duct sealing

All types of ducts – sheet metal, flexible and rigid fiberglass duct board… - are prone to air leaks, particularly the most common type of ducts: sheet metal...

Duct inspection

You may start with a visual inspection of your ducts. You may look for disconnected components and leaks in the connections and joints between the ductwork and the air handling unit and the plenums...

Professional tests

But to test the airtighness of your ducts, and to fully assess their leakage problems, you need a 1) special fan and test called blower test door or a 2) specific duct blower test.

The test is required by some Energy Efficiency Incentive Programs (Energy Star Programs, for instance), and is also performed by some contractors for a very low fee or as part of a bid to seal ductwork. Professional tests will take about an hour.

Duct Sealing materials

Use duct sealant (mastic) to seal the seams and connections of ducts. In some cases, some tapes (metal-backed foil tape, for instance, may also be used). Common duct tape is a bad option, that you should avoid.

See:
Duct Sealing Materials: Mastic.
Duct tapes

What to Seal

Basically you need to seal the openings in the air handler units, the register boot, the joints where ducts of different shapes meet, and alls seams and connections between the different parts of the ducts.

Seams in the air handling unit, plenums and rectangular ductwork should be carefully and properly sealed. Pay special attention to hard to reach places, and disconnected parts.

Other Leaky Areas

Ductwork located in the living space (including the air handler) isn’t a first priority in terms of air sealing, and usually doesn't need to be insulated.

Only drywall-to-ductwork and other similar connections to unheated areas have to be sealed.

Sheet Metal Ducts

Sheet metal ducts – the most common type of ducts - have more connections and joints than flexible ducts, and are largely used both on return and supply parts. All their seams, joints and connections have potential leakage and should be properly sealed.

Ducts sealing Many of these ducts aren’t sealed during construction, making sealing during installation more pertinent. Use mastic to get a superior sealing, and screws to fasten the joints.

This type of ducts should be insulated when located in unconditioned spaces.

Image credit: DOE

Flex Duct / Flexible Non-metallic Ducts

Flexible non-metallic ducts are factory-insulated and have few joints and connections and, consequently, low leakage potential. They are used as return ducts, and also in run-outs/curved surfaces (in this case with metal collars bridging the flexible duct to supply trunks, branches and plenums often made from duct board or sheet metal).

Though they have low leakage potential, they are easily crushed, torn or otherwise damaged during installation. For this reason installation is often more important than sealing. Pay attention to installation and to the possible damages associated to it. Sealing should be done with mastic.

Flex Ducts

Fiberglass duct boards

Fiberglass duct boards are factory-insulated and sealed. Connections should be fastened and sealed with mastic (or approved tapes).