Duct Sealing in the Attic and Basement

Torn or Disconnected Attic Ducts

Ducts can leak in many ways. Tears in flexible duct material or ducts that have come
loose are very common. If the leak occurs on the return side of the duct system (the side
that pulls air from the house back to the air handler to be re-conditioned), then
contaminated, unconditioned air from the attic gets pulled into the system. After being
heated or cooled, it then goes into the house. If the leak is in a supply duct, conditioned
air leaks into the attic and doesn't make it to the part of the house where you want it.

Leaks in the duct system also make infiltration problems worse because of the pressure
imbalance. For example, if you have a bad leak in the return duct but the supply ducts are
OK, the attic pressure can be lower than the house pressure. This will force conditioned
air from the house into the attic through whatever holes it can find.

















Torn duct in attic                                                      Same duct after being repaired
                                            and sealed with mastic.

Leaky Plenum or Ducts

In addition to torn or disconnected ducts, there can be a lot of leakage wherever two
pieces of duct or plenum are connected. All joints and seams should be sealed with
mastic. Tape is inadequate because attic conditions will cause the adhesive to fail in a
relatively short time.

















Plenum seams and joints sealed with mastic
Duct Sealing
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