Supply Air Plenum and Humidifier Installation

[thapranksta]

I had purchased a whole house humidifier to install on my furnace. When I went to measure everything and figure out what the best installation technique would ultimately be in my attic, I discovered that the supply plenum above my furnace and AC coil was not a rigid rectangular duct as I have often seen them depicted in my research. The supply plenum is made of some soft material (cardboard maybe?) wrapped in aluminum tape. Also to add on to that, my furnace has a return supply duct going into each side so there is no true return plenum. This was installed 6 years ago. Is this normal?

Shot of side of furnace, AC coil, and the "faux" supply plenum


Faux plenum only


Trane XV80 Furnace with return duct going in. The other side also has a return duct.

Outside of that, if this is normal practice where would I install a bypass or powered humidifier? I'll save my other questions around water supply for later.

Thanks.

 

[Guzzle]

Use an ice pick or a magnet to check if there is metal ductwork under that insulating wrap. Almost certainly there is.

If not, if you cut large pieces of cardboard or Masonite to exact size, a sheet metal shop can make you one or two pieces that cover the wrap & anchor to existing metal ductwork, to support the humidifier.
The flanges to fasten these can be on the sheets or you can use aluminum extruded angles.

Cut the humidifier holes first.

 

[Sparky617]

It is likely a fiberglass duct. I had them in my first house. It is a foil wrapped rigid fiberglass board about 1" thick.

https://www.jm.com/en/hvac/duct-board/

 

[thapranksta]

Yeah, it's definitely not metal. I took a magnet which stuck to everything nearby made of metal but not the supply plenum. I would think it is fiberglass board as suggested.