Sealed and Insulated Flex Ducts

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Scope

Air seal and insulate flex ducts

Air seal and insulate flex ducts.

  • Design a compact duct layout with short, straight runs and minimal bends. Design the home to include ducts within conditioned space if possible. (The DOE Zero Energy Ready Home program requires that heating/cooling system supply ducts be installed within the conditioned space of the home.)
  • Install flex duct that is pre-insulated with a layer of fiberglass blanket insulation covered with a foil or plastic vapor barrier.
  • Use insulated duct for all supply and return ducts located in unconditioned space. Meet code or program minimum R-value requirements. See the Compliance tab for R-value minimum requirements specified in the IRC, IECC, DOE Zero Energy Ready Home program, and ENERGY STAR criteria.
  • Connect flex duct to boots, trunk lines, and return boxes by pulling flex duct over the raised bead on the metal connection collar and fastening with a nylon draw band pulled tight with a tensioning tool. Seal the duct-collar junctions with mastic.
  • Pull insulation wrapped in vapor barrier over the connections and seal seams with both UL-181-approved tape and mastic. Do not compress the insulation.
  • Ensure that all flex duct connections are air sealed for the following, regardless of whether they are located in conditioned or unconditioned space:
    • supply ducts (connections to boots, trunk ducts, other ducts, i.e., duct splicing); 
    • return ducts (connections to return box); 
    • ventilation ducting (connections to exhaust fans, for ERV/HRV dedicated ducts)
    • pressure balancing (connections for jump ducts, dedicated returns).
  • Ensure that insulation completely covers ducts and connections without gaps, voids, or compressions.
  • Consider insulating all metal ductwork located in conditioned space, including supply and return ducts, duct boots, and exhaust ducts, to minimize condensation.

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

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. If ducts are located in an unconditioned space, such as a vented attic or vented crawlspace, they should be sealed and insulated to prevent heat loss due to air leaks and conduction and to provide some protection against harsh conditions.

Flex duct consists of a plastic inner liner attached to a metal coil that comes already covered with a layer of fiberglass blanket insulation, which is covered by foil or plastic vapor barrier. It is typically available in R-4, R-6, and R-8. The 2009 IECC (Section 403.2) requires that supply ducts located in unconditioned space be insulated to at least R-8 (return ducts can be insulated to R-6).

Insulation

Whenever ductwork is located in unconditioned spaces, thermal insulation with a vapor barrier is a must to prevent unnecessary heat gain or loss through the duct walls and to prevent condensation from forming on the ducts themselves. For the insulation to work properly, it must be fully aligned and in contact with the walls of the duct system. 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 duct’s thermal and vapor barriers are not properly aligned along the entire length of the duct. Because the insulation is an integral part of the duct with flex ducts, insulation alignment is not an issue along the duct length but can be a problem at connections, if the insulation layers are not properly connected.

Sealing

Duct leakage is a double hit on the utility bill: 1) duct leaks are an uncontrolled loss of conditioned air to the outdoors and 2) duct leakage drives building infiltration. For example, if a home had a 2.5-ton (30,000 BTU/H) cooling system moving 1,000 CFM (cubic feet per minute) of air and the ducts had 10% leakage (which is typical in code-built homes), the leakage rate would be 100 CFM. Each cubic foot of air carries with it 30 BTUs/H, so 3,000 BTUs of conditioned air would be lost to the outdoors each hour.

Duct leakage is an infiltration driver; it can negatively or positively pressurize the house depending on where the ducts are leaking, pulling outside air in through cracks in the building envelope or pushing conditioned air out. If the duct leakage is in the supply-side ducts, the house will be negatively pressurized compared to outdoors. If all the leakage is on the return side, the building will be positive with respect to outdoors. The field technician should understand this concept to help accurately detect any sources of duct leakage.

Unlike solid sheet metal and ductboard, flex duct is essentially seamless along its length and at bends so, as long as the ducting is not torn or punctured, the only place where air sealing is a concern is at connections. If connected properly, a correctly designed flex duct system can provide efficient air delivery for many years.

While there are several types of connections, all flex duct connections require similar practice to ensure proper air sealing, regardless of whether the flex duct is connected directly to the air handler box, to a trunk line, to a junction box connected to the trunk line, or to a supply register. All connections use a round metal connector called a collar, which is either purchased at a supply house or made in the HVAC contractors’ metal shop. A raised metal bead encircles the collar at the end (see Figure 1). The raised bead can be located anywhere from 2 inches to 1/2 inch from the end of the collar. When the flex duct is pulled over this bead and secured with a nylon draw band, the raised bead prevents the duct from slipping off the end of the collar.

Figure 1 shows three types of starting collars and two register boxes (also known as duct boots) with collars attached. Collar A attaches flex duct or round metal duct to a rectangular metal box such as the air handler box or a metal trunk line. Collar B is for attaching a flex duct to a rectangular ductboard box or trunk line. Collar C is for attaching round flex duct or round metal duct to a round trunk line. Images D and E show uninsulated and insulated register boxes that are purchased with the collars pre-attached. If uninsulated boxes are purchased, they can be insulated on site by wrapping them with R-8 blanket insulation. Notice that all five collars have the raised bead. This type of connection (with raised beads on each end) can also be used to splice two pieces of flex duct together, a practice used to save on materials costs rather than throwing away short pieces of duct.

Collars that are specifically made for flexible duct have a raised bead to prevent the duct from slipping off

Figure 1 - Collars that are specifically made for flexible duct have a raised bead to prevent the duct from slipping off.  Reference

After the inner liner and coil are secured, the insulation and outer liner are secured and the seam is made air tight with metal tape and mastic.  The metal tape should meet the requirements of the Underwriters Laboratory UL-181, UL-181A, or 181B (CEC 2005). Regular cloth-backed duct tape should not be used because it can dry out and fail quickly.  Mastic is a thick, gooey, non-hardening substance that is spread onto the duct seams with a paintbrush or a putty knife.

For more on flex duct installation, see No Kinks or Sharp Bends in Flex Duct Installation, No Excessive Coiled or Looped Flex Ducts, Sufficient Cavity Space for Flex Ducts, Support at Intervals for Flex Ducts, Building Cavities not Used as Supply or Return Ducts, and HVAC Ducts Shall Not Be Run in Exterior Walls.

For more on duct insulation levels see Insulation Levels for Ducts in Unconditioned Space.  Duct installation standards and installation guidance are also provided in Manual D Residential Duct Systems, 2009 Edition, published by the Air Conditioning Contractors of America (see Appendix 17 for a detailed discussion of duct installation) and The Flexible Duct Performance Standards (Fifth Edition) published by the Air Diffusion Council (ADC).

How to Seal and Insulate Flex Duct at a Collar Connection

   1.  Pull the end of the flex duct over the end of the collar, pulling the flex duct core at least 2 inches past the raised bead.
 
   2.  Push back the outer liner and insulation.

To attach the flex duct to a main trunk duct or any other connection, the flex duct is pulled over the connecting collar at least 2 inches past the raised bead, then the insulation is pulled back.

Figure 2 - To attach the flex duct to a main trunk duct or any other connection, the flex duct is pulled over the connecting collar at least 2 inches past the raised bead, then the insulation is pulled back.  Reference

   3.  Wrap the nylon draw band around the duct over the inner liner, above the raised bead.

A nylon draw band and tensioning tool are used to secure the inner coil of the pre-insulated flexible duct

Figure 3 - A nylon draw band and tensioning tool are used to secure the inner coil of the pre-insulated flexible duct.  Reference

   4.  Thread the draw band through its locking system and tighten the nylon draw band with a tool specifically designed for this use, commonly known as a tensioning tool.

A nylon draw band and tensioning tool are used to secure the inner liner of the pre-insulated flexible duct.

Figure 4 - A nylon draw band and tensioning tool are used to secure the inner liner of the pre-insulated flexible duct.  Reference

   5.   Seal the connection with mastic. Paint the mastic generously over the draw band, bridging from the inner liner to the collar.

After securing the inner coil, cover the draw band and the seam with a generous amount of mastic

Figure 5 - After securing the inner coil, cover the draw band and the seam with a generous amount of mastic.  Reference

   6.  Pull the insulation and outer liner of the flex duct up over the collar. Make sure the flex duct insulation comes in full contact with the insulation on the trunk line or fitting (such as a duct boot) that it is being attached to. Pull the outer liner of the duct up to the outer liner of the trunk line or fitting.

7.  Tape the outer liner in place with UL-181- approved foil tape. Do not use a draw band to secure the flex duct insulation. It will compress the insulation, which could create cold spots along the exterior of the duct and possible condensation issues.

Pull the insulation and outer liner of the flex duct over the collar to come in full contact with the liner and insulation of the trunk line or fitting and tape in place. 

Figure 6 - Pull the insulation and outer liner of the flex duct over the collar to come in full contact with the liner and insulation of the trunk line or fitting and tape in place.  Reference

UL-181 approved tape

Figure 7 - Use only Underwriters Laboratory UL-181 approved tape.  Reference

   8.  Paint mastic over the foil tape making sure that the mastic bridges across the foil tape from the trunk line or fitting to the duct.

The tape is covered with mastic to ensure an airtight seal between the duct and the fitting

Figure 8 - The tape is covered with mastic to ensure an airtight seal between the duct and the fitting.  Reference

How to Seal and Insulate Flex Duct at a Splice

   1.  Connect each end of the duct to a round sheet metal connecter of the correct diameter that is purchased or fabricated. If fabricated, it should be made with the machined raised bead at each end of the pipe to secure the connections.

Two pieces of flex duct are spliced together with a metal sleeve, nylon draw bands, mastic, metal tape, and more mastic

Figure 9 - Two pieces of flex duct can be spliced together with a metal sleeve. Pull the ducts over the raised beads at each end of the connector, fasten the inner coils with nylon draw bands, mastic the seams, then pull together the insulation and outer liners, tape the seams together, and cover the tape with more mastic.  Reference

   2.  Join each piece of flex duct to the connector and secure the inner liner using a draw band, as described in steps 2 through 6 above.
   3.  Pull the insulation together and ensure that it meets all the way around the pipe.
   4.  Pull together the exterior liner and attach with foil tape.
   5.  Seal the connection with mastic.

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 all connections are properly fastened and sealed, preferably with mastic, and completely insulated. The following connections should be inspected: the main supply trunk to the branch duct, the branch duct to the duct boots, duct splices, the return ducts to the return box, jump duct connections, and exhaust fan and ERV/HRV connections. HVAC ducts should be tested for air leakage and proper air flow with a duct blaster test. This test should be done before drywalling when any air leaks can still be accessed and sealed. The ducts should be inspected to confirm that the proper amount of insulation is installed (minimum R-8 for supply ducts and R-6 for all other ducts) and that the insulation is not compressed by tight strapping, by framing members or other obstacles, or by excessive bending.

Climate

No climate specific information applies.

Training

Right and Wrong Images

Presentations

None Available

Videos

  1. Sealed and Insulated Flex Ducts
    Publication Date: July, 2015
    Courtesy Of: Train2Build

    Video describing how to seal and insulate flex ducts.

CAD Images

None Available

Compliance

ENERGY STAR Certification Homes

ENERGY STAR Certified Homes (Version 3/3.1, Revision 08), Rater Field Checklist:

6. Duct Quality Installation 
6.3 All supply and return ducts in unconditioned space, including connections to trunk ducts, are insulated to ≥ R-635

35. Item 6.3 does not apply to ducts that are a part of local mechanical exhaust and exhaust-only whole-house ventilation systems. EPA recommends, but does not require, that all metal ductwork not encompassed by Section 6 (e.g., exhaust ducts, duct boots, ducts in conditioned space) also be insulated and that insulation be sealed to duct boots to prevent condensation.

Builders Responsibilities:  It is the exclusive responsibility of builders to ensure that each certified home is constructed to meet these requirements. While builders are not required to maintain documentation demonstrating compliance for each individual certified home, builders are required to develop a process to ensure compliance for each certified home (e.g., incorporate these requirements into the Scope of Work for relevant sub-contractors, require the site supervisor to inspect each home for these requirements, and / or sub-contract the verification of these requirements to a Rater). In the event that the EPA determines that a certified home was constructed without meeting these requirements, the home may be decertified. 

ENERGY STAR Revision 08 requirements are required for homes permitted starting 07/01/2016.

DOE Zero Energy Ready Home

Exhibit 1: Mandatory Requirements. 

The duct distribution system is located within the home’s thermal and air barrier boundary or optimized to achieve comparable performance.16

16 Exceptions and alternative compliance paths to locating 100% of forced-air ducts in home’s thermal and air barrier boundary are:

a. Up to 10’ of total duct length is permitted to be outside of the home’s thermal and air barrier boundary.

b. Ducts are located in an unvented attic, regardless of whether this space is conditioned with a supply register.

c. Ducts are located in a vented attic with all of the following characteristics:

i. In Moist climates (Zones 1A, 2A, 3A, 4A, 5A, 6A and 7A per 2012 IECC Figure R301.1) and Marine climates (all “C” Zones per 2012 IECC Figure R301.1), minimum R-8 duct insulation with an additional minimum 1.5” of closed-cell spray foam insulation encapsulating the ducts; duct leakage to outdoors ≤ 3 CFM25 per 100 ft2 of conditioned floor area (in addition to meeting total duct leakage requirements from Section 4.1 of the ENERGY STAR HVAC Rater checklist); and ductwork buried under at least 2” of blown-in insulation.

ii. In Dry climates (all “B” Zones per 2012 IECC Figure R301.1), minimum R-8 duct insulation; duct leakage to outdoors ≤ 3 CFM25 per 100 ft2 of conditioned floor area (in addition to meeting total duct leakage requirements from Section 4.1 of the ENERGY STAR HVAC Rater checklist); and ductwork buried under at least 3.5” of blown-in insulation.

Note that in either of these designs the HVAC equipment must still be located within the home’s thermal and air barrier boundary.

d. Jump ducts which do not directly deliver conditioned air from the HVAC unit may be located in attics if all joints, including boot-to-drywall, are fully air sealed with mastic or foam, and the jump duct is fully buried under the attic insulation.

e. Ducts are located within an unvented crawl space.

f. Ducts are located in a basement which is within the home’s thermal boundary.

g. Ductless HVAC system is used.

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.*

Section 403.2.2 Sealing (Mandatory). All joints and seams of air ducts, air handlers, filter boxes, and building cavities used as return ducts are substantially airtight by means of tapes, mastics, liquid sealants, gasketing or other approved closure systems.This section also specifies duct leakage testing*

2009 IRC

Section M1601.3 describes duct insulation materials requirements including flame and smoke spread criteria and specifications for measuring and identifying insulation R values.

Section M1601.4.1 Joints and seams. Tapes, mastics, and fasteners are rated UL 181A or UL 181B and are labeled according to the duct construction. Metal duct connections with equipment and/or fittings are mechanically fastened. Crimp joints for round metal ducts have a contact lap of at least 1 1/2 inches and are fastened with a minimum of three equally spaced sheet-metal screws. Exceptions: a) Joint and seams covered with spray polyurethane foam. b) Where a partially inaccessible duct connection exists, mechanical fasteners can be equally spaced on the exposed portion of the joint so as to prevent a hinge effect. c) continuously welded and locking-type longitudinal joints and seams on ducts operating at less than 2 in. w.g. (500 Pa).*

Section N1103.2 Ducts. 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.* Air seal and test as described in Section N1103.2.2. 

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.*

Section R403.2.2 Sealing (Mandatory). All joints and seams of air ducts, air handlers, and filter boxes are substantially airtight by means of tapes, mastics, liquid sealants, gasketing or other approved closure systems. This section also specifies duct leakage testing.

2012 IRC

Section M1601.3 describes duct insulation materials requirements including flame and smoke spread criteria and specifications for measuring and identifying insulation R values. 

Section M1601.4.1 Joints and seams. Tapes, mastics, and fasteners are rated UL 181A or UL 181B and are labeled according to the duct construction. Metal duct connections with equipment and/or fittings are mechanically fastened. Crimp joints for round metal ducts have a contact lap of at least 1 1/2 inches and are fastened with a minimum of three equally spaced sheet-metal screws. Exceptions: a) Joint and seams covered with spray polyurethane foam. b) Where a partially inaccessible duct connection exists, mechanical fasteners can be equally spaced on the exposed portion of the joint so as to prevent a hinge effect. c) continuously welded and locking-type longitudinal joints and seams on ducts operating at less than 2 in. w.g. (500 Pa).*

Section N1103.2 Ducts. 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.* Air seal and test as described in Section N1103.2.2.*

2015 IECC

Section R403.3.1 Insulation (Prescriptive). Supply and return ducts in attics are insulated to a minimum of R-8 when the diameter of the duct is ≥ 3 inches and R-6 when the diameter of the duct is < 3 inches. Supply and return in ducts in other parts of the building must be insulated to a minimum of R-6 when the diameter of the duct is ≥ 3 inches and R-4.2 when the diameter of the duct is < 3 inches. This requirement doesn’t apply to ducts located completely inside the building’s thermal envelope.*

Section R403.2.2 Sealing (Mandatory). All ducts, air handlers, and filter boxes are air sealed and joints and seams must comply with the International Mechanical Code or International Residential Code where applicable. Exceptions: Additional joint sealing is not required when the ducts are covered with air-impermeable spray foam. Ducts that have a static pressure of < 2 inches, continuously welded joints and seams, and locking type joints that are not the snap-lock or button-lock type do not need additional closure systems.*

Section R403.3.3 and R403.3.4 specify duct leakage testing.

2015 IRC

Section M1601.3 describes duct insulation materials requirements including flame and smoke spread criteria and specifications for measuring and identifying insulation R values. 

Section M1601.4.1 Joints and seams. Tapes, mastics, and fasteners are rated UL 181A or UL 181B and are labeled according to the duct construction. Metal duct connections with equipment and/or fittings are mechanically fastened. Crimp joints for round metal ducts have a contact lap of at least 1 1/2 inches and are fastened with a minimum of three equally spaced sheet-metal screws. Exceptions: a) Joint and seams covered with spray polyurethane foam. b) Where a partially inaccessible duct connection exists, mechanical fasteners can be equally spaced on the exposed portion of the joint so as to prevent a hinge effect. c) continuously welded and locking-type longitudinal joints and seams on ducts operating at less than 2 in. w.g. (500 Pa).*

Section N1103.2 Ducts are insulated and tested as specified in the 2015 IECC.

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

 

More Info.

Case Studies

None Available

References and Resources*

  1. 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.

  2. Author(s): DOE
    Organization(s): DOE
    Publication Date: May, 2015

    Standard requirements for DOE's Zero Energy Ready Home national program certification.

  3. Author(s): EPA
    Organization(s): EPA
    Publication Date: September, 2015

    Document outlining the program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 08).

  4. Author(s): Air Diffusion Council
    Organization(s): Air Diffusion Council
    Publication Date: January, 2010

    Standard providing a comprehensive approach to evaluating, selecting, specifying and installing flexible duct in HVAC systems.

None Available

Building Science-to-Sales Translator

High-R Duct Insulation =
Energy Saving Comfort Delivery System Insulation

Technical Description: 

Uninsulated ducts can experience significant heat loss or heat gain through the walls of the ducts, especially if the ducts are located in an uninsulated attic or crawlspace. Energy-saving comfort delivery system insulation protects the conditioned air flowing through the ducts from heat loss or heat gain when traveling from the comfort equipment to the living spaces of the home.

Alternate Terms

Energy Saving Comfort Delivery System Insulation
Sales Message
Energy saving comfort delivery system insulation reduces the heat gain and lost getting heating and cooling to each room. What this means to you is less wasted energy getting comfort to each room. Wouldn’t you agree it’s important that your comfort delivery system is insulated for optimum performance?
Last Updated: 03/14/2016

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