Concrete Block Insulation

Written by  The U.S. Department of Energy's Office of EERE

Insulated concrete blocks can accommodate many walls in a home. Their cores are filled with insulation (except for those cells requiring structural steel reinforcing and concrete infill), which raises the average wall R-value. The better concrete masonry units reduce the area of connecting webs as much as possible.

Concrete Block Insulation





Topic Summary

Types of Concrete Block Insulation

There are several ways to incorporate foam insulation — such as polystyrene, polyisocyanurate or polyiso, and polyurethane — into concrete blocks. The hollow cores of concrete blocks can be filled by pouring and/or injecting loose foam beads or liquid foam. Some manufacturers make concrete blocks that accommodate rigid foam inserts.

Some block makers coat polystyrene beads with a thin film of concrete. The concrete serves to bond the polystyrene while providing limited structural integrity. The most common group of ingredients are expanded polystyrene mixed with Portland cement, sand, and chemical additives. These make surface-bonded wall assemblies with a wall R-value of R-1 per inch thickness. Polystyrene inserts placed in the block cores increase the unit thermal resistance to about R-2 per inch.

In the United States, two varieties of solid, precast autoclaved concrete masonry units are now available: autoclaved aerated concrete (AAC), and autoclaved cellular concrete (ACC). This class of material has been commonly used in European construction since the late 1940s. Air makes up 80% (by volume) of the material. Autoclaved concrete has ten times the insulating value of conventional concrete. The R-1.1 per inch blocks are large, light, and have a flat surface that looks like a hard, fine sponge. Mastic or a thin mortar is used to construct a wall, and the wall then often gets a layer of stucco as the finish. Autoclaved concrete is easily sawed, nailed, and shaped with ordinary tools. Since the material absorbs water readily, it requires protection from moisture.

Precast autoclaved cellular concrete uses fly ash instead of high-silica sand as its distinguishing component. Fly ash is a waste ash produced from burning coal in electric power plants. The fly ash is the material that differentiates ACC from AAC.

Hollow-core units made with a mix of concrete and wood chips are also available. They are installed by stacking the units without using mortar (dry-stacking). Structural stability comes from the concrete fill and appropriate rebar throughout for structural walls. One potential problem with this type of unit is that the wood is subject to the effects of moisture and insects.

Concrete blocks are also sometimes filled with vermiculite or perlite pellets.


Concrete block insulation is typically installed for new home construction or in homes undergoing major renovation. Since installation involves masonry skills, it's best to have a certified cement mason do it.

When using masonry blocks for a foundation wall, filling the block cores with high-pressure foam works better than most poured-in insulations, like polystyrene beads or vermiculite or perlite pellets.

Note that even though filling the block cavities and special block designs improve a block wall's thermal characteristics, it doesn't reduce heat movement very much when compared to insulation installed over the surface of the blocks either on the exterior or interior of the foundation walls. Field studies and computer simulations have shown that core-filling of any type offers little fuel savings since the majority of heat is conducted through the solid parts of the walls such as block webs and mortar joints.

Article source: The U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE). For the most up-to-date information please visit the EERE website.

Last modified on Tue, Oct 12, 2010
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