts

Inspect these places in the duct system and seal them as needed.

ü       Plenum joint at air handler: Technicians might have had problems sealing these joints because of a lack of space. Seal these plenum connections thoroughly even if you must cut an access hole in the plenum. Use silicone caulk­ing or foil tape instead of mastic and fabric mesh here for future access — furnace replacement, for example.

ü       Joints at branch takeoffs: Seal these important joints with a thick layer of mastic. Fabric mesh tape should cover gaps and reinforce the seal at gaps.

ü       Joints in sectioned elbows: Known as gores, these are usu­ally leaky and require sealing with duct mastic.

ü       Tabbed sleeves: Attach the sleeve to the main duct with 3-to-5 screws and apply mastic plentifully. Or better, remove the tabbed sleeve and replace it with a manufactured take­off.

ü       Flexduct-to-metal joints: Apply a 2-inch band of mastic to the end of the metal connector. Attach the flexduct’s inner liner with a plastic strap, tightening it with a strap ten­sioner. Attach the insulation and outer liner with another strap.

ü       Damaged flex duct: Replace flex duct when it is punctured, deteriorated, or otherwise damaged.

ü       Deteriorating ductboard facing: Replace ductboard, prefer­ably with metal ducting, when the facing deteriorates because this deterioration leads to a lot of air leakage.

ü       Consider closing supply and return registers in unoccu­pied basements or crawl spaces.

ü       Seal penetrations made by wires or pipes traveling through ducts.

ü       Seal the joint between the boot and the ceiling, wall, or floor between conditioned and unconditioned areas.

ü       Support the ducts and duct joints with duct hangers at least every 5 feet or as necessary to prevent sagging of more than one-half inch.

8.15.4   Materials for Duct Sealing

Duct mastic is the best duct-sealing material because of its supe­rior durability and adhesion. Apply mastic at least ¹/₁₆-inch thick, and use reinforcing mesh for all joints wider than ¹/₈-inch or joints that may move. Install screws to prevent joint move­ment or separation.

Aluminum foil or cloth duct tape aren’t good materials for duct sealing because their adhesive often fails. Consider covering tape with mastic to prevent tape’s adhesive from drying out and failing.