Your plenum will typically be located in an unheated area of the house such as the attic or garage.
How to seal air handler plenum in attic.
Anyhow the return box and the other box that has the ducts coming out of it are made of sheet metal.
Because the plenum is where the greatest air pressure differential is found it is therefore the 1 place to seal to save energy and cash.
If an air handler is in a garage attic crawlspace or closet it might experience less air flow and humid air.
Humid air and air flow.
My mother has had me going up into her attic to seal the leaky ducts and parts of the plenum air handler that join up to the furnace and coil units.
And plenum and return air connectors.
Where air conditioning pipes enter the housing grommets may be missing or leaky.
Common places to find them are at ill fitting cover panels or those with missing screws.
Leaks in hvac air handlers can usually be seen felt and heard.
Then finish up by sealing the access hatch with self sticking foam weatherstrip photos 10 and 11.
This is a common occurrence in florida garages.
The entrance of the attached air filter insertion point base.