HVAC Supply Plenum Mold?

[Midpack]

So during a routine furnace checkup today on our 8 year old system, I'm shown significant amounts of mold in the supply plenum just downstream of the furnace. The plenum is insulated on the inside, water condenses there, and mold ensues. The tech was not suggesting it was dangerous mold, but DW who has been suffering with allergies for the last several years latched on to that as a cause...could be, although DW's allergist has reported she's basically allergic to everything.

I would have thought the sheet metal plenum would have been insulated on the outside. The tech agreed, and when I asked why ours wasn't he said 'builders will use premade plenums and they are pre insulated on the inside - cause it's cheaper/faster.' I wouldnt' doubt that.

So they can replace the plenum about 4 feet with 6 drops, and insulate from the outside for $3100. Or add a scrubber to remove contaminants for $1600. While I'd like to go the cheaper route, I'd prefer a permanent fix over another system to maintain (scrubber).

Just wondered if anyone has been down this road before? So I may get another quote, and I will certainly do some research. From what I've read so far insulating ductwork from the outside is usually preferable, but that's not a unanimous consensus.

 

[wdwwildbill]

I live in New Orleans and that is a big issue we have here with so much humidity. I can tell you almost all plenums in our area are insulated on inside. Yes cheaper and easier but old school duct wrap is fading away.
What you see is most likely mildew and not bad mold. We use ultra violet lights ( also called scrubbers ) to clean up the plenums and also the evaporator coils. They defintely work but you are correct, It's another thing to maintain with changing the bulbs every 2 years at apx $ 200.00 Even with a new plenum you will have growth on evaporator and sometimes the blower as well. So UV's can be a good product to keep the growth down. $ 1600.00 does sound a bit pricey though.....