INSULATION TYPES and SELECTION PROCESSES
by Garth Liseth, Crossroads C&I Distributors Inc.
photos courtesy of Apollo Sheet Metal Ltd.

A SMACNA contractor has an incredible lifetime of technical information that he must continually learn, commit to memory and successfully master in his everyday work environment. Not that long ago he was given one more piece of the puzzle to deal with when he was faced with his shop doing its own internal duct insulation.

I can remember, in this marketplace not that many years ago, when Heat & Frost (Local 118) Insulators used to collect a shop’s metal duct work and fittings and have them lined and returned the next day. The insulators usually knew which insulation products they preferred as well as the performance parameters of the various available products, and had long-time connections with their wholesalers on a multitude of other mechanical insulation products. Granted, this was not the most efficient method and it was inevitable that one day it would change. With the gradual change away from this practice, I think the selection process of liner materials became more price driven and that usually the installation of
insulation in the sheet metal shop was often left to an apprentice.

I don’t think that anyone ever trained the sheet metal trade on selection practices for duct liners and since this was usually an entry-level function and if there was a material preference, this issue may not have been relayed to senior personnel. In the past there have been poor installation practices often caused by poor material selection, handling damage and resultant failures in the field. I think that both the mechanical insulation industry, consultants and sheet metal shops may have lost the reason and importance of duct liner usage and for the last few decades we have noticed that less liner is being specifi ed. Some consultants, after failures and fi bre erosion problems, have even taken it upon themselves to specify non-existent composites in an attempt to circumvent possible problems. This practice has made the suppliers grow prematurely grey.

So why should duct insulation be used? Duct liner is designed for application to the interiors of commercial and residential sheet metal heating, ventilating, and air-conditioning ducts operating at internal air temperatures not exceeding 121° C. Duct liner helps to attenuate the noise generated by central air handling equipment, air movement within the ducts, and occupant cross-talk. It also serves to reduce heat loss or gain through sheet metal duct surfaces and helps to prevent the formation of water vapour condensation both inside the duct and on its exterior surface. Fibrous glass duct liner insulations are manufactured from glass fi bres bonded with thermosetting resin. They are dimensionally stable, withstand abuse, and provide a cleanable, fi re-resistant interior surface designed to minimize air friction loss.

The major fi breglass insulation manufacturers have answered many of the earlier issues with the former drawbacks of their products and most liners now have the air stream surface faced with a coated mat that is bonded to the substrate. This stronger mat surface will also contain a water repellent EPA registered anti-microbial coating in order to reduce the potential of any mould growths and the edges are factory coated in order to seal the fi bres. Even with all these changes, we often see the consultant still pulling out some old specification reference to neoprene coating. I may be corrected but this style of product hasn’t been marketed for at least 20 years!

Why are some fibrous products friendlier to the touch than others? To answer this we must look at manufacturing technologies. Glass can be fiberized in a number of methods but the two most common methods are rotary and flamed-attenuation.

(1) The rotary process has molten glass running into a spinner or hollow cylindrical container with small holes drilled in the periphery. The spinner rotates at a very high speed, and the molten glass is thrown out by centrifugal force through the holes along the periphery. The fibres are then collected, sprayed with binder, formed into blankets and cured in an oven. Rotary glass blankets are heavier as the fibres are small and therefore more mass is required to achieve the thermals.

(2) The flame-attenuated method is more sophisticated and expensive method that utilizes raw glass that is first made into the form of marbles to facilitate inspection and handling. In producing fibres, the marbles are automatically fed into small pots, which are heated to very high temperatures. The molten glass is drawn through holes in platinum bushings installed in the bottom of the pots. As the filaments are pulled downward by rollers, they are drawn into very fine, controlled diameters. Then blasts from gas jet burners further attenuate the strands into long and even finer diameters. After being sprayed with binder they are deposited on a moving belt to form blanket, which is cured to the desired thickness
and density. The enormous amount of air entrapped by the extremely fine fibres made by this method produces insulation having the ultimate in conductivity versus density ratio. Insulation made from these fibres, having the same conductivity as those produced by any other method, will be lighter in weight and friendliness of feel. The latter property is important to those who have to work constantly with
these materials.

Another form of fibreglass liner is formed with textile glass fibres bonded with a thermosetting resin and again faced with a mat facing that prevents moisture from penetrating the air stream surface of the product. This product is usually heavier in nature with slightly poorer thermal conductivities and sound absorption capabilities. One very positive feature with a textile liner is that there are less loose fibres in fabrication when the material is cut, but the substate is more difficult to cut and seal.

One very positive feature with a textile liner is that there are less loose fibres in fabrication
when the material is cut, but the substate is more difficult to cut and seal.

Other products are also available such as cotton duct liners, reflective bubble pack insulations, elastomeric foams and other plastic composites but each of these present a number of inherent problems, are not readily available to the market and do not provide the same cost effective, thermal and sound absorption capabilities of fibreglass duct liners. When used correctly in modern air handling systems, fibrous glass duct liners improve the quality of the indoor environment by maintaining a consistent air temperature throughout the system, reducing condensation and absorbing noise from
the system’s operation.

The fabricator must ensure proper spacing and compression of mechanical fasteners as well as achieving sufficient (90% minimum) area coverage of an approved adhesive.

It behooves the industry to properly install fibreglass duct liner to the applicable standards and to ensure the continuity of industry support. The fabricator must ensure proper spacing and compression of mechanical fasteners as well as achieving sufficient (90% minimum) area coverage of an approved adhesive. All damaged areas of the air stream surface shall be repaired with an approved mastic. Metal
nosings shall be securely installed over transversely oriented liner edges facing the air stream at fan discharge, at access doors, and at any interval of lined duct preceded by unlined duct. Duct liner specifications abound but the best practices are found in the NAIMA Fiberglass Duct Liner Standard (obtainable from their website <www.naima.org>. Most manufacturers’ recommendations usually recommend that their products be installed in accordance with this Standard and that the SMACNA HVAC Duct Construction Standard be observed.

The following are some helpful websites that discuss duct liners:

www.naima.org
www.tiac.ca
www.insulation.org
www.bcica.org

Garth Liseth has been actively involved in the mechanical insulation field since 1973 and is a Past President of the Thermal Insulation Association of Canada. Garth is employed by Crossroads C & I Distributors Inc. in their Burnaby, B.C. office.

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Official Journal of Record for SMACNA-BC