Perlite Insulating Concrete Roof Decks

Perlite insulating concrete roofdecks offer architects, contractors and building owners several important benefits which include: 

  • Excellent Insulating Values
  • Excellent Fire Ratings
  • Lightweight
  • Slope to Drain
  • Monolithic Roofing Base


Insulating values of a perlite concrete roofdeck depends on the density of the concrete mix that is prepared.  Typical U values range from 0.21 Btu/h*ft2*F (1.19 W/m2*K) for a 2 inch (50.8 mm) 27 lb/ft3 (432 kg/m3) to 0.12 Btu/h*ft2*F (0.6 W/m2*K) for a 4 inch (101.6 mm) thick roofdeck. The table included on the side menu bar provides a range of typical insulating values.


There are a number of different perlite concrete roofdeck designs that offer fire ratings of from 1 hour to 3 hours. A typical 3 hour rated roofdeck design consists of 2 inch (50.8 mm) minimum thickness of perlite concrete on 28 gauge galvanized corrugated steel supported by steel joists 4 feet (1.22 m) on-center.  Ceilings of 7/8 inch (22.2 mm) perlite gypsum plaster on expanded metal lath that is attached to 3/4 inch (19.1 mm) furring channels wire-tied to the lower chord of the joists.


Perlite insulating concrete is light in weight, ranging from 20-40 lb/ft3 (320-640 kg/m3) depending on the desired density and insulating value. Because of its lightweight, there can be economies in the design of the structure. Typical density is 27 lb/ft3 (432 kg/m3).


Standing water on a roofdeck is destructive to roofing membranes. Perlite concrete may be easily sloped to drain to internal drains or to the roof edge. Perlite concrete may be provided by transit mix or it may be mixed on-site and pumped or lifted by crane to the roof level. Once placed, it is simply screened to the desire thickness and slope.


Perlite concrete roofdecks offer several important benefits in roofing.  As perlite concrete roofdecks are seamless, they provide a monolithic base for roofing that is smooth and even. In addition, roofing membranes may be adhered directly to the perlite concrete roofdeck or base sheets for built-up roofing may be nailed directly to the perlite concrete using one of several different design fasteners.  Roofing can be applied as soon as the perlite insulating concrete can carry construction traffic and is sufficiently dry to develop adhesion with hot asphalt or pitch.  Under normal conditions, this is about three days. Time will vary depending on the weather.

When reroofing, the existing roof insulation should be inspected for soundness and moisture content. Particular attention should be paid to the support structure to determine if it is capable of safely supporting new roof loads. All loose gravel and dirt should be swept off the existing roof. For optimum roofing performance, it is recommended the drainage be provided by sloping the perlite concrete.

Perlite for Underfloor Insulation

Why Perlite Insulation?

Perlite underfloor insulation is an inorganic product that does not rot, support combustion nor provide a habitat for rodents. Because of its neutral pH, the product does not foster corrosion in piping and electrical wiring that may be in the underfloor area.

When used for underfloor insulation, a water repellent, dust suppressed perlite specially produced for this application is used.

Underfloor Applications

Perlite underfloor insulation may be used under floating concrete floors, asphalt floors and floating board floors. It is especially useful when used to level floors and greatly reduces sound transmission from floor to floor, from floors to walls and from underfloor piping systems.

Floating Floors

Perlite underfloor insulation is most frequently used under floating concrete 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. Oil paper must be used if a concrete floating floor is to be poured. A plastic film is used beneath the perlite to prevent seepage of material between cracks in floors and walls. Poured concrete floors, asphalt floors or wood floors can be laid over the insulation in a conventional manner.

This system is particularly useful when underfloor heating is employed as the insulation is dimensionally stable under varying temperatures and is not combustible. Perlite underfloor insulation as thick as 12 inches (30 cm) has been applied, but 2-4 inches (6-10 cm) is the normal. Underfloor insulation over 4 inches (10 cm) should be compacted. A unique feature of perlite insulation is that particles will interlock when pressed together and settling or volume changes will not take place.

Levelling of Floors

A special property of perlite is its ability to flow around any roughness, unevenness or exposed installations on the base floor. This enables the easy and rapid installation of level floors. In addition, because perlite underfloor insulation can be made to resist water penetration, it will not absorb or hold water permanently should there be a plumbing leak. This characteristic of perlite underfloor insulation will limit damage to a local area. As perlite is inorganic with a neutral pH, it will not corrode piping or electrical or communications conduits beneath the floor.

Sound Insulation

An area of increasing importance today is sound insulation. In construction there are three concerns: low sound transmission in construction components, low transmission between floors and walls and low transmission of sound from footsteps. Perlite underfloor insulation performs in much the same manner as multiwall systems with bulk insulation to achieve reductions in sound transmission.

With perlite underfloor insulation systems, sound transmission from floors to walls is reduced by using expansion strips at the intersection of floors and walls. Sound transmission from plumbing systems is also reduced when piping is surrounded by perlite insulation within the floor.

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