Duct Board vs Sheet Metal:


Rising steel prices highlight duct board benefits

Price, energy savings, acoustical performance all favor fiber glass option


Rising steel prices(SHELBYVILLE, Ind., April 29, 2004)  The superior thermal and acoustical performance of fiber glass duct board have always commanded a higher price than bare sheet metal ducts – until a dramatic spike in steel prices.  The increase in steel costs in recent months has hit the commercial, industrial and residential construction markets especially hard, and has made fiber glass duct board the performance choice and a lower-cost option than even bare sheet metal.
Many sheet metal contractors are reporting price increases of up to 250% in the last 90 days, driven by increased world demand for steel, coke shortages and the devaluation of the U.S. dollar. Commercial Duct Systems (CDS) of Charlotte, North Carolina, is one of many fabricators to experience triple-digit metal price increases.
“I recently quoted a condominium project at about $60,000 for the metal,” said Rick Nagy, owner of CDS.  “Now the same job is $90,000 for the metal alone. I’m doing it in 1” (Knauf) Duct Board AGM for about $55,000.  The owner is getting superior performance at a cost that is now a savings – the price increases are really encouraging people to take a look at duct board.”
Fiber glass duct board has always been a competitive option compared to sheet metal lined or wrapped with insulation.  However, bare sheet metal ducts were often specified where cost was the primary consideration, even though they presented many problems – thermal performance and noise transmission primary among them. The steel price upswing has erased even this advantage for sheet metal ductwork.  Now the superior performance of fiber glass is also the lower cost option.
“We’ve said all along that fiber glass duct board cuts utility bills and dramatically reduces HVAC-related noise – cross-talk between offices and the ‘booming and crackling’ that come as sheet metal ducts expand and contract,” said Mark Smith, Knauf Insulation’s Marketing Manager for Air Handling Products.  “Now that duct board is actually less expensive than sheet metal, why choose a less effective system?”
Knauf Insulation’s duct board products are known for their thermal and acoustical performance, as well as their durability on the job site.  Knauf’s Duct Board AGM adds a lightweight mat facing to the airstream surface of standard duct board, providing a smooth interior surface for optimum performance.  Duct Board M, Knauf’s premium duct product, offers a heavier black mat facing on the airstream surface for added durability. The exteriors of both products are faced with a durable FSK facing for superior condensation control and aesthetic appeal. 
Both products are independently certified by the GREENGUARD ENVIRONMENTAL INSTITUTE to meet the industry’s toughest indoor air quality standards for respirable particles and volatile organic compounds (VOCs), including formaldehyde. GREENGUARD Certification standards are based on recognized IAQ criteria used by the State of Washington, the U.S. EPA, OSHA and the World Health Organization to provide specifiers with IAQ peace of mind.
Knauf Duct Board M and AGM can also be formed into the Knauf Max 10 System, a 10-sided configuration that offers efficient airflow competitive with round sheet metal ductwork, but with improved energy efficiency and noise control.  Color PVC jacketing is available for the Max 10 system to offer a clean, finished appearance that blends with any open ceiling or décor.  Cost savings are even greater compared to spiral metal ducts, making Max 10 an attractive alternative for all exposed commercial applications.
“Knauf offers a variety of choices for creating a cost-effective duct system with superior thermal and acoustical performance,” said Smith.  “Rising steel prices have tremendously impacted the construction industry, and these increases will remain into the near future. Even though steel costs will gradually decrease, they will never again reach the level where bare sheet metal can compete with fiber glass. We clearly have a superior product at a lower cost.”
Knauf Insulation is a leading U.S. manufacturer of thermal and acoustical insulations for residential, commercial, industrial, marine, OEM and metal building applications.  For more information about Max 10, Duct Board-M or other Knauf’s products, visit the Knauf Web site (www.KnaufUSA.com) or contact Knauf Insulation, One Knauf Drive, Shelbyville, IN 46176; 800.825.4434, ext. 8212.

Specs on duct board ;





Ducts are used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. These needed airflows include, for example, supply air, return air, and exhaust air. Ducts also deliver, most commonly as part of the supply air, ventilation air. As such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort.

A duct system is often called ductwork. Planning ('laying out'), sizing, optimizing, detailing, and finding the pressure losses through a duct system is called duct design


Ducts can be made out of a variety of materials.


 Polyurethane and Phenolic insulation panels (pre insulated aluminum ducts)


Traditionally, air ductwork is made of sheet metal which is installed first and then lagged with insulation as a secondary operation. Ductwork manufactured from rigid insulation panels does not need any further insulation and is installed in a single fix. Light weight and installation speed are among the features of pre insulated aluminum ductwork, also custom or special shapes of ducts can be easily fabricated in the shop or on site.

The ductwork construction starts with the tracing of the duct outline onto the aluminum pre insulated panel, then the parts are typically cut at 45 degree, bent if required to obtain the different fittings (i.e. elbows, tapers) and finally assembled with glue. Aluminum tape is applied to all seams where the external surface of the aluminum foil has been cut. A variety of flanges are available to suit various installation requirements. All internal joints are sealed with sealant.

Among the various types of rigid polyurethane foam panels available, a new water formulated panel stands out. In this particular panel, the foaming process is obtained through the use of water instead of the CFC, HCFC, HFC and HC gasses. The foam panels are then coated with aluminum sheets on either side, with thicknesses that can vary from 80 micrometers for indoor use to 200 micrometers for external use in order to guarantee the high mechanical characteristics of the duct

A rigid phenolic insulation ductwork system is available and complies with the UL 181 standard for class 1 air ductwork.


 Fiberglass duct board (pre insulated non metallic ductwork)


Also the fiberglass panels provide built-in thermal insulation and the interior surface absorbs sound, helping to provide quiet operation of the HVAC system. The duct board is formed by sliding a specially-designed knife along the board using a straightedge as a guide; the knife automatically trims out a "valley" with 45° sides; the valley does not quite penetrate the entire depth of the duct board, providing a thin section that acts as a hinge. The duct board can then be folded along the valleys to produce 90° folds, making the rectangular duct shape in the fabricator's desired size. The duct is then closed with staples and special aluminum or similar 'metal-backed' tape. Commonly available duct tape should not be used on air ducts, metal, fiberglass, or otherwise, that are intended for long-term use; the adhesive on so called 'duct tape' dries and releases with time.


 Flexible Ducting


Flexible ducts, known as flex, have a variety of configurations, but for HVAC applications, they are typically flexible plastic over a metal wire coil to make round, flexible duct. In the United States, the insulation is usually glass wool, but other markets such as Australia, use both polyester fiber and glass wool for thermal insulation. A protective layer surrounds the insulation, and is usually composed of polyethylene or metalized PET. Flexible duct is very convenient for attaching supply air outlets to the rigid ductwork. However, the pressure loss through flex is higher than for most other types of ducts. As such, designers and installers attempt to keep their installed lengths (runs) short, e.g., less than 15 feet or so, and to minimize turns. Kinks in flex must be avoided. Some flexible duct markets prefer to avoid using flexible duct on the return air portions of HVAC systems, however flexible duct can tolerate moderate negative pressures - the UL181 test requires a negative pressure of 200 Pa.




Fabric ducting, also known as air socks, duct socks or textile ducts, are designed for even air distribution throughout the entire length. Usually made of special permeable polyester material, fabric ducts act like a conventional system with much more diffusers.

Fabric ducts are normally used where even air distribution is essential. Due to the nature or the air distribution, textile ducts are not usually concealed within false ceilings, because of this, they are manufactured in various colors to coordinate with the interior.

Fabric ducts fitted above a ceiling will need to be pvc coated so that no maintenance is required to maintain performance.

Because air passes through the body of a textile duct, it is impossible for condensation to form on the ducts surface, they can therefore be used where air is to be supplied below the dew point, without needing to be insulated.

 Duct system components

Besides the ducts themselves, complete ducting systems contain many other components.


 Vibration isolators


A duct system often begins at an air handler. The blowers in the air handlers can create substantial vibration and the large area of the duct system would transmit this noise and vibration to the inhabitants of the building. To avoid this, vibration isolators (flexible sections) are normally inserted into the duct immediately before and after the air handler. The rubberized canvas-like material of these sections allow the air handler to vibrate without transmitting much vibration to the attached ducts.




Downstream of the air handler, the supply air trunk duct will commonly fork, providing air to many individual air outlets such as diffusers, grilles, and registers. When the system is designed with a main duct branching into many subsidiary branch ducts, fittings called take-offs allow a small portion of the flow in the main duct to be diverted into each branch duct. Take-offs may be fitted into round or rectangular openings cut into the wall of the main duct. The take-off commonly has many small metal tabs that are then bent to retain the take-off on the main duct; round versions are called spin-in fittings. Other take-off designs use a snap-in attachment method, sometimes coupled with an adhesive foam gasket to provide improved sealing. The outlet of the take-off then connects to the rectangular, oval, or round branch duct.


 Stacks, boots, and heads


Ducts, especially in homes, must often allow air to travel vertically within relatively thin walls. These vertical ducts are called stacks and are formed with either very wide and relatively thin rectangular sections or oval sections. At the bottom of the stack, a stack boot provides a transition from an ordinary large round or rectangular duct to the thin wall-mounted duct. At the top, a stack head can provide a transition back to ordinary ducting while a register head allows the transition to a wall-mounted air register.


 Volume Control Dampers


Ducting systems must often provide a method of adjusting the volume of air flow to various parts of the system. VCD's (Volume Control Dampers - Not To Be confused with Smoke/Fire Dampers) provide this function. Besides the regulation provided at the registers or diffusers that spread air into individual rooms, dampers can be fitted within the ducts themselves. These dampers may be manual or automatic. Zone dampers provide automatic control in simple systems while VAVs allow control in sophisticated systems.


 Smoke/Fire Dampers


Smoke and Fire dampers are found in ductwork, where the duct passes through a firewall or fire curtain. Smoke dampers are automated with the use of a mechanical motor often referred to as an Actuator. A probe connected to the motor is installed in the run of duct, and detects smoke within the duct system which has been extracted from a room, or which is being supplied from the AHU (Air Handling Unit) or elsewhere within the run. Once smoke is detected within the duct, the Actuator triggers the motor release and the smoke damper will automatically close until manually re-opened.

You will also find Fire dampers in the same places as smoke dampers, depending on the application of the area after the firewall. Unlike smoke dampers, they are not triggered by any electrical system, which is perfect in the event of an electrical failure where the Smoke dampers would fail to close. A fire damper is held open by a bar crossing the corrugated screen, which will break and allow the damper to close when air in the duct is above a certain temperature. This again will then have to be manually re-opened.




Plenums are the central distribution and collection units for an HVAC system. The return plenum carries the air from several large return grills (vents) to a central air handler. The supply plenum directs air from the central unit to the rooms which the system is designed to heat or cool.


 Terminal units


While single-zone constant air volume systems typically don't have them, other types of air distribution systems often have terminal units in the branch ducts. Usually there is one terminal unit per thermal zone. Some types of terminal units are VAV 'boxes' of either single or dual duct, fan-powered mixing boxes of either parallel or series arrangement, and induction terminal units. Terminal units may also include either, or both, a heating or cooling coil.


 Air terminals


'Air terminals' are the supply air outlets and 'return' or 'exhaust air inlets'. For supply, diffusers are most common, but grilles, and for very small HVAC systems such as in residences, 'registers' are also used widely. Return or 'exhaust grilles' are used primarily for appearance reasons, but some also incorporate an air filter and are known as 'filter returns'.

 Duct cleaning

The position of the U.S. Environmental Protection Agency (EPA) is that "If no one in your household suffers from allergies or unexplained symptoms or illnesses and if, after a visual inspection of the inside of the ducts, you see no indication that your air ducts are contaminated with large deposits of dust or mold (no musty odor or visible mold growth), having your air ducts cleaned is probably unnecessary."

Studies by the EPA and the Canadian Mortgage and Housing Corporation (CMHC) in the 1990s has lead CMHC to conclude that "duct cleaning will not usually change the quality of the air you breathe, nor will it significantly affect airflows or heating costs".