Perlite for Insulation

by Nick Gromicko, CMI®

Perlite granulesPerlite is a naturally occurring siliceous rock used for thermal insulation in buildings. 
The United States is the world's largest producer and consumer of perlite. Other leading countries that produce perlite include China, Greece, Japan, Hungary, Armenia, Italy, Mexico, the Philippines, and Turkey. You may recognize it as the small white pebbles used in potting soil to improve aeration and moisture retention. 
After it is mined, perlite is heated to approximately 1,600° F (871° C), which causes its water content to vaporize and create the myriad tiny bubbles that account for the mineral’s unique physical properties. Perlite insulation is produced in a granular form as well as a powder form, but some manufacturers combine it with gypsum or other materials to turn it into an insulating board. In addition to its use as an insulator in buildings, perlite is used for the insulation of low-temperature equipment, such as super-cold storage and cryogenic tanks, as well as in food-processing applications.
Physical Properties and Identification

Insulation found in homes may be made of perlite if it possesses the following qualities:
  • its snowy white to grayish-white in color. The crude rock ranges from transparent light gray to glossy black, but the expanded form found in homes is easily identified by its white color;
  • it is lightweight. Expanded perlite can be manufactured to weigh as little as 2 pounds per cubic foot; and/or
  • its grain sizes may vary, but they generally are not larger than ¼-inch in diameter.

Perlite's Performance as an Insulator

Perlite is widely used as loose-fill insulation, especially in masonry construction, because of the following qualities that make it desirable:
  • low toxicity. According to the Perlite Institute, “No test result or information indicates that perlite poses any health risk.” Other insulators, such as asbestos, vermiculite (which may contain asbestos), and fiberglass are more hazardous;
  • chemical inertness, meaning it will not corrode piping, electrical or communications conduits. Perlite has a pH of around 7, which is similar to fresh water;
  • pliability. As a loose-fill insulator in masonry construction, perlite can be poured into the cavities of concrete block where it completely fills all crevices, cores, mortar areas and ear holes. The mineral will flow around any roughness, unevenness or exposed installations. It supports its own weight and will not settle or bridge;Perlite cement
  • high fire rating. Underwriters Laboratories has found that a two-hour rated, 8-inch (20.32-cm) concrete block wall is improved to four hours when cores are filled with silicone-treated perlite;
  • decay- and vermin-resistant;
  • sound attenuation. Perlite loose-fill insulation has the ability to fill all voids, mortar lines and ear holes, thus enabling it to reduce airborne sound transmission through walls. Lightweight, 8-inch (20-cm) masonry block filled with perlite achieves a sound transmission class of 51, exceeding HUD sound transmission standards;
  • moisture-resistant, making it useful for use in areas exposed to water or dampness, such as in floor leveling compounds and under-floor insulation; and
  • all natural. The U.S. Department of Energy advocates perlite as a green building material.

Some common applications for perlite as an insulator in buildings include:

  • in the cores of hollow-masonry unit walls;
  • in the cavities between masonry walls;
  • between exterior masonry walls and interior furring;
  • for under-floor insulation and the leveling of old floors. In this application, perlite insulation is poured on the original floor surface, screeded to proper depth, covered with corrugated cardboard or lightweight boards, and a layer of oil paper;
  • in ceiling tiles;
  • as fireproofing around chimneys, doors, rooms and safes; and
  • for roof decking.


  • With an R-value of 2.7, perlite under-performs other insulators, such as fiberglass, rockwool and cellulose. It outperforms others, however, such as vermiculite, loose-fill wood products, and straw.
  • Perlite is not acceptable for applications where it will be directly exposed to continuous temperatures of 200° F.
  • Perlite should not be used on exterior surfaces that are regularly exposed to water or moisture. Where contact with excessive water or moisture is expected, Portland cement plaster is recommended.
  • Perlite plasters are not recommended over radiant heating panels because of their insulating values.
  • The maximum temperature that perlite can withstand is 2,300º F (1,260º C).
In summary, perlite is an all-natural, safe mineral that is used as insulation in buildings.