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Flexible ductwork demands careful workmanship


Gloria Fitzgerald of Westminster has concerns about the ductwork in her new air conditioning system:

"I recently had AC installed in my house and the ducts are all soft ducts from the attic to the first floor and extending down into my basement area. Would this pass inspection if I were to sell my home or should the ducts have been hard ducts?"

Your system appears to have been installed using insulated, flexible ductwork, often referred to as flex duct. Although there are drawbacks to flex duct, it will pass muster with most inspectors if the installation has reasonably good workmanship.

Flex duct comes in long, flexible sections which are cut to the desired length. Its flexibility is allowed by a spiral wire core, similar to the fake snakes-in-the-can party gag you once may have had sprung on you.

The spiral core is incorporated in a layer of plastic film, forming the tube that the air flows through. A layer of fiberglass surrounds the tube, providing thermal insulation to keep the air in the duct cold or hot. The fiberglass has an outer sleeve of plastic film or metal foil over it, which serves as a vapor barrier. The vapor barrier keeps humidity from reaching the inner core of the duct, where it would otherwise condense into droplets of water when cold air is flowing through the duct.

Although traditional rigid, metal ductwork is superior in many respects, flex duct has become very popular with heating and air conditioning installers, and few building codes put restrictions on its use.

Reasons for its popularity include its ease of installation and low installation cost. Disadvantages include greater resistance to air flow because of the spiral contour at its interior surface and its tendency to constrict at bends or to sag between supports if not carefully installed. Also, marginal or poor workmanship at connections in flex duct results in leaky or disconnected joints developing over time.

John Lederer of Frosty Refrigeration Co. Inc., a regional heating and air conditioning installer, says that his company uses flex duct in applications over relatively short distances, such as branches off trunk ducts.

He says that ductwork designs prepared by engineers and architects will generally limit flex duct sections to 8 feet in length because of its drawbacks in longer runs. Lederer does not use flex duct for vertical runs, such as those from the attic to lower floors described in Gloria's letter. In a vertical configuration, the weight of the duct creates a greater tendency for kinking at turns and for over-stressed connections.

Workmanship will determine whether the ductwork installed in Gloria's house performs well and passes scrutiny in the future.

Signs of superior workmanship include hangers supporting suspended flex duct at least every five feet, preferably with a 1 1/2 -inch or wider saddle where the duct rests on each support; no more than half-inch sag per foot of duct length between supports; connections that are both strapped and taped, or sealed with mastic, at the collars where duct sections begin and end; and rigid supports installed at bends in the flex duct to prevent it from becoming crimped.

Inspector's Eye

Dean Uhler has been a home inspector for more than 12 years and is president of Baltimore-based Boswell Building Surveys Inc. Uhler is a member of the American Society of Home Inspectors (ASHI) and is the treasurer of the Greater Baltimore Chapter of ASHI.

Questions, with name, address and daytime telephone number, about homes and home inspections can be faxed to 410-783-2517, e-mailed to or mailed to Inspector's Eye, Second Floor, 501 N. Calvert St., Baltimore, Md. 21278-0001.

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