Fiberglass Roof Insulation

by Alvin Nunnikhoven, RRC

In continuing our feature on insulations, this article will focus on the use of rigid fiberglass insulation boards. Fiberglass insulation is manufactured in two forms, batt rolls and rigid boards. Fiberglass batts are more commonly used in residential construction, while rigid boards are used as a substrate for low-slope roof applications.

Rigid fiberglass insulation boards are composed of inorganic glass fibers and were first used in commercial roofing systems in 1941. No surface treatment is provided on the bottom of the insulation, while the top surface is faced with a glass fiber reinforced, asphalt kraft paper. The top surface treatment offers some impact resistance, while also providing a suitable mopping surface for conventional built-up roof applications.

Fiberglass insulation is dimensionally stable and will not expand with heat or swell when exposed to moisture. Additionally, fiberglass is flexible, chemically inert, and fire resistant. Its flexibility will allow the insulation to conform to minor deck irregularities when compared to other rigid, less flexible insulations. Fiberglass may be used as a thermal barrier over steel roof decks because of its excellent fire-resistance properties.

Fiberglass insulation is most prevalently used under built-up and modified bitumen roof systems. Its use with single-ply roof systems is rather limited due to its low compressive strength and the incompatibility of its asphalt kraft paper and pvc based single-ply sheet membranes. Because of its low compressive strength of 12 pounds per square inch, fiberglass insulation may not be suited for use on roofs subjected to concentrated or extensive foot traffic. However, fiberglass will accommodate normal maintenance traffic.

Fiberglass insulation is quite versatile in the means of attachment to the substrate. It may be mechanically attached to steel and wood decks, applied in hot asphalt to concrete decks or nailed base sheets on gypsum and lightweight concrete decks, or applied in adhesive to concrete decks. Depending upon the attachment selected, a Factory Mutual 1-60 or 1-90 wind uplift rating can be achieved. When fiberglass insulation is to be installed over steel decking, the width of the rib openings will be a determining factor in the thickness selected. The maximum flute span is 1 3/4" for a 3/4" thick board, and up to a 4" flute span for boards having 1 5/8" or greater thickness.

Fiberglass insulation is available in 4' x 4' and 4' x 8' sizes. Available thicknesses and thermal values are illustrated in the table on page 5. The thicknesses will vary in availability by manufacturer. Only two manufacturers of fiberglass insulation are currently listed in the NRCA 1998 Commercial Low Slope Roofing Materials Guide.

The thermal retention values for fiberglass insulation after 1, 5 and 10 years of service are estimated at 100%. These values assume a continuous mean temperature differential and constant moisture content are maintained. However, studies have shown that the original thermal value can be restored if wet insulation, caused by roof membrane damage, is dried by venting or other physical means. We have found instances where wet fiberglass insulation, installed over a steel roof deck, has dried by the venting of water vapor through the deck flutes to the building interior.

Tapered fiberglass insulation is available but is not produced by the insulation manufacturers. Fiberglass insulation blocks are purchased by independent fabricators and cut to the desired slope and size. Common slopes available are 1/8", 1/4" and 1/2" per foot. The number of independent fabricators producing tapered fiberglass is limited, meaning the lead time of fabrication and delivery can vary greatly.

The cost of flat fiberglass insulation is approximately $0.50 per square foot per inch thickness. The average cost of tapered fiberglass is approximately $1.50 - $1.75 per square foot. Many factors can affect the cost of fiberglass insulation, either flat or tapered, such as project size, time of construction, design and installation complexities, tapered fabricator, etc.

At Benchmark, we recommend fiberglass insulation be installed in a two layer application for built-up (or modified bitumen) membranes. The bottom layer of insulation is mechanically attached over a steel deck or applied in hot asphalt on concrete decks. The second layer of fiberglass is applied in hot asphalt over the bottom layer, with the joints of the second layer staggered from those of the bottom layer. With fiberglass insulation, we do not recommend mechanically attaching both layers together. Compression of the insulation adjacent to the fastener heads could place undue stress on and possibly rupture the membrane.

While fiberglass insulation is quite versatile and can accommodate a wide variety of design parameters, the design criteria for each individual roof will ultimately determine if fiberglass is appropriate for use.