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Covering Ductwork With Insulation.  Can You Do It?

ductwork in attic

This flexible ductwork was not covered with Closed-Cell Spray Foam before insulated was added.  This Presents A Potential Mold Hazard.

HVAC Ductwork is typically visible in your attic (especially flexible ductwork).  Today’s flexible ductwork is covered with a shiny metallic cover — this is its vapor-barrier.  The shiny vapor-barrier will protect the ductwork from sweating as long as it’s exposed to the surrounding air.

The reason ductwork is exposed is if it sweats in the summer (when a/c is on) the surrounding air will dry the sweat.   If ductwork is covered in insulation without an additional vapor barrier, there is insufficient air flow inside the insulation to dry sweating ductwork.

Flexible ductwork often sits above the attic floor.

correctly installed flexible ductwork

Exposed ductwork is great for mitigating a potential mold issue, but not great for HVAC System efficiency and performance.  A Rule Of Thumb: For every 10 further from the furnace, the a/c temperature at the duct temperature will be 1-2 degrees higher.  Why?  There is an additional 10 feet of ductwork in a blazing hot attic.

Watch The Movie!  View this YouTube Video Showing The Steps In Insulating Ductwork.  YouTube — Insulating Attic Ductwork

The Good News:  You can cover flexible or rigid ductwork with blown insulation but ONLY once it is enclosed with Closed-Cell Spray Foam First.


Spray Foam also acts as a Vapor-Barrier.  Covering ductwork with insulation is a great idea, if it’s done properly.   If you cover ductwork with attic insulation (after adding Closed-Cell Spray Foam) so the ductwork will not sweat in summer when the A/C is running.

The graphic below shows how much insulation is required with the spray foam surrounding the ductwork (see yellow in diagram).

insulation level diagram for flexible ductwork

These are the components in flexible ductwork:

photo of flexible ductwork with all components identified

NOTE: The Vapor-Barrier attached to flexible ductwork is designed for ductwork which is surrounded by air.


This article gives insight into why a/c runs all the time during quite hot weather.  This may be due to ductwork having no more than R-8 Insualtion (which is typically required by Building Code).  This article describes covering ductwork with Closed-Cell Spray Foam, then adding attic insulation over it.  The Spray-Foam acts as a vapor barrier.

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