Is it acceptable to seal a plenum attachment to the air handler with mastic from inside the air handler only?
Does the mastic affect air quality?
Thanks in advance,
Mastic and membrane the inside and silver tape the outside. If you are 4 piecing it as a transition from some other size duct its the only way Id do it.
If its a duct that is the same size as the furnace/coil outlet, you could simply mastic inside or even just tape the outside.
Hope this helps...