Thread: A/C Duct Mastic

Is there any code or standard that requires mastic to be applied to the back side of the wall/ceiling where the flex duct attaches to the diffuser assembly?

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Originally Posted by Larry Hood

Is there any code or standard that requires mastic to be applied to the back side of the wall/ceiling where the flex duct attaches to the diffuser assembly?

I have never seen mastic used with flex-duct. I have seen mastic tape, but never true mastic on flex-duct.

http://apps1.eere.energy.gov/buildin...ct_systems.pdf

Here's what is seen in many NE TX homes

Barry,

Same here in Florida at lateral connections to fiberglas trunck.At the end of the flex where the flex attaches to (what I call the diffuser boot) .The transition piece was written up for no mastic sealing around flange on the attic side of the dry wall penetration.

Originally Posted by Larry Hood

Is there any code or standard that requires mastic to be applied to the back side of the wall/ceiling where the flex duct attaches to the diffuser assembly?

Originally Posted by Larry Hood

Barry,

Same here in Florida at lateral connections to fiberglas trunck.At the end of the flex where the flex attaches to (what I call the diffuser boot) .The transition piece was written up for no mastic sealing around flange on the attic side of the dry wall penetration.

No requirement for that, besides, sealing from the back side does not keep the cold air off the drywall.

The pieces should fit such that there is no leakage out the front, which includes where the boot penetrates through the ceiling - if it does leak there then they need to seal/mastic the front side, not the back side.

Jerry,

That was my arguement too, and I am not sure I have one the battle or lost it with the county inspector.Each time we get ahead of him he comes up with another surprise.I will send you a picture at a later date win or lose.Thanks for the input.

Originally Posted by Larry Hood

That was my arguement too, and I am not sure I have one the battle or lost it with the county inspector.Each time we get ahead of him he comes up with another surprise.

Larry,

From the FBC-Residential:
- N1110.ABC.3.4.2 Terminal fittings and air ducts to building envelope components, approved closure systems. Terminal fittings and air ducts which penetrate the building envelope shall be mechanically attached to the structure and sealed to the envelope component penetrated and shall use one of the following closure systems/materials which conform to the approved closure and mechanical application requirements of Section N1110.ABC.3.0:
- - 1. Mastics or mastic-plus-embedded fabrics.
- - 2. Gaskets used in terminal fitting/grille assemblies which compress the gasket material between the fitting and the wall, ceiling or floor sheathing.

- N1110.ABC.3.0 General. All enclosures which form the primary air containment passageways for air distribution systems shall be considered ducts or plenum chambers and shall be constructed and sealed in accordance with the applicable criteria of this section.
- - N1110.ABC.3.0.1 Mechanical fastening. All joints between sections of air ducts and plenums, between intermediate and terminal fittings and other components of air distribution systems, and between subsections of these components shall be mechanically fastened to secure the sections independently of the closure system(s).
- - N1110.ABC.3.0.2 Sealing. Air distribution system components shall be sealed with approved closure systems.
- - N1110.ABC.3.0.3 Space provided. Sufficient space shall be provided adjacent to all mechanical components located in or forming a part of the air distribution system to assure adequate access for: (1) construction and sealing in accordance with the requirements of Section N1110.ABC.3; (2) inspection; and (3) cleaning and maintenance. A minimum of 4 inches (102 mm) is considered sufficient space around air-handling units.
- - - Exception: Retrofit or replacement units not part of a renovation are exempt from the minimum clearance requirement.
- - N1110.ABC.3.0.4 Product application. Closure products shall be applied to the air barriers of air distribution system components being joined in order to form a continuous barrier or they may be applied in accordance with the manufacturer's instructions or appropriate industry installation standard where more restrictive.
- - N1110.ABC.3.0.5 Surface preparation. The surfaces upon which closure products are to be applied shall be clean and dry in accordance with the manufacturer's installation instructions.
- - N1110.ABC.3.0.6 Approved mechanical attachments. Approved mechanical attachments for air distribution system components include screws, rivets, welds, interlocking joints crimped and rolled, staples, twist in (screw attachment), and compression systems created by bend tabs or screw tabs and flanges or by clinching straps. Mechanical attachments shall be selected to be appropriate to the duct system type.
- - N1110.ABC.3.0.7 Approved closure systems. The following closure systems and materials are approved for air distribution construction and sealing for the applications and pressure classes prescribed in Sections N1110.ABC.3.1 through N1110.ABC.3.8:
- - - 1. Metal closures.
- - - - a. Welds applied continuously along seams or joints through which air could leak.
- - - - b. Snaplock seams, and grooved, standing, double-corner, single-corner and Pittsburgh-lock seams, as defined by SMACNA, as well as all other rolled mechanical seams. All seams shall be rolled or crimped
- - - 2. Factory-made flexible air ducts shall have the installed thickness and calculated R -values determined in accordance with Paragraph 3.4 of the ADC Standard, Flexible Duct Performance & Installation Standards.
- - - 3. Gasketing, which achieves a 25/50 flame spread/smoke-density-development rating under ASTM E 84 or UL 723, provided that it is used only between mated surfaces which are mechanically fastened with sufficient force to compress the gasket and to fill all voids and cracks through which air leakage would otherwise occur.
- - - 4. Mastic closures. Mastics shall be placed over the entire joint between mated surfaces. Mastics shall not be diluted. Approved mastics include the following:
- - - - a. Mastic or mastic-plus-embedded fabric systems applied to fibrous glass ductboard that are listed and labeled in accordance with UL 181A, Part III.
- - - - b. Mastic or mastic-plus-embedded fabric systems applied to nonmetal flexible duct that are listed and labeled in accordance with UL 181B, Part II.
- - - - c. Mastic ribbons, which achieve a 25/50 flame spread/smoke-density-development rating under ASTM E 84 or UL 723, provided that they may be used only in flange-joints and lap-joints, such that the mastic resides between two parallel surfaces of the air barrier and that those surfaces are mechanically fastened.
- - - 5. Tapes. Tapes shall be applied such that they extend not less than 1 inch onto each of the mated surfaces and shall totally cover the joint. When used on rectangular ducts, tapes shall be used only on joints between parallel rigid surfaces and on right angle joints. Approved tapes include the following:
- - - - a. Pressure-sensitive tapes.
- - - - - 1) Pressure-sensitive tapes applied to fibrous glass ductboard that are listed and labeled in accordance with UL 181A, Part I.
- - - - - 2) Pressure-sensitive tapes applied to nonmetal flexible duct that are listed and labeled in accordance with UL 181B, Part I.
- - - - b. Heat-activated tapes applied to fibrous glass ductboard that are listed and labeled in accordance with UL 181A, Part II.
- - - 6. Aerosol sealant. Such sealants shall be installed by manufacturer-certified installers following manufacturer instructions and shall achieve 25/50 flame spread/smoke-density-development ratings under ASTM E 84 or UL 723.

This "4. Mastic closures. Mastics shall be placed over the entire joint between mated surfaces." does not mean to mastic the backside of the drywall and 'glue on' the boot, the boot is mechanically fastened and the flange-to-drywall lip is there to back-up the screws holding the grille in place. The space around the boot penetrating rim is supposed to be tight fitting and almost no space, with that annular space sealed with mastic (i.e., from the front side below the ceiling).

The gasketed mechanical closure is the gasket on the backside of the grille which gets compressed between the grille and the drywall.

If the mastic is done properly, no air will leak out the back side or out the front side between the ceiling and the grille - all the air will exit through the grille.