Volume 36, June 1999: Insulation Attachment

Perspectives
A Quarterly Information Source from Benchmark, Inc.
Volume 36 June 1999

Insulation Attachment

One of the more important performance characteristics of a roofing system is the attachment of the insulation to the substrate. Poorly attached insulation causes splits and ridges to develop in the roof membrane. Most importantly, a roof with poorly attached insulation is susceptible to wind uplift damage and blow-off.

Properly fastened insulation stabilizes the differences in thermal induced movement between the deck and the roof membrane. Restrained insulation minimizes the potential for splitting and ridging that occur when the roof membrane moves unrestrained. Properly fastened insulation is resistant to wind uplift forces and interior building pressures, keeping the roof secure on the building.

There are three primary methods of insulation attachment:
Mechanical Attachment - Mechanical fasteners consist of screws for metal decks, and various types of nails for gypsum and wood decks. Each of the fasteners range in size and thickness depending on the requirements of the system.
Adhesive Attachment - As the name implies, adhesive attachment involves gluing the insulation to the deck surface. Types of adhesives range from asphalt to low rise polyurethane foams. Adhesives are almost exclusively used on solid deck surfaces such as concrete, or the recover of an existing smooth roof. If you are insured by Factory Mutual, check with them before using an adhesive on a metal deck.
Ballast - Adding weight over the top of a loose laid membrane is the premise of ballast attachment. This weight can be in the form of round rocks applied at 10-15 pounds per square foot, concrete pavers of an equivalent weight, or an interlocking paver system that is lighter, but locked together to form a monolithic surface on the roof.

Determining how the insulation is going to be attached is dependent upon the type of deck and the type of roof assembly that is being installed. If a built-up, modified or fully adhered system is installed, the attachment of the insulation is the single most critical component of the assembly. In mechanically attached single-ply membranes, the insulation attachment is not as critical, as the insulation is not used to secure the roof membrane. In this case, fastening of the insulation is required to secure the insulation only to prevent it from moving and buckling under the membrane.

The success of good insulation attachment is dependent upon two factors:
Fastener density
Pull-out resistance

Each of these factors is dependent upon the other. The pull-out resistance determines the fastener spacing. Low pull-out strength from the substrate requires that fastener density be increased.

Fastener pull-out tests are usually performed on nailable decks prior to construction to determine the pull-out resistance in pounds. Typically the results from these tests reveal pull-outs in the 30-90 pound range. Using ANSI 7 or Factory Mutual data, this number is correlated to the anticipated wind speeds for the local area and building height. Once this correlation is made, the proper fastener, fastener spacing and fastener layout can be determined.

For a typical 20 or 22 gauge steel deck in good condition, additional testing is usually not required. The majority of manufacturers have tested their systems on these types of substrates and have a fastener pattern based on the pull-out resistance commonly found on this thickness of steel deck. Typically, this resistance is in excess of 400 pounds. If the deck is less than 22 gauge, tests are required, as additional fasteners will probably be necessary.

Hot asphalt by nature is an excellent adhesive to concrete decks. It is important that the deck be properly prepared prior to the installation of the insulation. This includes thoroughly cleaning the deck of all loose debris and coating the deck with an asphalt primer to ensure a proper bond. During installation, it is also important that the insulation boards be walked into the asphalt, as most decks contain some variations in surface height. By walking the boards in, pressure applied to the board assures bonding between the board and the deck surface.

Insulation being placed
over a sprayed adhesive.

Adhesives are most commonly used when asphalt is not desirable. Short of performing test patches, there is not a test to determine if an adhesive will perform on a particular substrate. One must rely on the past performance of a particular adhesive and make sure that proper deck preparation and adhesive installation is followed.

Never use 4' x 8' sheets of insulation with any type of adhesive or asphalt attachment. This size of insulation has a tendency to bridge imperfections in the deck surface, resulting in little or no contact with the adhesive.

Securement of the insulation by the use of ballast is determined by wind zone, building height and type of ballast. Typical ballast securement requires 10-15 pounds per square foot of material be placed on the roof to hold the insulation and membrane in place. When considering a ballast application, it's important that the structural capacity of the building be determined.

In mechanically attached membrane systems the insulation is not used for system securement. Fasteners placed in the insulation are used to keep the insulation from moving under the membrane. In most cases, the minimum number of fasteners per 4' x 8' sheets of insulation is 4. However, it has been our experience that certain insulations have a tendency to curl if not restrained by the use of additional fasteners.

The attachment of the insulation is critical to the proper performance of built-up, modified and fully adhered single-ply membranes. Improper securement can result in shortened service life up to catastrophic failure of the roof system. Before installation of any system, determine what the requirements are, either from the system manufacturer, local building codes, or your insurance representatives.