Shafts (e.g., Duct, Flue)

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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 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 1 - Spray foam air seals the boot to the ceiling Reference

2. Add insulation to the specified attic insulation depth.

Caulk air seals the boot to the ceiling

Figure 2 - Caulk air seals the boot to the ceiling  Reference

 

 

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 3 - Install wood framing cross pieces in the attic rafter bays on each side of the duct chase Reference

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 4 - Seal all wood framing joints surrounding the chase with sealant and lay a bead of sealant along top edge of chase framing Reference

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 5 - Cut plywood, rigid foam, or drywall to fit around duct. Fasten to framing and caulk edges and seams Reference

 

 

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 6 - Install wood framing cross pieces in the attic rafter bays on each side of the duct chase Reference

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 7 - Seal all wood framing joints surrounding the chase with sealant and lay a bead of sealant on top edge of chase framing Reference

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 8 - Cut plywood, rigid foam, or drywall to fit around duct. Fasten to framing and caulk edges and seams Reference

 

 

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 9 - Ceiling opening for chimney chase Reference

Cut sheet metal to cover the chase opening. Fasten sheet metal to framing and seal edges with fire-rated caulk.

Figure 10 - Cut sheet metal to cover the chase opening. Fasten sheet metal to framing and seal edges with fire-rated caulk Reference

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 11 - Form a sheet metal shield around the flue pipe Reference

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

Fill area around shield with insulation

Figure 12 - Fill area around shield with insulation Reference

 

 

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 13 - Ceiling opening for chimney pipe chase Reference

Cut sheet metal to fit around flue, fasten to wood blocking, and seal with caulk

Figure 14 - Cut wood blocking to frame in flue pipe Reference

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 15 - Cut sheet metal to fit around flue, fasten to wood blocking, and seal with caulk Reference

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 16 - Form sheet metal shield around pipe keeping 3-inch clearance Reference

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

Cover shield with insulation to required attic insulation height

Figure 17 - Cover shield with insulation to required attic insulation height Reference

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.

Scope

Duct/flue shaft

Air Sealing

  1. Install a continuous rigid air barrier material to separate the exterior from the conditioned space.
  2. Using a saw or drill, cleanly cut all penetrating holes no more than 1 inch larger in diameter than the penetrating object to allow for proper air sealing.
  3. Seal all gaps, and holes to unconditioned space with caulk or foam.  Fibrous insulation is not an air barrier and cannot be used for sealing gaps.
  4. Use high temperature caulking along with flashing or UL-rated collars. Install them continuously around all combustion flues while maintaining proper clearance from combustion materials.

ENERGY STAR Notes:

ENERGY STAR recommends using a rigid air barrier, but it is not a requirement.

Training

Right and Wrong Images

Presentations

None Available

CAD Images

Compliance

ENERGY STAR Version 3, (Rev. 07)

Thermal Enclosure Checklist, Air Sealing. Penetrations to unconditioned space fully sealed with solid blocking or flashing as needed and gaps sealed with caulk or foam.

DOE Challenge Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.

2009 IECC

Section 402.4.1 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.*

2009 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 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.* Table R402.4.1.1, 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.*

2012 IRC

Table N1102.4.1.1 Air Barrier and Insulation Installation, Shafts/penetrations: Duct shafts, utility penetrations, and flue shafts opening to exterior or unconditioned space are air sealed.* Table N1102.4.1.1, 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.*

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

More Info.

Case Studies

  1. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: December 2009

    Case study about new home construction in the hot-humid climate, optimizing thermal enclosure, HVAC, water heating and lighting measures.

  2. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: October 2012

    Case study about one builder's conversion to high-performance building in the hot-humid regions of the Atlantic seaboard.

  3. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: April 2012

    Case study about a production builder that achieves HERS scores of 60, compared to 85 for other builders that build new homes to Florida code.

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): Lstiburek
    Organization(s): BSC
    Publication Date: January 2010

    Fact sheet providing detailed information about air sealing attics.

  4. Author(s): EPA
    Organization(s): EPA
    Publication Date: October 2011

    Guide describing details that serve as a visual reference for each of the line items in the Thermal Enclosure System Rater Checklist.

Last Updated: 08/15/2013

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