Perlite is a light gray, glassy volcanic rock with a vitreous, pearly luster and a characteristic concentric or perlitic fracture. It differs from other volcanic glasses principally in its combined water content, which produces the unusual characteristic of expanding up to 20 times its original volume when exposed to rapid controlled heating. The resulting expanded particles are spherical in shape, usually fluffy or frothy, highly porous due to a foam-like cellular internal structure, and have a very low density. Expanded perlite exhibits very low thermal conductivity, low sound transmission, high fire resistance, a large surface area, and low moisture retention. It is chemically inert, sterile, non-toxic and non-fibrous, free of organic impurities, and has a neutral pH. Perlite, obsidian perlite, and pumice (a related product) are regarded as essentially "environmentally safe" building materials. Expanded perlite can be manufactured to weigh as little as 2 pounds per cubic foot (32 kg/m³), making it adaptable for numerous applications.

Insulation
Reductions in heat transmission of masonry walls of 50% or more may be obtained with perlite loose fill insulation. Thermal performance tests using ASTM C236 Test Method for Steady State Thermal Performance of Building Assemblies by Means of a Guarded Hot Box have shown perlite to be a superior concrete block insulation when compared to vermiculite, EPS inserts, EPS beads, and foamed-in-place insulations.

Recent testing of an 8 inch medium weight concrete block wall filled with perlite loose-fill insulation using dynamic temperature conditions simulated by a National Bureau of Standards 24 hour (diumal) test cycle indicates that the measured total energy (total heat flow through a wall for a 24 hour period) was only 72% of the total energy predicted by steady state analysis.

Non-Combustible
The fusion point of perlite is 2,300°F (1260°C)

ASTM E84
Flame Spread: 0
Fuel Contribution: 0
Smoke Density: 0

4 Hour Fire Ratings
Underwriter's Laboratories Design No. U905 shows that a 2 hour rated 8, 10, or 12 inch concrete block wall is improved to 4 hours when cores are filled with water repellent-treated perlite.

Water Repellent
The non-flammable water repellent treatment significantly improves low water retention properties of perlite. Laboratory tests on water transmission by Structural Clay Products Research Foundation show that a cavity wall filled with treated perlite resists transmission of water to the interior wythe. Performance of overall wall was rated "excellent" in accordance with procedures established by the National Bureau of Standards in BMS 82. However, it should be noted that treated perlite insulation will not waterproof a poorly constructed masonry wall. Good construction practice should also incorporate the use of weep holes covered with copper, galvanized steel, or fiberglass screening.

Sound Reduction
Perlite loose-fill insulation has the ability to fill all voids, thus enabling it to reduce the airborne sound transmission through walls. Lightweight 8 in. (20 cm) masonry block filled with perlite achieves an STC of 51 which exceeds HUD sound transmission standards.

Economical
Perlite loose-fill masonry insulation offers excellent thermal and fire resistant properties at an economical cost. It is lightweight and pours easily and quickly without requiring special equipment or skills.

Standards, Specifications, and References
ASTM Specification C549 Perlite Loose Fill Insulation
ASTM Specification C520 Density of Granular Loose Fill Insulation
ASTM Test Method C236 Test Method for Steady State Thermal Performance of Building Assemblies by Means of a Guarded Hot Box
ASTM Test Method E84 Test for Surface Burning Characteristics of Building Materials
FHA Use of Materials Bulletin UM-37
GSA Commercial Item Description A-903 - Insulation, Thermal (Expanded Perlite)
Brick Institute of America Technical Notes No. 21A
National Concrete Masonry Association Tek 101A
Federal Specification HH-1-515D for: Smoldering Combustion/Critical Radiant Flux