Instructions / Assembly
Chapter 28 Ductwork
mastic is water-based, nonflammable, fiber-reinforced,
fast drying, and waterproof. Easy application, and
clean-up with soap and water are attractive features of
the material. Mastic can be applied by brush, trowel,
caulking gun, or with a gloved hand. It typically dries
to touch in 1 or 2 hours, but unusually wet conditions
may require 4-5 hours drying time. Insulation can be
installed over mastic that is still wet, but care is needed
to prevent excessive movement and loss of seal.
28.16.2 Fiberglass Reinforcing Membrane
Fiberglass reinforcing membrane is used with mastic
for sealing air leaks in insulation, air ducts, plenums, and
fittings. This inorganic woven fiberglass membrane,
Figure 28-5313, will not rot, is lightweight and conforms
to irregular surfaces. A typical roll is 3" wide x 50' long,
or 3" wide x 150' Jong, Figure 28-54. This product is
designed to be used with mastic as a reinforcement
when there is a gap of 1/4" or more. If the membrane
has a sticky side, apply it over the clean metal surface
before applying the mastic. If the membrane does not
have a sticky side, apply a Ihin layer of mastic and place
the membrane in this layer. Add another layer of mastic
until the membrane is completely covered.
28.17 How to Use Mastic
Before applying mastic or fiberglass reinforcing
membrane, the duct surface must be clean and dry, Use
a cloth to wipe dust from the surface of the duct,
Figure 28-54. If an oily film or grease is present, wipe
the duct clean with a damp cloth. If necessary, use a
grease-cutting cleaner on the cloth, then wipe thor-
oughly dry.
28.17.1 Sealing Joints with Less Than
1/4" Gaps
Load the brush with mastic, and coat the entire
joint with a continuous strip of the adhesive. Use the end
of the brush to work mastic into joint. Spread mastic at
least 1" on each side of the joint. Figure 28-55. The
mastic should be thick enough to hide the metal
surface of the duct (about 1/16" thick).
28.17.2 Sealing Joints with Gaps Greater
Than 1/4"
Tf the gap in the duct connection is wider than 1/4",
use fiberglass reinforcing membrane in addition to
maslic. Cut enough membrane to cover the joint, and
press the membrane over the joint, Figure 28-56. Tf
necessary, first brush a thin layer of mastic on the joint
to hold the membrane in place. Finally, cover the
membrane with mastic in a layer about 1/16" thick,
overlapping the membrane on each side.
Cloth
Bead
Figure 28-54. Mastic will not stick to a dirty surface. Wipe
dust off the surface, and clean any greasy residue.
Bead
Mastic
Metal
duct
Figure 28-55. Joints with a gap of less than 1/4" can be
sealed with mastic only.
Joint
Fiberglass
membrane
tape
Figure 28-56. Gaps of more than 1/4" require use of rein-
forcing membrane.
Summary
This chapter introduced the classifications of
forced-air furnaces, based on direction of airflow, and
described the difference between conventional and
perimeter systems. The major air distribution systems,