(Editor's note: The following excerpt is from ANSI/ASHRAE Standard 90.2-2001, "Energy-Efficient Design of Low-Rise Residential Buildings.")

Sizing and design. All air distribution systems shall be sized and designed in accordance with the ASHRAE Handbook - Fundamentals and the ACCA Manual D or other procedures based on the following:

Calculation of the supply air for each room shall be based on the greater of the heating load or sensible cooling load for that room.

Duct size shall be determined by the supply air requirements of each room, the available static pressure, and the total equivalent length of the various duct runs.

Friction loss data shall correspond to the type of material used in duct construction.

All thermal properties (U,C, and R) that are used to determine compliance with this standard, whether calculated or measured, shall be based on a mean temperature of 758F.

Installation. All ductwork shall be constructed, erected, and sealed in accordance with the following:

Sheet Metal and Air Conditioning Contractors National Association (SMACNA) Installation Standards for Residential Heating and Air-Conditioning Systems, 6th ed.

NAIMA Fibrous Glass Duct Construction Standards, AH116.

SMACNA HVAC Duct Construction Standards - Metal and Flexible, 1st ed.

SMACNA HVAC Duct Leakage Test Manual.

ADC Flexible Duct Performance and Installation Standards.

UL 181A, Closure Systems for Use with Rigid Air Ducts and Connectors.

SMACNA, Fibrous Glass Duct Construction Standards, 6th ed.

Balancing. The system design shall provide means for balancing the air distribution system unless the design procedure provides a system intended to operate within plus/minus 10% of design air quantities.

Insulation. All ducts, plenums, and enclosures installed in or on buildings shall be thermally insulated as stated in Table 6-2a. Table 6-2a does not apply to hvac equipment meeting the minimum efficiency requirements of 6.4. Where ducts are used for both heating and cooling, the insulation shall be required for the most severe condition. Where HDD65 exceeds 10,000 (e.g., Alaska), consideration should be given to putting all ductwork within the building envelope.

The level of insulation may not be sufficient to prevent surface condensation in humid areas. The designer should verify this by using the predicted surface temperature and anticipated dew-point temperature. Vapor retarders shall be installed on conditioned air-supply ducts in climates where the mean outdoor dew-point temperature in any month exceeds 608F. Vapor retarders shall have a water vapor permeance not exceeding 0.5 perm when tested in accordance with ASTM E96, Procedure A.

Duct insulation is not required in any of the following cases except where required to minimize condensation (6.3.1.4.3):

When the ductwork is within the conditioned space.

Where supply or return air ducts are used for heating only and are installed in basements or unvented crawl spaces (plenum) having insulated walls, and exhaust air ducts.

If ducts are installed in exterior walls, roofs, ceilings, or floors that separate heated or mechanically cooled spaces (or both) from unconditioned space or exterior ambient conditions, the assemblies shall be insulated in accordance with Section 5 and the insulation shall be installed between the ducts and the surface exposed to the unconditioned space or exterior ambient condition.

The required thermal resistances do not consider condensation. Additional insulation with vapor retarders may be required to prevent condensation under some conditions.

To order a complete copy of the standard contact ASHRAE Customer Service at 800-527-4723 (U.S. and Canada) or 404-636-8400; fax 404-321-5478; www.ashrae.org; or by mail at 1791 Tullie Circle NE, Atlanta, Ga. 30329.

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