Can lightweight concrete block replace thermal insulation?

How well does lightweight concrete block insulate?

How well a material insulates is measured by its thermal conductivity (λ, W/mK). The lower the value, the less heat passes through. In lightweight concrete, conductivity depends on the share of air pores and rises with density.

The dry-state design targets for Redblok density classes are: KB300 0.08–0.10 W/mK, KB400 0.10–0.12 W/mK, KB500 0.12–0.14 W/mK and KB600 0.14–0.17 W/mK. For KB700 and KB800, the classes designed for infill walls, the targets are 0.17–0.20 and 0.20–0.24 W/mK. These are design targets. Each class is confirmed by trial mixes and laboratory testing.

Common insulation materials such as EPS, XPS and stone wool sit around 0.030–0.040 W/mK. Comparing mid values, KB300 conducts about 0.09 / 0.035 ≈ 2.6 times more heat than a board, and KB400 about 0.11 / 0.035 ≈ 3.1 times more. Lightweight concrete block insulates far better than normal concrete. It still does not reach the level of an insulation board.

How is thermal resistance calculated?

The thermal resistance of a layer is R = thickness / λ, in m²K/W. Assumptions for this example: both materials are dry, KB400 takes the mid value of its design band, 0.11 W/mK, and the board is 0.035 W/mK. Internal and external surface resistances and other layers are left out.

10 cm of KB400 gives R = 0.10 / 0.11 ≈ 0.91 m²K/W. 10 cm of board gives R = 0.10 / 0.035 ≈ 2.86 m²K/W. At equal thickness, the board gives about 3.1 times more resistance. Matching the board's 10 cm with KB400 alone would take 2.86 × 0.11 ≈ 0.31 m, roughly 31 cm of block. That layer adds about 0.31 × 400 = 124 kg per m², based on dry density. The denser classes used in walls conduct more heat, so they would need an even thicker layer.

Why can't block replace façade insulation?

External wall insulation (mantolama in Türkiye, ETICS elsewhere) is a system fixed to the outside of a wall. It is made of insulation boards, anchors, mesh and render. Its job is to raise the wall's thermal resistance to the level required by TS 825, the Turkish thermal insulation standard for buildings, and to reduce thermal bridges. Lightweight concrete block cannot take its place. As the example shows, the same resistance needs about three times the thickness, and building an external wall that thick and that heavy is not practical.

The TS 825 calculation decides whether an external wall needs extra insulation. The block enters that calculation as added thermal resistance and can affect the board thickness required. In colder regions such as Eastern and Central Anatolia, the calculation generally calls for thicker insulation.

What Redblok really brings is a light wall. It is used in non-loadbearing interior partitions and in infill walls within a frame. Lower wall mass reduces the seismic mass entering the structure in an analysis under TBDY 2018, the Turkish Building Earthquake Code. Interior partitions usually carry no thermal insulation requirement. There, the block is chosen for its low weight.

How does moisture affect insulation value?

The conductivity values above are for dry material. When water replaces air in the pores, conductivity rises, because water conducts heat far better than air. The target water absorption by volume is 8–12% for KB300 and KB400 and 10–14% for KB500 and KB600. A block that gets wet cannot fully deliver the insulating contribution assumed in the calculation.

So moisture control is part of the block's thermal performance. Blocks wait on their pallets, off the ground and covered. Walls are not built with wet blocks. On external walls, the block face is protected from rain by render or cladding. Wet rooms get a waterproofing detail. In cold climates, the risk of water freezing in the pores is also considered. On walls in contact with the ground, damp protection is resolved in the design details.

Which value enters the TS 825 calculation?

TS 825 sets the thermal insulation rules and the calculation method for buildings. The value used there for the block should not be a generic table value or the design target in this article. It should be the conductivity measured in the laboratory on the mix that will actually be used. Porous lightweight concrete falls outside the scope of TS EN 206, so its performance is defined by the project specification and laboratory testing. Conductivity should be asked for explicitly in that specification.

The practical sequence is short. First, the wall's job and the density class are set. Then a trial mix is made. Dry density, compressive strength, water absorption and thermal conductivity are measured on samples. The engineer reviews the reported value for service and moisture conditions and enters it into the TS 825 calculation. The insulation board thickness follows from that result.

Additives and their ratios are specific to our company, and we use a separate mix for each density class. Thermal conductivity changes with the mix too, so the project value comes from the test report for that class.