WestCon Tribune

February 2008

January Review

Retrofitting With Lightweight Insulating Concrete

Presented By: Phil Dregger, Pacific Building Consultants, Concord

Westcon member, Phil Dregger gave an overview of the two major types of lightweight insulating concrete (LWIC) systems, explained some of the attributes and limitations associated with LWIC systems as compared to tapered rigid board insulation, and discussed lessons learned during several LWIC projects.

LWIC is a mixture of cement, water, and a lightweight aggregate (or a foam solution) and has been used successfully to provide positive drainage on roofs for over 70 years.  Modern LWIC systems consist of, from top down, a 1”-2” layer of insulating concrete, polystyrene insulation, a 1/8” slurry coat of insulating concrete, and a structural deck.

There are two types of insulating concrete materials used in LWIC systems - “aggregate” and “cellular” insulating concrete.

Aggregate insulating concrete consists of water cement and a lightweight aggregate (vermiculite or perlite).  Cellular insulating concrete consists of water, cement, and a proprietary foam solution.

Multiple stair-stepped layers of molded-bead expanded polystyrene (MEPS, ASTM C578, Type I) are used to add thermal resistance and reduce weight of the installed system.

Aggregate and cellular LWIC vary in physical properties.  For example aggregate LWIC is heavier on a “wet” unit weight basis than cellular since it uses more mix water. 

Aggregate LWIC was first introduced in the 1930’s and did not contain polystyrene insulation.   During the 1960’s, cellular LWIC was introduced as a lower cost option.  During the 1970’s, MEPS insulation was introduced as well as higher strength lightweight aggregate mixes which allowed the thickness of the top layer to be reduced from 2” to 1”.

Attributes of LWIC compared to most tapered rigid board insulation systems:

   • Less costly for larger projects and for complex taper

   • No asphalt fumes & no fasteners into structural deck.

   • Resistant to moisture deterioration.

   • Air impermeable (enhanced membrane wind resistance).

   • Readily accommodates irregular substrates

   • Readily reusable (green).

Note:  LWIC systems when listed as components of hourly rated roof/deck assemblies often have special requirements for compressive strength and/or wire reinforcements.

Limitations of LWIC compared to most tapered rigid board insulation systems:

   • Relatively heavyweight.

   • More costly on smaller jobs.

   • Greater thickness for R-value.

   • Deck needs to “sealed” before placement over occupied         space since “free water” in the mix may leak inside.

   • Retains water in roof system for extended periods

   • Has 2 to 5 day “open” time which can be problematic if  rain occurs.

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