Insulation Levels for Ducts in Unconditioned Spaces

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Description

Ideally ducts should be located in conditioned space, such as within a dropped ceiling, between floors, in an insulated basement or crawlspace, or in an unvented attic that is insulated along the roof line.

Whenever ductwork is located in unconditioned spaces, thermal insulation with a vapor barrier is a must to prevent unnecessary heat gain or heat loss through the duct walls and to prevent condensation from forming on the ducts themselves. Supply ducts located in unconditioned space should be insulated to at least R-8; all other ducts should be insulated to at least R-6.  For the insulation to work properly, it must be fully aligned and in contact with the ducting.  A typical vented attic with a dark shingle roof can reach summer temperatures of 140ºF. At the same time, the dew point temperature in the attic will be about the same as it is outdoors. In humid climates, "duct sweating" can become a significant problem if the ducts’ thermal and vapor barriers are not properly aligned along the entire length of the ducts, including at all duct connection points.

How to Install Ducts with Adequate Insulation Levels in Unconditioned Space:

  1. Determine duct layout at the design stage to allow for a compact duct layout with open duct chases or routes that provide adequate space and support so that ducts can be routed with no compressions and with as few bends as possible. 
  2. For flex ducts – select R-8 or higher flex duct for supply ducts; select R-6 or higher flex duct for return ducts and other ducts. Install flex ducts using the installation guidance provided in the following guides:  Sealed and Insulated Flex Ducts, No Kinks or Sharp Bends in Flex Duct Installation, No Excessive Coiled or Looped Flex DuctsSufficient Cavity Space for Flex Ducts, and Support at Intervals for Flex Ducts
  3. For metal ducts – select R-8 or higher insulation for supply ducts; select R-6 or higher insulation for return ducts and other ducts. Install metal ducts using the installation guidance provided in Sealed and Insulated Metal Ducts. See this guide or the insulation manufacturer’s recommendations for “stretch-out” guidance to ensure adequate insulation thickness.
  4. For fiberboard ducts – select R-8 or higher fiberboard for supply ducts; select R-6 or higher fiberboard for return ducts and other ducts. Install fiberboard ducts using the installation guidance provided in Sealed and Insulated Fiber Board Ducts.

Ensuring Success

After ducts are installed and before drywall is installed, the duct system should be visually inspected by a HERS rater to ensure that the ducts have the proper amount of insulation. Supply ducts located in unconditioned space should be insulated to at least R-8; all other ducts should be insulated to at least R-6.  The inspector should also confirm that the insulation is properly installed to fully cover the ducts along their entire lengths and that the insulation is not missing or compressed anywhere along the duct length.

Scope

Prescriptive Path: Supply ducts in unconditioned attic have insulation ≥ R-8. Performance Path: Supply ducts in unconditioned attic have insulation ≥ R-6

Duct Insulation and Air Sealing

  1. Install insulated ductwork, boxes, and boots in all unconditioned spaces to meet either the prescriptive or performance path.
  2. Install all ducts in unconditioned spaces without compressing the insulation.

Common unconditioned places include:

  • Basements
  • Vented Crawlspaces
  • Closed Crawlspaces
  • Bonus Room Attic Space

ENERGY STAR Notes:

  • Supply ducts in unconditioned attic have insulation >= R-8 (to comply with ENERGY STAR prescriptive path)
  • Supply ducts in unconditioned attic have insulation >= R-6 (to comply with ENERGY STAR performance path)
  • All other supply ducts and all return ducts in unconditioned space have insulation >= R-6

Training

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Presentations

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Compliance

ENERGY STAR Version 3, (Rev. 07)

HVAC System Quality Checklist, Duct Insulation. Supply ducts in unconditioned attic have insulation >= R-8 (prescriptive path). Supply ducts in unconditioned attic have insulation >= R-6 (performance path).  All other supply ducts and all return ducts in unconditioned space have insulation >= R-6.

DOE Challenge Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3. Ducts in vented attics must have a minimum of R-8 duct insulation with an additional minimum 1.5” of closed-cell spray foam insulation encapsulating the ducts; total duct leakage <= 3 CFM25 per 100 ft2 of conditioned floor area; and ductwork buried under at least 2” of blown-in insulation.

2009 IECC

Section 403.2.1 Insulation (Prescriptive). Supply ducts in attics are insulated to a minimum of R-8. All other ducts in unconditioned spaces or outside the building envelope are insulated to at least R-6.*

2009 IRC

Section N1103.2.1 Insulation. Supply ducts in attics are insulated to a minimum of R-8, and all other ducts are insulated to a minimum of R-6 except those ducts in conditioned space.*

2012 IECC

Section R403.2.1 Insulation (Prescriptive). Supply ducts in attics are insulated to a minimum of R-8. All other ducts in unconditioned spaces or outside the building envelope are insulated to at least R-6.*

2012 IRC

Section N1103.2.1 Insulation (Prescriptive). Supply ducts in attics are insulated to a minimum of R-8, and all other ducts are insulated to a minimum of R-6 except those ducts in conditioned space.*

*Due to copyright restrictions, exact code text is not provided.  For specific code text, refer to the applicable code.

More Info.

Case Studies

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References and Resources*

  1. Author(s): DOE
    Organization(s): DOE
    Publication Date: June 2013

    Standard requirements for DOE's Challenge Home national program certification.

  2. Author(s): EPA
    Organization(s): EPA
    Publication Date: June 2013

    Standard document containing the rater checklists and national program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 7).

  3. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: December 2013

    Standard outlining industry procedure for sizing residential duct systems.

Last Updated: 08/15/2013

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