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Homes now have much tighter construction. Duct leakage will have an impact on system performance and utility consumption. In addition, duct leakage can cause undesirable room air pressure differences. Duct leakage in return air ducts passing through an unconditioned attic or crawl space causes a positive pressure throughout the entire house, with air exfiltration through envelope cracks. Leakage in supply ducts passing through an unconditioned attic or crawl space can cause a negative pressure throughout the house with a reverse infiltration effect. Duct leakage from ductwork located in the conditioned space may not cause infiltration problems, but can cause zone to zone air balance and temperature control problems. Insulation Types Mechanically Cooled

Duct Location

A leaking duct system operating in combination with kitchen and bathroom exhaust fans, a fireplace, and with the added air buoyancy forces caused by wind and building height, can result in a significant system performance loss. A room under high pressurization may not allow conditioned supply air flow to enter from a supply register, while a negative pressure in a room may not allow an exhaust fan to exhaust odors or contaminates. 

Long term air infiltration or exfiltration driven by a significant duct leakage problem, may cause permanent mold and mildew damage to the structure when moisture migration is not properly controlled. Heating Zone3 I II III I II III I II III

C, V2, and W

Exterior of Building Unconditioned Attics, Crawl Spaces, and Garages

A and V2

In Walls, within Floor–Ceiling Spaces1

A and V2

Within the Conditioned Space or in Basements Cement Slab or within Ground

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Insulation Types Heating B&W C&W C&W B B C A A B

None Required

None Required

None Required

None Required

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2. 3.

Insulation may be omitted on that portion of a duct which is located within a wall or a floor–ceiling space where both sides of this space are exposed to conditioned air and where this space is not ventilated or otherwise exposed to unconditioned air. Vapor Barrier shall be required on conditioned air supply ducts only. Heating Degree Days Zone I – Below 4500 Degree Days Zone II – 4501 to 8000 Degree Days Zone III – Over 8000 Degree Days

Type A–B C

NOTE: When design temperature indicates a dew point temperature less than 60EF (16EC) vapor barrier will not be required. INSULATION TYPES: A B C V

1” thick 0.75 lb./cu. ft. fiber blanket. 2” thick 0.75 lb./cu. ft. mineral fiber blanket. 3” thick 0.75 lb./cu. ft. Mineral fiber blanket. Vapor Barrier with perm rating not greater than 0.05 perms. W Weather Proof.

Description 1” thick 1.5 to 3 lb./cu. ft. Mineral blanket (duct liner) 1–1/2” thick 3 to 10 lb./cu. ft. Mineral fiber board

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Smacna residential comfort system installation standards manual seventh edition  
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