Why is duct insulation usually foil covered?

[Stephen Tashiro]

I'm curious why duct insulation is usually foil covered. My conjectured explanation is that ductwork is often left exposed (e.g. in basements or shops) so some sort of covering for the fiberglass is needed. I suppose foil helps keep ducts air tight, but if ducts are properly sealed, foil should not be needed for that purpose.

[Frank Pratt]

Foil faced insulation is used on the outside of ducts to provide a vapor barrier and protect the fiberglass. The unfaced stuff is what's used to line the inside of ducts. Inside there is no need for the protection or vapor barrier. And the unfaced insulation deadens sound travel through the ducts.

[Edward Weber]

It's a radiant heat barrier

[Jason Roehl]

It’s a vapor barrier. Flex-duct often has a polymer wrap, not foil. If the dew point is 65°F, and your A/C kicks out 55°F air, you’ll get condensation on the ductwork. If the fiberglass insulation is unfaced, it will become saturated, and perhaps start a science experiment…

[George Yetka]

Everything above and it is also a material that gives the insulation a good amount of protection for the weight it adds. Also also the foil wont break down and fall apart. Commercially if it is a wrap then it is usually foil. but high use areas will be hard board often with a pvc jacket applied on top.

[Brian Elfert]

I would think the covering is there to encapsulate the fiberglass. You don't often see loose fiberglass insulation unless it was something like a DIY job of insulating a water heater with unfaced fiberglass insulation. Luckily, my current house has zero flex duct. I had forced air installed when I bought the house in 2014 and all of the duct work is metal. My previous house I had built in 2001 and for some reason the returns on the first floor had small lengths of flex duct before it transitioned to metal.

[Jason Roehl]

Everything above and it is also a material that gives the insulation a good amount of protection for the weight it adds. Also also the foil wont break down and fall apart. Commercially if it is a wrap then it is usually foil. but high use areas will be hard board often with a pvc jacket applied on top.

The duct insulation I have at work, installed by union insulators is paper-faced, or plastic-faced. It's a heavy, white-coated paper that can be easily painted, and anywhere they have exposed fiberglass (such as end cuts at a change in pipe size), it gets "painted" with a heavy mastic to seal it, and they have a self-adhesive tape for butt joints. Elbows typically get a pre-made PVC elbow that has a snap-together seam, and gets stuffed with loose fiberglass batting.