Air Sealing Duct and Flue Shafts

    Scope Images
    Image
    Air seal around all duct shafts and flues installed through ceilings, walls, or flooring to keep conditioned air from leaking into unconditioned space.
    Scope

    Air seal around all duct shafts and flues installed through ceilings, walls, and flooring to keep conditioned air from leaking into unconditioned space. 

    • To air seal around duct shafts, use caulk, canned foam, or rigid air barrier material cut to fit and caulked or foamed in place around the duct shafts. 
    • To air seal around combustion flues, use fire-rated caulk and UL-rated collars or sheet metal cut to fit and sealed with fire-rated caulk, while maintaining proper clearance between flue and combustible materials.
    • Construct a sheet metal shield around combustion flues to maintain 3 inches of clearance between the flue and attic insulation. Make the shield 4 inches higher than the expected insulation height.

    See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

    Description

    Install air barriers around duct and flue shafts in the attic floor to prevent air leakage between the living space and the attic and to allow full insulation levels to be installed around the duct or flue. Air barrier material around duct shafts can include thin sheet goods such as rigid insulation, dry wall, OSB, or plywood. Air barriers around flue shafts should be made of a heat-resistant material such as sheet metal. These materials may be installed by insulators, framers, or drywallers. This task should be included in the contract for the appropriate trade depending on the workflow at specific job sites.

    Air barrier effectiveness is measured at the whole-house level. High-performance branding programs and the IECC code require that builders meet specified infiltration rates at the whole-house level. See the “compliance” tab for these specified infiltration rates. 

    How to Seal a Rigid Duct Chase 

    1. Install wood framing cross pieces in the attic rafter bays on each side of the duct chase, if needed.

    Install wood framing cross pieces in the attic rafter bays on each side of the duct chase.
    Figure 1. Install wood framing cross pieces in the attic rafter bays on each side of the duct chase (Courtesy of BSC). 

     

    2. Seal all wood framing joints surrounding the chase with sealant. Lay a generous continuous bead of sealant along the top edge of the chase framing.

    Seal all wood framing joints surrounding the chase with sealant and lay a bead of sealant along top edge of chase framing.
    Figure 2. Seal all wood framing joints surrounding the chase with sealant and lay a bead of sealant along top edge of chase framing (Courtesy of BSC). 

     

    3. Measure and cut the air-blocking material (plywood, rigid foam, or drywall) to fit around the duct. Place the blocking material on the framing leaving a 1/4-in. gap between the rigid duct and the material. Fasten the material in place with nails or screws. Seal the material to the duct with sealant. Also seal any joints in the blocking material. Cover the material with insulation to the specified attic insulation depth.

    Cut plywood, rigid foam, or drywall to fit around duct. Fasten to framing and caulk edges and seams.
    Figure 3. Cut plywood, rigid foam, or drywall to fit around duct. Fasten to framing and caulk edges and seams (Courtesy of BSC). 

     

    How to Air Seal a Flex Duct Chase

    1. Install wood framing cross pieces in the attic rafter bays on each side of the duct chase, if needed.

    Install wood framing cross pieces in the attic rafter bays on each side of the duct chase.
    Figure 4. Install wood framing cross pieces in the attic rafter bays on each side of the duct chase (Courtesy of BSC). 

     

    2. Seal all wood framing joints surrounding the chase with sealant. Lay a generous continuous bead of sealant along the top edge of the chase framing.

    Seal all wood framing joints surrounding the chase with sealant and lay a bead of sealant along top edge of chase framing.
    Figure 5. Seal all wood framing joints surrounding the chase with sealant and lay a bead of sealant on top edge of chase framing (Courtesy of BSC). 

     

    3. Measure and cut the air-blocking material (plywood, rigid foam, or drywall) to fit around the duct. Cut the material into two halves and then cut half circles in each to encompass the flex duct. Place the blocking material on the framing and in contact with the duct. Fasten the material with nails or screws. Seal the blocking material to the duct with sealant. Also seal the joints in the blocking material. Cover the blocking material with insulation to the required levels.

    Cut plywood, rigid foam, or drywall to fit around duct. Fasten to framing and caulk edges and seams.
    Figure 6. Cut plywood, rigid foam, or drywall to fit around duct. Fasten to framing and caulk edges and seams (Courtesy of BSC). 

     

    How to Air Seal a Metal Chimney or Flue Vent Pipe – Option 1 – Air Seal at the Bottom of the Framing

    1. Cut two pieces of sheet metal to cover the chase opening. Allow 1 inch of overlap. Fasten the sheet metal to the framing and seal all edges and seams with fire-rated caulk.

    Ceiling opening for chimney chase.
    Figure 7. Ceiling opening for chimney chase (Courtesy of BSC). 

     

    Cut sheet metal to cover the chase opening. Fasten sheet metal to framing and seal edges with fire-rated caulk.
    Figure 8. Cut sheet metal to cover the chase opening. Fasten sheet metal to framing and seal edges with fire-rated caulk (Courtesy of BSC). 

     

    2. Use sheet metal to make a shield that will wrap around the pipe with a 3-inch clearance. Fold in the tabs at the top and every other tab at the bottom to maintain a 3-inch clearance. With tabs folded, the shield should be 4 inches taller than the finished insulation level. Seal the edges together with fire-rated caulk.

    Form a sheet metal shield around the flue pipe.
    Figure 9. Form a sheet metal shield around the flue pipe (Courtesy of BSC). 

     

    3. Cover the shield with insulation to the required height. The insulation should cover the rafters.

    Fill area around shield with insulation.
    Figure 10. Fill area around shield with insulation (Courtesy of BSC). 

     

    How to Air Seal a Metal Chimney or Flue Vent Pipe – Option 2 – Air Seal at the Top of the Framing

    1. Cut two pieces of framing lumber equal in height to the ceiling joists. Fasten wood cross pieces to joists keeping at least 3 inches of clearance to the pipe. Caulk this wood blocking to the framing.

    Ceiling opening for chimney pipe chase.
    Figure 11. Ceiling opening for chimney pipe chase (Courtesy of BSC). 

     

    Cut sheet metal to fit around flue, fasten to wood blocking, and seal with caulk.
    Figure 12. Cut wood blocking to frame in flue pipe (Courtesy of BSC). 

     

    2. Cut two pieces of sheet metal or aluminum flashing to fit around the chimney pipe with 1 inch of overlap. Fasten the sheet metal to the framing and seal all edges and seams with fire-rated caulk.

    Form sheet metal shield around pipe keeping 3-inch clearance.
    Figure 13. Cut sheet metal to fit around flue, fasten to wood blocking, and seal with caulk (Courtesy of BSC). 

     

    3. Use sheet metal to make a shield that will wrap around the pipe with a 3-inch clearance. Fold in tabs at the top and every other tab at the bottom to maintain a 3-inch clearance. With tabs folded, the shield should be 4 inches taller than finished insulation level. Seal the edges together with fire-rated caulk.

    Form sheet metal shield around pipe keeping 3-inch clearance.
    Figure 14. Form sheet metal shield around pipe keeping 3-inch clearance (Courtesy of BSC). 

     

    4. Cover the shield with insulation to the required height.

    Cover shield with insulation to required attic insulation height.
    Figure 15. Cover shield with insulation to required attic insulation height (Courtesy of BSC). 

     

    How to Seal a Duct Boot to the Ceiling

    1. Seal all sides of the duct boot to the gypsum board with spray foam or caulk. Apply mastic or metal tape to all duct seams and joints.

    Spray foam air seals the boot to the ceiling.
    Figure 16. Spray foam air seals the boot to the ceiling (Courtesy of BSC). 

     

    2. Add insulation to the specified attic insulation depth.

    Caulk air seals the boot to the ceiling.
    Figure 17. Caulk air seals the boot to the ceiling (Courtesy of BSC). 

     

    Ensuring Success

    Blower door testing, which is conducted as part of the whole-house energy performance test-out, may help indicate whether duct and flue openings to unconditioned space (such as an attic) have been successfully sealed. An infrared camera can be used in conjunction with the blower door testing to detect air leakage and heat loss at the duct and flue shaft openings, if a sufficient temperature difference exists between the unconditioned and the conditioned space of the house. An experienced technician can also check for air leaks beneath the knee walls with a smoke pencil or by feeling with the back of the hand.

    Climate

    No climate specific information applies.

    Right and Wrong Images
    Image
    Wrong – Chase not capped
    Wrong – Chase not capped
    Image
    Right – Chase capped with rigid air barrier and duct work penetrations properly sealed
    Right – Chase capped with rigid air barrier and duct work penetrations properly sealed
    Image
    Wrong – Penetration hole is larger than duct and not sealed
    Wrong – Penetration hole is larger than duct and not sealed
    Image
    Right – Neatly cut and sealed penetration
    Right – Neatly cut and sealed penetration
    Image
    Wrong – Fibrous insulation does not air seal
    Wrong – Fibrous insulation does not air seal
    Image
    Right – Penetrations have been neatly cut and properly sealed with foam
    Right – Penetrations have been neatly cut and properly sealed with foam
    Image
    Wrong – Vent sleeve not completely sealed
    Wrong – Vent sleeve not completely sealed
    Image
    Wrong - Penetrations in walls for ducts should be air sealed to reduce air leakage.
    Wrong - Penetrations in walls for ducts should be air sealed to reduce air leakage.
    Image
    Right- This flue pipe has been air sealed with sheet metal and fire-rated caulk and an insulation dam has been constructed to keep insulation from touching the hot flue pipe
    Right- This flue pipe has been air sealed with sheet metal and fire-rated caulk and an insulation dam has been constructed to keep insulation from touching the hot flue pipe
    Image
    Right – Several potential sources of air leakage into the attic have been air sealed; canned spray foam was used to seal around duct boots, along seams in the drywall, and along top plates.
    Right – Several potential sources of air leakage into the attic have been air sealed; canned spray foam was used to seal around duct boots, along seams in the drywall, and along top plates.
    Videos
    CAD
    CAD Files
    Air seal at chase walls - plan
    Air seal at chase walls - plan
    Download: DWG PDF
    Air seal at duct boot
    Air seal at duct boot
    Download: DWG PDF

    Compliance

    The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

     

    ENERGY STAR Certified Homes, Version 3/3.1 (Rev. 09)

    National Rater Field Checklist

    Thermal Enclosure System.
    4. Air Sealing (Unless otherwise noted below, “sealed” indicates the use of caulk, foam, or equivalent material). 
    4.1 Ducts, flues, shafts, plumbing, piping, wiring, exhaust fans, & other penetrations to unconditioned space sealed, with blocking / flashing as needed. 

    Please see the ENERGY STAR Certified Homes Implementation Timeline for the program version and revision currently applicable in in your state.

     

    DOE Zero Energy Ready Home (Revision 07)

    Exhibit 1 Mandatory Requirements.
    Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.

     

    2009 International Energy Conservation Code (IECC)

    Section 402.4.1. The building thermal envelope shall be durably sealed to limit infiltration...including all joints, seams, and penetrations. 

    2012, 2015, and 2018 IECC

    Table R402.4.1.1 Air Barrier and Insulation Installation, Shafts/penetrations: Duct shafts, utility penetrations, and flue shafts opening to exterior or unconditioned space are air sealed.

    2021 IECC

    Table R402.4.1.1 Shafts, Penetrations: Air Barrier and Insulation Installation, Shafts/penetrations: Duct shafts, utility penetrations, and flue shafts opening to exterior or unconditioned space are air sealed. Insulation shall be fitted around shafts and penetrations to maintain the required R-value.

    Retrofit:  2009, 2012, 2015, 2018,  and 2021 IECC

    Section R101.4.3 (in 2009 and 2012). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

    Chapter 5 (in 2015, 2018, 2021). The provisions of this chapter shall control the alteration, repair, addition, and change of occupancy of existing buildings and structures.

     

    2009 International Residential Code (IRC) 

    Section N1102.4.1 Building thermal envelope. Joints (including rim joist junctions), attic access openings, penetrations, and all other such openings in the building envelope that are sources of air leakage are sealed with caulk, gasketed, weatherstripped or otherwise sealed with an air barrier material, suitable film or solid material.

    2012, 2015, and 2018 IRC

    Table N1102.4.1.1 Shafts, Penetrations: Air Barrier and Insulation Installation, Shafts/penetrations: Duct shafts, utility penetrations, and flue shafts opening to exterior or unconditioned space are air sealed. General Requirements: Air barrier and thermal barrier: A continuous air barrier is installed in the building envelope including rim joists and exposed edges of insulation. Breaks or joints in the air barrier are sealed. Air permeable insulation is not used as a sealing material. 

    2021 IRC

    Table N1102.4.1.1 Shafts, Penetrations: Air Barrier and Insulation Installation, Shafts/penetrations: Duct shafts, utility penetrations, and flue shafts opening to exterior or unconditioned space are air sealed. Insulation shall be fitted around shafts and penetrations to maintain the required R-value.

    Retrofit:  2009, 2012, 2015, 2018,  and 2021 IRC

    Section R102.7.1 Additions, alterations, or repairs. Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with the requirements of this code, unless otherwise stated. (See code for additional requirements and exceptions.)

    Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

    Existing Homes

    SCOPE

    In existing homes, air seal and insulate around new or existing duct and flue shafts to minimize air leakage to and from unconditioned attics.  

    • Remove any existing insulation from around the duct or flue shaft. Inspect the attic floor around the duct and flue shafts for air and water leaks. Repair or replace any damaged materials. Add air sealing and/or insulation as needed.
    • See the Scope and Description tab for additional instructions.

    For more information on conditions that may be encountered when working in existing attics, see the Pre-Retrofit Assessment of Attics, Ceilings, and Roofs. Also see the U.S. Department of Energy’s Standard Work Specifications (SWS) guidance on air sealing chases and ceiling penetrations.

    DESCRIPTION

    1. Inspect around ducts and flue shafts for water damage and air leaks. Repair or replace any damaged materials. Add air sealing and/or insulation, as needed.
    2. See the Description tab for additional installation information.

    COMPLIANCE

    Alterations

    2009 IECC and 2009 IRC, 2012 IECC and 2012 IRC 

    2012 IECC, Section R101.4.3 / 2012 IRC N1101.3 and 2009 IECC 101.4.3 / 2009 IRC N1101.4.3 Alterations – General.  Alterations to an existing building or portion of a building should comply with the provisions of the code as they relate to new construction without requiring unaltered portion(s) of the existing building to comply with this code.

    2015 IECC and 2015 IRC, 2018 IECC and 2018 IRC

    2015 IECC/2015 IRC, Section R501.1.1/N1107.1.1 Existing Buildings – General.  Alterations to an existing building or portion of a building should comply with Sections R502/N1108, R503/N1109, or R504/N1110.  Unaltered portions of the existing building are not required to comply.

    R503.1/N1109.1 Alterations. General.  Alterations to any building or structure should comply with the requirements of the code for new construction.  Alterations should not negatively impact conformance of a building or structure to the provisions of this code; that is, code conformance should be the same as existed for the building or structure prior to the alteration.  Alterations should not create an unsafe or hazardous condition or overload existing building systems.  Alterations should be such that the altered building or structure uses no more energy than the existing building or structure prior to the alteration.

    See Compliance tab. 

    More Info.

    Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

    Case Studies
    References and Resources*
    Author(s)
    U.S. Environmental Protection Agency
    Organization(s)
    EPA
    Publication Date
    Description
    Guide describing details that serve as a visual reference for each of the line items in the Thermal Enclosure System Rater Checklist.
    *For non-dated media, such as websites, the date listed is the date accessed.
    Contributors to this Guide

    The following authors and organizations contributed to the content in this Guide.

    Building Science Measures
    Building Science-to-Sales Translator

    Tight Air Sealed Home = Comprehensive Draft Protection

    Image(s)
    Technical Description

    Poorly air-sealed homes are less comfortable and cost more to maintain because they provide a pathway for drafts, cold spots, moisture, and insects into the home. Comprehensive draft protection includes a continuous air barrier around the whole house along with caulking and sealing in all holes and cracks. This includes around wiring, plumbing, ducts, and flues; where wall framing meets flooring; around windows; where drywall meets top plates and sill plates; where rim joists meet foundation walls and subfloors; etc. Spray foam insulation can be used at rim joists, floors above unconditioned space, and in attics to insulate and air seal at the same time.

    Comprehensive Draft Protection
    Sales Message

    Comprehensive draft protection minimizes air flow that can undermine a complete high-performance insulation system. What this means for you is less wasted energy along with enhanced comfort, health, quiet, and durability. Wouldn’t you agree it would be a shame to only get a partial return on your investment in advanced insulation?

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