Lightweight Insulating Concrete

Lightweight Insulated Concrete (also known as LWIC) is a material intended for use as an insulation and substrate for support and attachment of roofing membranes on slope roofs. The LWIC systems are composed of either aggregate or cellular concrete. The largest single use for LWIC is as a roofing base and thermal insulation for industrial and commercial buildings with low slope roofs. LWIC is “non-structural” in the traditional sense. There are other forms of concrete, including “lightweight”, that are structural in nature.

The LWIC installations are combinations of components, each of which has a specific function. The substrate, lightweight insulating concrete, and molded polystyrene (MEPS), creating an LWIC system. The Substrate may be galvanized metal deck (bottom-slotted preferred), structural concrete (poured-in-place or pre-cast), or a sound, existing built-up roof membrane. The function of cement in LWIC is to bind the components together and generate strength. The cement used in LWIC is Portland Cement.

Attributes Creating Continued Market Use

LWIC systems were known in coastal areas as having a performance history of resistance to high wind loads. The concrete slurry coat bonds the system to the structural substrate, helping to provide high wind load resistance.

Also, LWIC systems are re-usable supplying cost-effective insulation. Most LWIC systems that are re-roofed these days are left in place and are repaired only where necessary, many only adding a new membrane without adding the cost of new insulation when re-roofing occurs.

LWIC systems are proven performers in the market, with a track record of over 60 years of continuous use. The reduction in available labor and the continuing increase in rigid board replacement and disposal costs would indicate that LWIC will continue to be a competitive force in the market for many years to come.


Philosophy of Design

The basis of LWIC system design is the encapsulation of MEPS insulation with lightweight insulating concrete. In all designs, there is a layer of insulated concrete applied to the substrate, commonly referred to as the slurry coat. In most cases, MEPS boards are embedded in this first layer.

Thicknesses of MEPS board are stepped to create positive slope-to-drain. Slope is primarily created by placing stair-stepped thicknesses of eps board embedded in concrete slurry. Varying the topcoat thickness of LWIC creates the final positive slope.

Finally, a topcoat of LWIC is applied to create a smooth sloped monolithic surface for application of the roof membrane.


Expanded Polysterene (EPS) used in LWIC is the preferred material. It’s used in the form of boards in various thicknesses, generally from 1 to 16 inches. These boards are perforated with various configurations of holes and/or slots in order to allow the LWIC to flow through the boards and bind the system together.

The EPS functions as the primary insulating component of the system (nominal R-value, four per inch). This lightweight material is used to build thickness and generate slope-to-drain.


LWIC Roof Membrane Performance

LWIC have been in use for over 60 years. LWIC systems provide the designer with a versatile means of providing slope-to-drain. Their high compressive strength and dimensional stability provide support for the roof membrane system and minimize stresses imposed on the membrane from dimensional movement. Thermal stresses from temperature changes are also reduced due to the mass of the LWIC.


Air Entrainment and Water

In LWIC, the function of air entraining admixtures is to generate air cells that assist in density control and to create a homogenous mixture that does not segregate. Additionally, water provides the moisture necessary for cement hydration. It also creates fluidity necessary for pumping and finishing of the LWIC.