Generally, you will not need insulation for heating and air-conditioning
ducts that are enclosed in already well-insulated portions of
your house. But, if the ducts for either your heating or your
air-conditioning system run above the ceiling or below the floors,
they should be insulated.
Whether ducts are in the attic or beneath the floor, insulation
is installed in pretty much the same way using special duct insulation:
Seal seams with metal duct tape.
Cut the insulation so it's long enough to wrap around the duct
with a two-inch overlap.
Wrap the insulation around the duct with the vapor barrier on
the outside, faced away from the ducts.
If your ducts already have some insulation, check to see if any
moisture has collected in it. If so, replace it with new insulation.
Note: some duct systems have insulation on the inside that cannot
be seen. Check to make sure you do not have this type of duct
system before adding additional insulation
Once the ducts have been installed, and the trunks and runs established,
it's time to finish off the heating system by insulating the ducts.
The degree to which you insulate your ducts depends on local building
codes for energy efficiency, and whether the ducts are in conditioned
or non-conditioned areas. If a duct runs through an unheated crawl
space, an unheated basement, or garage, it must typically be insulated
to R-4 or greater. If ducts are long or poorly insulated, they
often result in cool blows, or gusts of air that have lost the
desired temperature. In such a case, running the heat can actually
cool a room rather than heat it. The same can be said for cool
conditioned air, which must maintain its temperature or fail to
cool the room at the other end.
Sealing and Insulating the Ducts
The first step toward fully insulated ducts is to seal all of
the points in the duct run. This is done with a mastic that is
brushed on then left to dry for at least 24 hours. The mastic
seals the joints and protects against air loss and drop in velocity.
How quickly the air moves through the ducts determines how well
it will hold its temperature. Well-sealed joints allow the ducts
to carry air at maximum capacity. Mastic is used because it will
not degrade or shrink away from the joint. Duct tape should not
be used for this purpose, since the tape degrades and decomposes,
leaving the joint leaky and unprotected.
Once the ducts have been sealed, an insulating jacket is wrapped
around the ducts and plenum. Like wall insulation, duct insulation
is typically made of fiberglass. Insulators may use rigid insulation
or faced fiberglass. The fiberglass surface faces in toward the
duct, while the foil face protects the outside of the wrap, preventing
conductive heat loss and protecting the fiberglass within. The
insulation is applied in sheets that are cut to fit and seamed,
usually along the sides. While some installers use foil tape for
this purpose, most insist on attaching the pieces to one another
with a staple gun. Staples are less likely to degrade or lose
grip over time.
Insulation is applied to ductwork to enhance thermal performance
and prevent condensation and dripping. Duct thermal performance
needs enhancement since air transported through a supply duct
is at a temperature different than that of the surroundings. Insulation
reduces the rate of thermal loss to those surroundings. Without
insulation, the air would need extra heating or cooling in order
to arrive at the design supply air temperature. Return air ducts
only need to be insulated if they pass through environments that
adversely affect the return air temperature. Exhaust air ducts
normally do not need insulation. Supply air ducts may be left
un-insulated if they run exposed through the space being conditioned;
this arrangement also reduces system first cost.
Insulation prevents condensation and dripping from ducts. Un-insulated
cold air ducts very often have surface temperatures below the
local dew point. At this temperature, condensate will form and
eventually drip off, causing an uncontrolled accumulation of moisture
on the outside surface of the duct. Duct insulation eliminates
the formation of condensate and consequently prevents rusting
Extra heating (or cooling) energy required to compensate for reduced
thermal performance of un-insulated duct has a negative effect
on the HVAC system's life-cycle cost. Therefore, duct insulation
always presents an optimization problem. Since insulated duct
costs much more than un-insulated, the recommended air velocity
becomes a key factor in optimization. For instance, a higher air
velocity reduces duct surface area and thus insulation cost.
Because of the relatively small temperature differences between
supply air ducts and the spaces through which they ductwork are
routed, a one-inch-thick fiberglass blanket is almost always sufficient.
Insulation should be wrapped around the duct's exterior. A protective
cover with a vapor barrier such as an aluminum foil, referred
to as FKS, should be included in insulation specifications. Care
must be exercised to protect exterior insulation integrity where
insulation comes in contact with hangers, supports, and other
structural members. Interior duct insulation (lining) should not
be used in laboratory or cleanroom applications because the insulation
tends to entrain microscopic particles into the airflow.
Special consideration must be given to ducts exposed to weather.
Lagging materials or heavy metal covers over the insulation are
commonly used to protect ductwork. A life-cycle cost analysis
may be necessary to determine optimum insulation thickness when
ducts encounter temperature extremes.