AC duct connection, not sealed.

[RICHARD TAN]

Hi Jerry,

Do the inside of the ac flex duct required to sealed with mastic and tape? In the old day I always see the inside of the duct sealed with mastic and tape? Has the code change? And expose fiberglass noted.

[Jerry Peck - Codeman]

Hi Richard,

This has been the code for many years, this is from the 2004 FBC, Residential, it is also in the energy efficiency code chapter. There are a couple of methods to chose from. (underlining and bold are mine)
- 603.5 Flexible air ducts and flexible air connectors.
- - Flexible air ducts, both metallic and nonmetallic, shall comply with Sections 603.5.1, 603.5.1.1 and 603.5.3 through 603.5.5. Flexible air connectors, both metallic and nonmetallic, shall comply with Sections 603.5.2 through 603.5.5.
- - 603.5.6 Flexible air duct systems, nonmetal.
- - - Flexible nonmetal ducts shall be joined to all other air distribution system components by either terminal or intermediate fittings. All duct collar fittings shall have a minimum 5/ 8 inch (.63 mm) integral flange for sealing to other components and a minimum 3-inch (76 mm) shaft for insertion into the inner duct core.
Flexible ducts having porous inner cores shall not be used.
- - - - Exception: Ducts having a nonporous liner between the porous inner core and the outer jacket. Fastening and sealing requirements shall be applied to such intermediate liners.
All joints of flexible ducts to fittings and fittings to other air distribution system components shall be mechanically attached and sealed as specified in Sections 603.5.6.1 through 603.5.6.6.
- - - 603.5.6.1 Duct core to duct fitting, mechanical attachment.
- - - - The reinforced core shall be mechanically attached to the duct fitting by a drawband installed directly over the wire-reinforced core and the duct fitting. The duct fitting shall extend a minimum of 2 inches (51 mm) into each section of duct core. When the flexible duct is larger than 12 inches (305 mm) in diameter or the design pressure exceeds 1 inch water gage, the drawband shall be secured by a raised bead or indented groove on the fitting.
- - - 603.5.6.2 Duct core to duct fitting, approved closure systems.
- - - - The reinforced lining shall be sealed to the duct fitting using one of the following sealing materials which conforms to the approved closure and mechanical attachment requirements of Section 603.1:
- - - - - 1. Gasketing.
- - - - - 2. Mastic, mastic-plus-embedded fabric, or mastic ribbons.
- - - - - 3. Pressure-sensitive tape.
- - - - - 4. Aerosol sealants, provided that their use is consistent with UL 181.
- - - 603.5.6.3 Duct outer jacket to duct collar fitting.
- - - - The outer jacket of a flexible duct section shall be secured at the juncture of the air distribution system component and intermediate or terminal fitting in such a way as to prevent excess condensation. The outer jacket of a flexible duct section shall not be interposed between the flange of the duct fitting and the flexible duct, rigid fibrous glass duct board, or sheet metal to which it is mated.
- - - 603.5.6.4 Duct collar fitting to rigid duct, mechanical attachment.
- - - - The duct collar fitting shall be mechanically attached to the rigid duct board or sheet metal by appropriate mechanical fasteners; either screws, spin-in flanges, or dovetail flanges.
- - - 603.5.6.5 Duct collar fitting to rigid duct, approved closure systems.
- - - - The duct collar fitting’s integral flange shall be sealed to the rigid duct board or sheet metal using one of the following closure systems/materials which conforms to the approved closure and mechanical attachment standards of Section 603.1:
- - - - - 1. Gasketing.
- - - - - 2. Mastic or mastic-plus-embedded fabric.
- - - - - 3. Mastic ribbons when used to attach a duct collar to sheet metal.
- - - - - 4. Pressure-sensitive tape.
- - - - - 5. Aerosol sealants, provided that their use is consistent with UL 181.
- - - 603.5.6.6 Flexible duct installation and support.
- - - - Flexible ducts shall be configured and supported so as to prevent the use of excess duct material, prevent duct dislocation or damage, and prevent constriction of the duct below the rated duct diameter in accordance with the following requirements:
- - - - - 1. Ducts shall be installed fully extended. The total extended length of duct material shall not exceed 5 percent of the minimum required length for that run.
- - - - - 2. Bends shall maintain a center line radius of not less than one duct diameter.
- - - - - 3. Terminal devices shall be supported independently of the flexible duct.
- - - - - 4. Horizontal duct shall be supported at intervals not greater than 5 feet (1524 mm). Duct sag between supports shall not exceed ½ inch (12.7 mm) per foot of length. Supports shall be provided within ½ feet (152 mm) of intermediate fittings and between intermediate fittings and bends. Ceiling joists and rigid duct or equipment may be considered to be supports.
- - - - - 5. Vertical duct shall be stabilized with support straps at intervals not greater than 6 feet (1829 mm).
- - - - - 6. Hangers, saddles and other supports shall meet the duct manufacturer’s recommendations and shall be of sufficient width to prevent restriction of the internal duct diameter. In no case shall the material supporting flexible duct that is in direct contact with it be less than 1½ inches (38 mm) wide.

Many times you will see a combination of pressure-sensitive tape and mastic over it.

[RICHARD TAN]

Which requirement apply to flex duct installation in Florida ADC or FBC? Many builder refer to to ADC aThe outer jacket of a flexible duct section shall not be interposed between the flange of the duct fitting and the flexible duct, rigid fibrous glass duct board, or sheet metal to which it is mated. I have found many plastic white strap on the outside of the flex duct installation.

603.5.6.3 Duct outer jacket to duct collar fitting.
- - - - The outer jacket of a flexible duct section shall be secured at the juncture of the air distribution system component and intermediate or terminal fitting in such a way as to prevent excess condensation. The outer jacket of a flexible duct section shall not be interposed between the flange of the duct fitting and the flexible duct, rigid fibrous glass duct board, or sheet metal to which it is mated.

[Jerry Peck - Codeman]

Hi Richard,

Which requirement apply to flex duct installation in Florida ADC or FBC?

If there is a conflict, the most restrictive shall apply.

From the 2004 Florida Building Code, Building (as the FBC, Residential refers to the FBC, Building for the administrative sections). (underlining and bold are mine)
- SECTION 102
- - APPLICABILITY
- - - 102.1 General.
- - - - Where, in any specific case, different sections of this code specify different materials, methods of construction or other requirements, the most restrictive shall govern. Where there is a conflict between a general requirement and a specific requirement, the specific requirement shall be applicable.

- M1601.6.4 Flexible air duct systems, nonmetal.
- - Flexible nonmetal ducts shall be joined to all other air distribution system components by either terminal or intermediate fittings. All duct collar fittings shall have a minimum 5/8 inch (16 mm) integral flange for sealing to other components and a minimum 3-inch (76 mm) shaft for insertion into the inner duct core.
- - - Flexible ducts having porous inner cores shall not be used.
- - - - Exception: Ducts having a nonporous liner between the porous inner core and the outer jacket. Fastening and sealing requirements shall be applied to such intermediate liners.
- - - All joints of flexible ducts to fittings and fittings to other air distribution system components shall be mechanically attached and sealed as specified in Sections M1601.6.4.1 through M1601.6.4.6.
- - M1601.6.4.1 Duct core to duct fitting, mechanical attachment.
- - - The reinforced core shall be mechanically attached to the duct fitting by a drawband installed directly over the wire-reinforced core and the duct fitting. The duct fitting shall extend a minimum of 2 inches (51 mm) into each section of duct core. When the flexible duct is larger than 12 inches (305 mm) in diameter or the design pressure exceeds 1 inch water gauge, the drawband shall be secured by a raised bead or indented groove on the fitting.
- - M1601.6.4.2 Duct core to duct fitting, approved closure systems.
- - - The reinforced lining shall be sealed to the duct fitting using one of the following sealing materials which conforms to the approved closure and mechanical attachment requirements of Section M1601.3:
- - - - 1. Gasketing.
- - - - 2. Mastic, mastic-plus-embedded fabric or mastic ribbons.
- - - - 3. Pressure-sensitive tape.
- - - - 4. Aerosol sealants, provided that their use is consistent with UL 181.
- - M1601.6.4.3 Duct outer jacket to duct collar fitting.
- - - The outer jacket of a flexible duct section shall be secured at the juncture of the air distribution system component and intermediate or terminal fitting in such a way as to prevent excess condensation. The outer jacket of a flexible duct section shall not be interposed between the flange of the duct fitting and the flexible duct, rigid fibrous glass duct board, or sheet metal to which it is mated.
- - M1601.6.4.4 Duct collar fitting to rigid duct, mechanical attachment.
- - - The duct collar fitting shall be mechanically attached to the rigid duct board or sheet metal by appropriate mechanical fasteners, either screws, spin-in flanges, or dovetail flanges.
- - M1601.6.4.5 Duct collar fitting to rigid duct, approved closure systems.
- - - The duct collar fitting’s integral flange shall be sealed to the rigid duct board or sheet metal using one of the following closure systems/materials which conforms to the approved closure and mechanical attachment standards of Section M1601.3:
- - - - 1. Gasketing.
- - - - 2. Mastic or mastic-plus-embedded fabric.
- - - - 3. Mastic ribbons when used to attach a duct collar to sheet metal.
- - - - 4. Pressure-sensitive tape.
- - - - 5. Aerosol sealants, provided that their use is consistent with UL 181.
- - M1601.6.4.6 Flexible duct installation and support.
- - - Flexible ducts shall be configured and supported so as to prevent the use of excess duct material, prevent duct dislocation or damage, and prevent constriction of the duct below the rated duct diameter in accordance with the following requirements:
- - - - 1. Ducts shall be installed fully extended. The total extended length of duct material shall not exceed 5 percent of the minimum required length for that run.
- - - - 2. Bends shall maintain a center line radius of not less than one duct diameter.
- - - - 3. Terminal devices shall be supported independently of the flexible duct.
- - - - 4. Horizontal duct shall be supported at intervals not greater than 5 feet (1524 mm). Duct sag between supports shall not exceed 1/2 inch (12.7 mm) per foot of length. Supports shall be provided within 1½ feet (457 mm) of intermediate fittings and between intermediate fittings and bends. Ceiling joists and rigid duct or equipment may be considered to be supports.
- - - - 5. Vertical duct shall be stabilized with support straps at intervals not greater than 6 feet (1829 mm).
- - - - 6. Hangers, saddles and other supports shall meet the duct manufacturer’s recommendations and shall be of sufficient width to prevent restriction of the internal duct diameter. In no case shall the material supporting flexible duct that is in direct contact with it be less than 1½ inches (38 mm) wide.

You will have noticed that the wording is the same in the above references as in my previous post, but the referenced section numbers are different - this is because my previous reference was accidentally from the FBC, Mechanical and these references are from the FBC, Residential. The only thing which changes are the code section reference numbers.

[RICHARD TAN]

M1601.6.4.3 Duct outer jacket to duct collar fitting.
- - - The outer jacket of a flexible duct section shall be secured at the juncture of the air distribution system component and intermediate or terminal fitting in such a way as to prevent excess condensation. The outer jacket of a flexible duct section shall not be interposed between the flange of the duct fitting and the flexible duct, rigid fibrous glass duct board, or sheet metal to which it is mated.

Can you define interposed? if interposed, is strapped to the outside of the flex duct not allowed by the duct fitting?

[Jerry Peck - Codeman]

Hi Richard,

Can you define interposed? if interposed, is strapped to the outside of the flex duct not allowed by the duct fitting?

See attached drawing.