Fiber board used for the fabrication of supply and return air duct work for HVAC systems is constructed of resin-bonded strands of glass fibers (e.g., fiberglass) that are formed into flat sheets, typically 4 foot wide by 10 foot long or 4 foot wide by 8 foot long; it typically comes in thicknesses of 1, 1.5, and 2 inches. The exterior surface of the sheet is covered with a reinforced foil laminate air/vapor barrier. The interior surface that is in contact with air flow is not as smooth as metal because it consists of bare bonded fiberglass (Figure 1). Fibrous glass duct boards are available with a durable, hard interior surface treatment to prevent eroding of the bonded fiberglass; however, this type of fiber board is very expensive. Many brands of fiber board are also available with an antimicrobial coating on the interior surface. Fiber board ducting can be used for heating and cooling ducts; it is not recommended for ventilation ducts, especially in humid climates.
Figure 1 - Fiberglass duct board typically comes in sheets 4 foot wide by 10 foot long.
One advantage of fiber board is the fact that it is inherently insulating; the fibrous glass board carries an R-value of 4.3 per inch, so thicknesses of 1, 1.5, and 2 inches yield R-values of 4.3, 6.5, and 8.7. The product itself is air tight so the air tightness of the overall fiber board duct system is dependent on the care with which the sections are fabricated and the joints and seams are cut and sealed. The durability of the fiber board is also contingent on the care taken in constructing the ducts. The fabricator must be careful to not fracture the bonded fibers. A crack in the bonded fiberglass may not be noticed if it is concealed by the reinforced foil laminate but, over time, it can lead to the erosion of the glass fibers, creating a durability issue and compromising the insulation value of the duct.
Fiber board ducts have been tested for safe use at velocities up to 5,000 feet per minute and temperatures up to 250°F (121°C) (NAIMA 2010).
HVAC contractors who use fiber board ducts typically purchase duct board sheets by the carton (each carton contains 5 or 6 sheets) and fabricate the ducts themselves in the shop or on site. The sheets are available with straight edges or with factory-cut male-female shiplapped edges along each of the long sides (Figure 2). The sheets are cut and folded to the designed duct dimensions by the fabricator. Each 4-foot section is fitted together using the shiplapped ends (Figure 3).
Figure 2 - Fiber board sheets are available with factory-cut male/female shiplapped ends along the long side of each sheet.
Figure 3 - A length of rectangular HVAC duct assembled using duct board. Note the shiplapped end for fitting to the next section of duct. The gray interior on this fiber board indicates an antimicrobial coating.
Fiber board ducts are fabricated using either specially designed hand tools or grooving and closure machines. The duct sections are assembled by matching factory or field-cut shiplapped male and female ends. The shiplapping ensures close-fitting, smooth joints when duct sections are joined together. Duct fittings such as tees, offsets, elbows, and transitions are also fabricated from these sections or from flat duct board.
Typical tools used to fabricate fiber board duct systems in the field include a yardstick or tape measure, straight edge, duct board knife, marking pen, and edge-cutting tools. If hand cutting, the tools used should be fiber board cutting tools specifically designed for the intended cuts, to ensure a consistent connection. The cutting tools come in different sizes depending on the thickness of the fiber board being used: 1 inch, 1.5 inch, or 2 inch. The tool manufacturers color-code the tools red, orange, or grey to identify which type of edge or end cut the tool will make: the red tool cuts a V-shaped groove, the orange tool cuts a shiplap, and the grey tool makes a final or female shiplap (see Figure 4). When forming the inside corners of the duct, the fabricator can use either the V-groove or modified shiplap method. Some manufacturers recommend the modified shiplap method as providing a more stable corner (Knauf 2006). Many installers prefer the V-groove method for straight duct sections because it is much faster to assemble. They use the shiplap method for assembling and connecting fittings, branches, and other modifications.
Fiber board duct can also be machine fabricated in the shop using a grooving machine. A grooving machine can be set up to provide the same kinds of cuts as the hand tools, with the advantage that it can make multiple cuts at one time with consistency and precision.
In addition to the seam and edge cuts, the fiberglass is cut away 1.5 inches from the edge on one side leaving a foil flap that will serve to cover the finished seam. After the cuts are made, the fiber board is folded together to form a rectangle, and the flap is pulled taut over the opposite edge, then stapled in place using out-clinching staples. All seams and joints are sealed with UL-181A heat-sealing tape, pressure-sensitive tape, or mastic and fiber mesh tape.
When connecting fiber duct board sections to sheet metal components such as equipment flanges, fasteners such as sheet metal screws and washers are used to carry the mechanical load. Mastic and glass fiber fabric are typically used to air seal the connections at these points. However, approved pressure-sensitive aluminum foil tape or heat-activated aluminum foil/scrim tape may be used to seal the fiber board to the sheet metal when the operating pressure is less than 1 in. w.g. (250 Pa) and when sheet metal surfaces are cleaned in accordance with tape manufacturers’ instructions. Closure details may be found in Section IV of the North American Insulation Manufacturers Association Fibrous Glass Duct Construction Standard (NAIMA 2010).
Figure 4 - With a marker, a knife, measuring tape, a ruler, and a straight edge, plus edge-cutting tools (which are color-coded red, orange, and grey by the manufacturer to indicate the cut design), the installer can cut and fabricate a complete fiber board duct system.
Example instructions for the field fabrication and assembly of one section of fiber board duct are provided below. Detailed instructions and dimensions for hand cutting and machine cutting fiber board ducts in several configurations are available on line from insulation manufacturers such as Knauf and Johns Manville and from the North American Insulation Manufacturers Association.
How to Field Fabricate a Fiber Board Duct with Inside Dimensions of 14x6 Inches
1. Lay out a piece of 10x4 foot piece of fiber board, 1.5 inches thick, foil side down.
2. Starting at the left edge, mark out center lines for each corner, plus the line for the start of the stapling flap (on the left). Because you will only need 52.5 inches of length to make a duct with inside dimensions of 14x6 inches, you could mark up and cut the 120-inch fiber board to get two 52.5-inch pieces with 15 inches left over.
3. On the right edge of the board (opposite the end that has the stapling flap), cut the closure joint rabbet with a gray (female shiplap) tool. Use fiber board cutting tools specifically designed for the intended cut and for the thickness of fiber board you are using. Know your tools - cutting tools have a lip where the tool rests against the straight edge to cut a straight line. The distance from the lip to the cutting blade must be taken into account when centering the blade for the cut and this distance varies among tool brands.
4. Use a red V-groove tool to cut the grooves on the first three centerlines for the corner folds.
5. Prepare the stapling flap by making a perpendicular knife cut through the insulation 1.5 inches in from the end, taking care not to cut the foil facing. Then, strip the fiberglass insulation from the facing.
Figure 5 - The red cutting tool is used to cut a "V-groove" in the fiber board allowing each corner of the assembled duct to form a 90-degree bend, the gray tool is used to cut a shiplap joint at one edge, and the duct knife is used to trim and clean the fiberglass from the foil at the opposite edge so it can be used as a flap to cover the seam when the duct is formed.
6. Fold the fiber board to assemble the duct section, making sure that the corners meet and fold correctly for a snug fit. Be sure to properly seat the flush edge in the rabbet cut at the closure joint.
7. To staple the seam, pull the flap taught over the opposite edge and, while holding the seam closed, slightly flatten the duct (about 30° beyond square). Staple the seam using out-clinching staples, starting at the center and working out toward each end of the seam, stapling one-half inch in from the edge at 2-inch intervals.
8. Connect this section to other sections, making sure the male and female shiplapped ends of each section are firmly seated. Smooth the stapling flap over the adjoining section and staple at 2-inch intervals.
9. Seal all longitudinal and circumferential joints using one of three approved methods:
Seal the seams with UL-181A-H heat-activated aluminum foil/scrim tape. Center the tape strip over the edge of the stapling flap, press down, and heat seal following the tape manufacturers’ instructions.
Seal the seams with UL-181A-P pressure-sensitive foil tape. Use tape that is at least 1 inch wider than the thickness of the fiber board. Center the tape over the edge of the stapling flap. Press down and rub in carefully using a squeegee or similar tool until the scrim pattern from the duct board facing shows through the tape. Heat seal if the temperature is below 40°F (4°C).
Seal the seams with UL-181A mastic and fiber mesh tape. Brush on a 4-inch-wide coating of mastic over the stapled flap. Embed a strip of 3-inch-wide open-mesh glass fiber fabric tape in the mastic. Apply an additional coat of mastic over the tape to cover and fill in the mesh.
10. Where sheet metal connections are made with the fiber board duct, connect the fiberboard to the metal with flat head sheet metal screws that are one-half inch longer than the thickness of the fiber board and use 1-inch outer-diameter washers. Seal the metal-fiber board seams with mastic or approved tapes after cleaning the sheet metal surfaces of any oily residue per the tape manufacturers’ instructions.
Figure 6 - To assemble and seal the fiber board ducts, use outward-clinched staples, and UL-181A approved heat-activated or pressure-sensitive tape or mastic and fiber mesh tape.