Concrete blocks are used to build home foundations and walls, and there are several ways to insulate them. If the cores aren’t filled with steel and concrete for structural reasons, they can be filled with insulation, which raises the average wall R-value. Field studies and computer simulations have shown, however, that core filling of any type offers little fuel savings, because heat is readily conducted through the solid parts of the walls such as block webs and mortar joints.

It is more effective to install insulation over the surface of the blocks either on the exterior or interior of the foundation walls. Placing insulation on the exterior has the added advantage of containing the thermal mass of the blocks within the conditioned space, which can moderate indoor temperatures.

Some manufacturers incorporate polystyrene beads into concrete blocks, while others make concrete blocks that accommodate rigid foam inserts.

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 material contains about 80% air by volume and has been commonly used in Europe since the late 1940s. Autoclaved concrete has ten times the insulating value of conventional concrete. The blocks are large, light, and easily sawed, nailed, and shaped with ordinary tools. The material absorbs water readily, so it requires protection from moisture. Precast ACC uses fly ash instead of high-silica sand, which distinguishes it from AAC. Fly ash is a waste ash produced from burning coal in electric power plants.

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) and filling the cores with concrete and structural steel. One potential problem with this type of unit is that the wood is subject to the effects of moisture and insects.

Concrete block walls are typically insulated or built with insulating concrete blocks during new home construction or major renovations. Block walls in existing homes can be insulated from the inside. Go to insulation materials for more information about the products commonly used to insulate concrete block.