Perlite Roof Insulation

by: Kevin Ernster, RRC

In our last issue of Perspectives, we highlighted the importance of insulation as a component of the roof system. The type of insulation used in a roof system influences the overall performance of the system in terms of fire resistance, wind uplift resistance and cost. In some cases, selection of an incompatible insulation will lead to the premature failure of the roof membrane. A wide variety of insulations are available for installation in roof systems. In this issue, we focus on the characteristics of perlite insulation.

Perlite insulation is classified as an inorganic, rigid board insulation. It is available in 2' x 4' and 4' x 4' boards with common thickness of 1/2" to 3". We do not recommend the use of the 1/2" board due to reported problems with high moisture content and difficulty in handling, as it tends to break easily. Perlite has excellent fire resistance and can be used with other less fire resistant insulations to qualify a roof system for an acceptable fire rating.

Perlite insulation is composed of expanded volcanic glass and wood fibers bonded with asphaltic binders. This forms a rigid board that is light in weight, dimensionally stable and high in compressive strength. At one time, perlite was the most common insulation used in roofing. Although still popular, its low R-value of 2.78 per inch, and tendency to absorb moisture has lessened its popularity.

Perlite is not recommended for use under mechanically attached or fully adhered single-ply membranes. When used under a mechanically attached single-ply, perlite insulation has a tendency to absorb the condensation that typically forms on the bottom side of the single-ply sheet. Over time, the insulation will become saturated and completely break down. As perlite is a friable material (it breaks down into fine powder rubbed), it sometimes breaks down from continued rubbing under the bellowing action of mechanically attached systems. Due to its poor interlaminar strength (4 lbs./inch peel), it's also a poor substrate for fully adhered single-ply membranes. Because single-ply membranes are highly flexible, the movement of the membrane promotes delamination of the insulation in the fully adhered application.

If R-value is not of concern, perlite is a good insulation under built-up roof systems, particularly where there is extensive foot traffic. With a compressive strength of 35 pounds per square inch, it can accommodate normal traffic encountered from construction, routine HVAC maintenance, and traffic normally found in industrial applications. In many cases, as with a metal deck, perlite will be installed under other less fire resistant types of insulation to achieve a fire rating. Perlite is also required as an overlay board in built-up applications where a foam insulation is used to achieve a high R-value. This type of application results from the inability of the foam insulation to absorb various vapors created in the application process. If the perlite is not installed, blistering of the built-up (or modified bitumen) membrane, when installed directly over foam insulation, may result as vapors expand. The perlite has the ability to absorb and diffuse these vapors. It should be noted that the great majority of built-up and modified manufacturers require the use of an overlay board of perlite or wood fiber over foam insulations.

In order to overcome the need to overlay a foam insulation, composite board insulation was developed. Composite board typically consists of a 1/2" or 3/4" layer of perlite bonded to a minimum of 1" foam insulation in the manufacturing process. This preassembled board reduces labor costs at the time of installation, as a two-step process is reduced to a one-step process.

Perlite is versatile in the means of attachment to the substrate. It can be mechanically attached to steel decks, nailed to gypsum and lightweight concrete decks, applied in hot asphalt to concrete decks or applied in adhesive to both concrete and metal decks. In each of these cases, a wind rating of up to a Factory Mutual 1-90 can be achieved depending on the type of fastener and the fastening pattern. When installing over a metal roof deck, the width of the flutes will often determine the thickness of the insulation. Typically 3/4" of thickness will span a flute of up to 1". A 1" board will span up to 1/58", and a 11/2" thick board will span up to a 2 5/8" flute.

At Benchmark, we typically specify perlite insulation for built-up roof systems in a two layer application. If a higher R-value is required, then the bottom layer of insulation is usually a foam board that is mechanically attached in a steel deck application, and applied in hot asphalt on concrete decks. If the R-value of perlite is acceptable, it is applied as the bottom layer of insulation. The second layer of perlite is applied in hot asphalt with the joints of the top layer staggered from those of the bottom layer. We do not typically specify mechanically attaching both layers together, due to thermo-bridging of the mechanical fasteners. This is also one of our concerns with the use of composite insulation.

Perlite insulation is a versatile product that can serve a variety of purposes in your roofing system. However, it is only one of several insulations that are available for today's commercial roofing systems. The criteria for each individual project will ultimately determine if perlite is appropriate for your project.