AIR DISTRIBUTION SYSTEM DESIGN
F
I B E R G L A S S
D
U C T
B
O A R D
Fiberglass duct board is insulated and sealed as part of its
construction. It is usually used to form rectangular supply and
return trunks, branches, and plenums, although it can be used
for runouts as well. Connections should be mechanically
fastened using shiplap or V-groove joints and stapling and
sealed with pressure-activated tapes and mastic. Fiberglass duct
board provides excellent sound attenuation, but its longevity is
highly dependent on its closure and fastening systems.
F
L E X I B L E
N
O N M E T A L L I C
D
U C T
Flexible nonmetallic duct (or flex duct) consists of a duct inner
liner supported on the inside by a helix wire coil and covered
by blanket insulation with a flexible vapor-barrier jacket on the
outside. Flex duct is often used for runouts, with metal collars
used to connect the flexible duct to supply plenums, trunks,
and branches constructed from sheet metal or duct board. Flex
duct is also commonly used as a return duct. Flex duct is
factory-insulated and has fewer duct connections and joints.
However, these connections and joints must be mechanically
fastened using straps and sealed using mastic. Flex duct is
easily torn, crushed, pinched, or damaged during installation. It
has the highest resistance to air flow. Consequently, if used, it
must be properly specified and installed.
D U C T A N D R E G I S T E R L O C A T I O N S
Locating the air handler unit and air distribution system inside
the conditioned space of the house is the best way to improve
duct system efficiency and is highly recommended. With this
design, any duct leakage will be to the inside of the house. It
will not significantly affect the energy efficiency of the heating
and cooling system because the conditioned air remains inside
the house, although air distribution may suffer. Also, ducts
located inside the conditioned space need minimal insulation
(in hot and humid climates), if any at all. The cost of moving
ducts into the conditioned space can be offset by smaller
heating and cooling equipment, smaller and less duct work,
reduced duct insulation, and lower operating costs.
There are several methods for locating ducts inside the
conditioned space.
·
Place the ducts in a furred-down chase below the ceiling (e.g.,
dropped ceiling in a hallway), a chase furred-up in the attic, or
other such chases. These chases must be specially
constructed, air-sealed, and insulated to ensure they are not
connected to unconditioned spaces.
·
Locate ducts between the floors of a multi-story home (run
through the floor trusses or joists). The exterior walls of these
floor cavities must be insulated and sealed to ensure they are
within the conditioned space. Holes in the cavity for wiring,
plumbing, etc., must be sealed to prevent air exchange with
unconditioned spaces.
·
Locate ducts in a specially-constructed sealed and insulated
crawlspace (where the walls of the crawlspace are insulated
rather than the ceiling).
Ducts should not be run in exterior walls as a means of moving
them into the conditioned space because this reduces the
amount of insulation that can be applied to the duct and the
wall itself.
A supply outlet is positioned to mix conditioned air with room air
and is responsible for most of the air movement within a room.
Occupant comfort requires that supply register locations be
carefully selected for each room. In cold climates, perimeter floor
outlets that blanket portions of the exterior wall (usually
windows) with supply air are generally preferred. However, in
today's better insulated homes, the need to locate outlets near
DUCTS INSIDE CONDITIONED SPACE