ductwork in attics
heat escape duct work attic unconditioned

If a furnace is installed in the attic it should be surrounded by a completely air tight / fireproof  room as well as insulated. It is essential to create a tight well insulated room to avoid heat loss resulting in the underside of the sheathing temperature warmer than outside air. 

This warm sheathing / roof deck will melt snow prematurely resulting in icicles and potential for ice damming like we saw in the February 2021. Unprecedented amounts of snowfall triggered excessive snow melt on poorly insulated homes across Chicagoland triggering a underserved response of ice dam removal services over a 5 day period.

Ductwork installed in attics in humid climates which are accessible can benefit from closed cell spray foam. 1” is all the protection needed to meet ductwork insulation standards while also providing a more important role of air tight seams and a moisture barrier. This technique is helpful in reducing energy consumption.

Ductwork in Attics

Attics are traditionally unconditioned ( meaning not intentionally heated or cooled by electric or natural gas.)This Means The Ductwork in the Attic is sending paid conditioned air through a hostile environment. 

Well balanced active or passive attic ventilation  targets the same temperature as outside.  This prevents all sorts of issues such as icicles, mold and humidity in winter, and heat buildup from radiation for a sweltering hot upstairs  in the summer. 

The attic is not an ideal location for ductwork. Particularly heating. Ideally the heating source would be as low as possible in the room such as floor vents ( and the home such as the basement)  as the heat will rise naturally through the conditioned space. 

Whenever possible, it is best to avoid running ductwork through the unconditioned attic. Here is a few reasons 

  • opposing temperature is attempting to be delivered to a conditioned space through the hostile unconditioned attic creating unintentional heat escape
  • Ductwork (particularly the return) negative pressure sucks attic air dust pollutants, etc through the seams and gaps 
  • Ductwork temperature differences can create excess humidity and moisture


Ductwork in an unconditioned space should be in direct contact with the ceiling / attic floor.

Duct insulation should include a vapor barrier.

Ductwork should be secure.

Ductwork should prevent any air escape at the seams.

Ductwork should be covered with 1" of closed cell spray foam if accessible to seal all seams, provide a thermal barrier, as well as a moisture barrier.

Sealing Ductwork Saves Energy, Saves Money Month After Month, Improves Efficiency, and Comfort.

After ducts are sealed they are buried deeply under loose fill cellulose insulation. This prevents additional thermal transfer effectively separating cool air in the summer and warm air in the winter from contact with opposing outdoor temps.

If the ductwork going through unconditioned spaces is inaccessible, we recommend a licensed experienced HVAC contractor seal the ductwork from the inside with an aerosol sealing procedure.

The cool air is running through a hot attic in the summertime. Average temperatures of the roof deck on the inside surface is 130-150 Degrees. 

Similarly, in the winter the unconditioned vented attic are similar to outside temperatures. Opposing heat supply is 120-130 but a large percentage of heat is lost by the surface temperature of the ductwork. 

Ductwork is leaky at the seams by default, so sealing all the duct connections and adding insulation will improve efficiency, lower heating and cooling costs, and make the home more comfortable.  


If a furnace room is installed in the attic it is imperative to have a complete pressure barrier and preferably a thermal barrier similar to R21 ( exterior walls recommended for region 5), as well as a fire barrier. This prevents unintentional heat escape in winter and a hostile hot room environment in summer. 


Fill out the contact form on our main page to set an appointment for a free assessment of your attic, and keep in mind the variables of ductwork in attics, making an informed decision for improvement has never been so easy. 

Attic Complete
Green Attic

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