HVAC Duct System Installation – Part 2

Duct Materials

  • The most common types of air ductwork are:
    • Galvanized-steel sheet metal:
      • Comes in large, flat sheets
      • Common sizes are 4 ft wide and 8–10 ft long
      • Offers the least amount of resistance to airflow
      • The thickness of the metal is called its gauge
    • Spiral metal:
      • Made from long strips of narrow metal and fabricated with spiral seams
      • Used in commercial applications
      • Frequently used where the ductwork will be exposed
      • Requires less support than other types of duct due to its inherent rigidity
    • Fiberglass ductboard:
      • A rigid material made of compressed fiberglass with an outer vapor barrier
    • Flexible duct:
      • The most common duct material used today in residential duct systems
      • Has a spiral metal wire for support
      • A smooth plastic inner liner

Fiberglass Ductboard

  • Advantages:
    • Good insulator for both heat and sound, reducing duct losses and providing sound-absorbing qualities
    • Less expensive than metal
    • Fabricating ductboard duct is generally easier than fabricating sheet metal duct
  • Disadvantages:
    • Less durable than sheet metal duct
    • Has a higher resistance to airflow than sheet metal because of its rougher interior surface

Flexible Duct

  • Advantages:
    • The least expensive duct material
    • The easiest to install
    • Very quiet because its soft sides absorb sound
  • Disadvantages:
    • The easiest to install incorrectly
    • Tight radius turns
    • Improper support
    • Soft, undulating sides have the highest resistance to airflow of all of the most commonly used duct materials

Insulating Sheet Metal Ductwork

  • Sheet-metal ductwork can be insulated on the inside with duct liner or on the outside with duct wrap.
  • Duct liner is applied to the inside of the duct.
  • The most common duct wrap is fiberglass with an outer vapor barrier.

Sheet-Metal Duct

Galvanized sheet-metal duct has been the gold standard of duct systems for years.

One advantage of metal duct is that it has structural integrity, making it much easier to hang than flexible duct.

The first step in installing a takeoff is to cut a hole in the duct using an adjustable hole cutter. Holes can also be cut using aviation snips.

Takeoffs are connected to metal duct using either dovetails or flanges. Flange fittings have a flange that fits on the outside of the duct.

Duct Insulation

  • Metal duct must be insulated.
  • Round duct is insulated with duct wrap.
    • The most common type of duct wrap is made of fiberglass with an outer vapor barrier of vinyl, aluminum foil, or aluminum foil reinforced with Kraft-back paper.
  • When the job is done:
    • No metal should be showing
    • No fiberglass should be showing
    • The vapor barrier should cover the entire duct system
  • The duct wrap should be cut in pieces that are large enough to wrap around the duct and have some overlap.

Duct Liner

Rectangular duct, plenums, and return-air boxes may be lined with insulation rather than wrapped. Duct liner is glued to the inside of the duct and connected with mechanical fasteners that compress the insulation. Duct liner is also fiberglass, but it does not have a vapor barrier.

Lining the duct makes a system quieter and is usually easier than wrapping the duct.

When a liner is used, the duct must be made larger to compensate for the space the liner takes up.

Bubble Wrap

Bubble wrap is a new form of duct insulation. It is similar to the bubble packing material with outer layers of either Mylar or aluminum.

Bubble wrap may be used as liner or as duct wrap. Bubble wrap is typically not applied directly to the duct.

Installers like bubble wrap because it does not irritate their skin and cause itching like fiberglass.

As a new product, it does not have a long track record of success like fiberglass.


Ductboard is an alternative to sheet metal duct. Ductboard is a rigid fiberglass insulating board with an outer vapor barrier used to construct plenums and ductwork.

Quality ductboard should be UL 181 rated, and the tapes used to fabricate it should be UL 181A rated.

Ductboard has several advantages. It is relatively low in both material and installation cost. It does not need insulating because it is inherently thermally efficient. It is generally easier to fabricate and produces quieter systems.

Despite these advantages, many contractors still consider a ductboard duct system inferior to a metal duct system. It is less durable than metal but holds up well if fabricated correctly.

Ductboard has a higher resistance to airflow because of its relatively rough interior surface. This surface also causes it to collect dirt.


Ductwork is constructed by cutting grooves or slots in the fiberglass for each corner.

The tools are hand tools with razor-sharp knife blades that cut through the fiberglass to form the joints.

Types of joints used are the V-groove, shiplap, and modified shiplap.

Pressure-Sensitive Tape

  • Pressure-sensitive tape should be worked with a tool edge, and not just fingertip pressure.
  • Rub tape firmly with a plastic sealing tool until the facing reinforcement shows through the tape.
  • Avoid excessive pressure on the sealing tool, as this could cause the tape to be punctured at staple locations.
  • Pressure-sensitive tape should not be used below 50°F without heating.
  • Complete the bond by rubbing the tape firmly with the plastic sealing tool until the facing reinforcement shows through the tape clearly.

Heat-Activated Tape

  • A seaming iron should have a plate temperature between 550°F and 600°F.
  • Slowly pass the iron along the tape seam with sufficient pressure and dwell time to activate the adhesive.
  • Heat-indicator dots on the tape will darken to indicate when a satisfactory bond has been achieved.
  • Use a second pass of the iron to complete the bond.
  • Avoid puncturing the tape at staple locations with excessive pressure from the iron.
  • Allow all joints and seams to cool below a 150°F (66°C) surface temperature before any stress is applied.

Joining and Hanging Ductboard Sections

  • Joining:
    • Joining using shiplap joints:
      • The shiplap joint provides some structural rigidity to the joint.
    • The sections are sealed using either pressure-sensitive or heat-activated tape
  • Hanging ductboard:
    • It is lightweight, so hangers are not required to support a lot of weight.
    • Duct hangers should be at least 1 in. wide across the bottom.
    • A common arrangement is to use an inverted 1-in. metal channel that is suspended by wire, straps, or rods.
    • Hangers should not be placed directly under joints between two pieces of duct to avoid stressing the joints.
    • Hangers should be placed every 6 ft for duct that is less than 12-in. high or every 8 ft for ducts that are 12-in. high or larger.
  • Takeoffs:
    • The most common use of ductboard is for plenums and trunk lines in systems that use flex duct for branch ducts.
    • Takeoffs designed specifically for fiberduct should be used.
    • Two types are available:
      • Spin-in collars
      • Dovetail collars
    • Follow flex duct recommendations to connect to the takeoff.

Will Continue In Part 3.


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