Fibrous Glass Ductwork
One thing a contractor
won’t tell you is the
airside surface of
board is “rougher”
than metal: There are
“nooks and crannies”
to collect dirt; if
moisture enters the
duct system, this can
present an increased
risk of mold growth
because board will
accumulate more dirt
than metal.
Here's a return duct
that failed in warranty,
causing attic air to be
drawn into the system.
The homeowner had
observed the system
was running
continuously and not
cooling well. Our
phone rang after the
huge electric bill
came in.

We determined "Butt
Joints" were used
throughout, instead of
the required "Shiplaps"
(interlocking male and
female leading edges
that strengthen the
joints), substandard
tape had been used
(there's no red printing
documenting "UL181"),
support spacing
greatly exceeded
Industry Standard, and
the required staples
(another way of
strengthening the
joints) had been

We were not the least
surprised to
subsequently learn the
builder who hired us
had begun using a
different subcontractor.
This installer used the
correct tape, but he
didn’t provide the
required mechanical
fasteners (screws
through washers and
“board”, into sheet
metal angles secured
to the unit) to secure
the duct to the unit;
this duct simply fell off.
Furthermore the unit
was a gas furnace; the
installation presented
the 90B violation.

Three types of materials are used in the construction of main ducts: Flexible
Duct (
the most abused product in the industry), “Fibrous Glass” (fiberglass,
AKA "Duct board" or just "Board", the second most abused product) and
galvanized sheet metal. Flexible Duct Systems are DOA (good luck in you have
one of them); we'll deal with board  here, and with metal on another page.

Installation of board is governed by “NAIMA” (North American Insulation
Manufacturers Association, the people who make the product), in their
Fibrous Glass Residential Duct Construction Standards Third Edition 2002”.
NAIMA’s checklist is at the lower left of the page; ductwork fabricated and
installed to the Standard and passing the checklist is fine. The problem is we've
never seen any; contractors are profiting by ignoring the Standard and installing
substandard ductwork that can literally fall apart.

Duct construction is also governed by “NFPA” (National Fire Protection
Association), in their “
NFPA 90B Standard for the Installation of Warm Air
Heating and Air conditioning Systems 2006 Edition
” (in essence, a residential
duct construction standard). ¶ requires furnace plenums and supply ducts
within 36” of the furnace centerline to be constructed of metal.

IMPORTANT NOTE: Fiberglass supply ducts cannot be connected directly to a
warm air furnace; furthermore, any supply ductwork close to the furnace must be

Downflow heating and air conditioning systems (wherein vertical furnaces
resting on cooling coils draw return through their top and discharge through the
cooling coil beneath) present a unique problem: Condensate can overflow into
the supply plenum beneath the unit; this presents an opportunity for mold
growth. In ¶2.14 Moisture Control, NAIMA tells us “Duct systems which, in
service, are found to be wet should be replaced.” There’s another reason not to
use a fibrous glass plenum on downflow systems.

IMPORTANT NOTE: Try to avoid downflow systems, because of their increased
risk for mold growth within the supply ducts.

IMPORTANT NOTE: If you have a downflow system and an overflow occurs, have
the insides of the plenum and connecting supply ducts checked and cleaned as
indicated; if the plenum is board, correct your Code violation by replacing it with
metal and providing metal supply ducts within 36" on the furnace centerline.

More Fun with Board
The cooling coil
below the furnace
indicates a
"downflow" type.

Here we see several
deficiencies: The
38# air filter is not
supported, it's simply
resting on the duct;
there are all the
usual shortcuts - butt
joints, no staples, etc.
The auxiliary drain
connection (See that
blue thing left of the
PVC connection at
the coil?) is not
piped. That means if
the primary drain
stops up, the
fiberglass plenum
beneath the coil will
be wetted (the
auxiliary connection
cannot function to
relieve the overflow);
and that's quite likely
to happen at some
point in the life of the
this system.
You know you've been had....

When the company that installed
your system and has serviced it
for years starts patching their
substandard work with multiple
layers of cloth duct tape.

You've really been had...

When they charge you to do it.
You know you've been had...

When your duct fittings start
falling off because the installer
used fittings intended for sheet
metal ducts on fibrous glass
You know you've been had...

When your contractor's
housewarming present is
"Fiber Shed" forever. (Think
about it).

Here the correct fittings are
used; "tabs" are extended into
the duct and bent over the
opening, assuring a good
mechanical connection.
You know you've been had...

When you start air conditioning
your attic.

This is another case of the
required fasteners being
omitted, and another 90B
violation (it was a furnace).
You know you've been had...

When you pay a guy $20K and
he can't make two "modules"
of duct the same size.

Note the use of butt joints;
there were no staples, so the
only thing keeping these ducts
together was a thin layer of
You know you've been had...

When the joint with the
highest negative pressure in
your system is not sealed as
required by Code.

When there's no duct fitting at
your filter to evenly distribute
the air across the filter, also
as required by Code (they just
ran a duct down and cut a
hole in the side of it - this
arrangement also creates a
huge resistance to flow).
It's always good to have a little fun; BUT do remember that these things were
done to fellow human beings by people who should have known better.

Life Expectancy for Ductwork

UL runs six different tests for structural integrity as a part of their approval
process: Puncture Test, Static Load Test, Pressure and Collapse Tests,
Impact Test, Leakage Test and Erosion Test; in these tests, properly fabricated
fibrous glass ducts are tested to assure they are strong enough to withstand
conditions expected in the real world. Fibrous glass ductwork fabricated and
installed to Industry Standard has a good chance of meeting the 30 year
service life everyone expects of ductwork.

We're not aware of any tests run on the type of substandard work documented
herein, and we'd be quite surprised to see such specimens come close to
passing. Indeed, this type of work can begin to fall apart soon after completion.

Fire Safety

In fire situations, ducts can deliver deadly smoke and products of combustion
instead of breathable air; ducts that separate in fire situations can spread fire
and products of combustion, and can feed a fire. For those reasons, a great
deal of  attention is paid to how ducts are fabricated, installed and supported;  
that's why the Industry Standard was developed.


The decision as to what type of main ductwork to live with is yours and you'll
only make it once per home.
  • If you can’t be sure of a proposed board system being fabricated and
    installed to Industry Standard,
  • If you don't want to risk an argument about whether the work
    complies with Industry Standard,
  • If you don't want to worry about whether you duct system will fall
  • If you don't want to worry about whether your duct system is safe,
  • If you don't want to worry about whether or not fibers can be inhaled
    and whether or not that might be harmful for your health,

Then you might want to buy metal ductwork instead; end of story.
The lack of a
proper duct fitting
adds about 45 feet
in "Equivalent
Length" to the
duct system;
considering a
normal duct
system would be
around 100 feet,
that's huge.

It's also a Code
The air side surfaces
of duct board are
sealed against fiber
cut edges are not.
Industry Standard
Attaching Board
to Metal
This brain surgeon  
felt he should glue his
joints together using
silicone caulk

Note the shreds of
fiberglass along the
bottom of the duct.

Will fibers carried into
the breathing zone?
Here's what
happens when a
downflow coil
overflows into
fibrous glass duct:
All those black
spots are mold; a 93
year old occupant
suffered adverse
(Image coming)