Lined Duct vs Wrapped Insulation

[Golestan]

Good Morning,

A primary air duct (55 F) is going into a non-conditioned space, and after about 50 ft. in this space the duct feeds a series of discharge grilles which blow the cold air in to the space. What is the preferred type of insulation for this duct. Internally lined or wrapped on the outside? What about vapor barrier? Locaton is in the south.

Thanks,

 

[atlas06]

For performance, think cellular glass insulation over ductwork for such a condition.

If this was not a hot/humid area, you'd go double wall with insulation in between the walls, you get the looks of the exposed duct and it will not sweat.

By the way, cellular glass does look good when well applied. Costs more but no headaches for your hot/humid application, guaranteed.

 

[dblyle]

If the duct is exposed in the space, typical codes allow you to insulate with 1" of fiberglass insulation (duct wrap). If it is above a drop ceiling, you'll need at least an R-6 wrap (typically 1-1/2").

Duct wrap versus internally lined duct just depends on the application and the engineer. Lined duct will be cheaper to install (only have to touch the duct once and don't have to wait on city inspections, etc); however, a lot of people now use duct wrap because it is easier to keep the duct clean. If the duct is exposed, lined duct looks better.

 

[AbbyNormal]

unconditioned like an attic, warehouse or outside exposed to weather?

Most important is to seal the metal ductwork.

Internal insulation gives the fringe benefit of sound and being easier to weather proof.

If it is not exposed to the elements have a look at kanuf kool duct system. I was impressed with it.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[EAJ82]

wouldn't an internally exposed duct (uninsulated on the inside) create more noise?

 

[dblyle]

Externally wrapped duct will transmit more noise. Typically, we allow for the contractor to internally line the first 10 foot of duct and then externally wrap the rest (assmuming the duct is inside).

Check your local codes for what is required in your area.

 

[Drazen]

1. Noise is selection criteria. Internal lining is preferred when you want to prevent noise transmission, especially lateral transmission. In your situation, unconditioned space doesn't seem important for this consideration. That means external insulation seems to be good choice.

2. Vapour barrier is mandatory for cold air ducts. Otherwise you will have inevitable humidity concentration on the outside of the duct as well as quick deterioration of insulation. If you select insulation specified as AC insulation (not heating systems insulation), it should contain vapor barrier on the outside.