Concrete Block vs Aerated Block — UK

Standard dense concrete blocks are typically 7.3 N/mm² (medium density) or 10.4 N/mm² (high density). Standard aerated blocks (e.g. Thermalite, Celcon) are typically 2.9-3.6 N/mm². This means dense blocks are roughly twice as strong, making them essential for load-bearing applications below DPC, retaining walls and heavy structural loads. Aerated blocks are adequate for most above-ground domestic walls.

Aerated blocks are far superior thermally. A 100mm Thermalite Turbo block has a thermal conductivity of 0.11 W/mK, while a dense concrete block is around 1.3-1.5 W/mK — about 12 times worse. In a cavity wall, using aerated blocks on the inner leaf significantly improves the wall U-value and helps meet Part L energy efficiency requirements.

A standard 440x215x100mm dense concrete block weighs approximately 20 kg, while the same size aerated block weighs only 8-10 kg. This makes a significant difference to laying speed, fatigue over a day, scaffolding loads and delivery logistics. Aerated blocks can be laid much faster, typically 50-80 blocks per day vs 40-60 for dense blocks.

Dense concrete blocks are slightly cheaper per unit — typically £1.00-1.50 each vs £1.50-2.50 for aerated blocks. However, the faster laying speed of lighter aerated blocks can offset the material cost difference. When thermal performance is factored in, aerated blocks often reduce the need for additional insulation, providing overall project savings.

Standard aerated blocks should not be used below DPC or in contact with the ground, as they absorb moisture and can degrade. Dense concrete blocks (or engineering bricks) should be used below DPC, for foundations, retaining walls and any application in contact with ground moisture. Some manufacturers offer moisture-resistant aerated blocks for specific below-DPC applications, but dense blocks are the standard choice.