What is an AAC block?

An AAC (autoclaved aerated concrete) block - sold under brand names Thermalite, Celcon, and Durox - is a lightweight masonry block made from a cementitious mix foamed with aluminium powder and cured under steam pressure (autoclaved). The process creates a cellular matrix with up to 80% air content, giving the block very low density (typically 400-750 kg/m3) and very good thermal insulation. AAC blocks are the standard material for the inner leaf of modern cavity walls and for internal partitions throughout the UK.

What are the different types of AAC block?

AAC blocks vary in density and strength. Low-density (Thermalite Shield, 400-480 kg/m3) have the best thermal performance (lambda value around 0.11 W/mK) but lower compressive strength (2.8-3.6 N/mm2) - used in inner leaf walling and non-loadbearing partitions. Medium-density (Thermalite Turbo, 500-600 kg/m3, around 3.5 N/mm2) balance thermal performance and strength. High-density (Thermalite Hi-Strength, 700-750 kg/m3, 7-8 N/mm2) are used for loadbearing walls and beam-and-block floor construction where strength is more important than maximum thermal performance.

Why are AAC blocks used for the inner leaf of cavity walls?

The inner leaf of a cavity wall significantly contributes to the overall U-value of the wall. A low-density AAC block (lambda 0.11 W/mK) performs far better thermally than a dense concrete block (lambda 1.13 W/mK) or a medium-density block (lambda 0.57 W/mK). Using AAC blocks for the inner leaf can reduce the thickness of cavity insulation needed to achieve a target U-value, or can achieve a lower U-value with the same insulation thickness. They are also light and easy to cut, making them fast to lay.

Can AAC blocks be used below ground?

Standard AAC blocks are not suitable for below-ground use - their cellular structure and relatively high water absorption make them vulnerable to frost damage and sulphate attack when in prolonged contact with ground moisture. Dense concrete blocks or engineering bricks should be used below DPC level. Some manufacturers produce denser, more moisture-resistant AAC products rated for below-ground use, but these should only be specified where the manufacturer's technical guidance confirms suitability for the specific exposure conditions.

Masonry noun

AAC Block

/ eɪ eɪ siː blɒk /

Also known as: autoclaved aerated concrete block, aircrete block, Thermalite, Celcon, Durox, lightweight block

Definition

An AAC block (autoclaved aerated concrete block, also called aircrete) is a lightweight masonry unit made from a cementitious mix foamed with aluminium powder and steam-cured under pressure, creating a cellular structure with up to 80% air content. This gives it very low density (400-750 kg/m3), good thermal insulation (lambda value 0.11-0.16 W/mK), and excellent workability. AAC blocks are the standard choice for the inner leaf of modern cavity walls and internal partitions in UK construction.

In practice

Low-density AAC blocks (Shield grade, ~430 kg/m3) have a lambda value of approximately 0.11 W/mK - significantly better than dense concrete (1.13 W/mK). Using them for the inner leaf of a 100mm cavity wall with 100mm partial-fill insulation can achieve a wall U-value of around 0.18-0.20 W/m2K, comfortably meeting Part L targets without a full-fill cavity. AAC blocks are soft and easily cut with a standard handsaw - overcuts are filled with the same thin-bed mortar used for laying.

Most AAC blocks are laid in thin-bed mortar (2-3mm bed rather than the 10mm standard for dense blocks), which eliminates the thermal bridge of the mortar bed. Thin-bed mortar systems require flat-faced blocks (AAC blocks are manufactured to tight dimensional tolerances for this reason) and a level, true base course. The first course must be set on standard mortar (10mm) to accommodate any variation in the base. Lintel and beam bearing on AAC inner leaf requires padstones or spreader plates where the bearing stress exceeds the block's compressive strength.

Building Regulations

Thermal (Part L)Lambda 0.11 W/mK (low density) aids wall U-value compliance

StandardBS EN 771-4 - Autoclaved aerated concrete masonry units

Below groundNot suitable - use dense block or engineering brick below DPC

Thin-bed mortarReduces mortar bridging - improves thermal performance vs standard bed

Approved Document L requires new dwellings to achieve limiting U-values for external walls (typically 0.18-0.26 W/m2K depending on specification route). AAC blocks in the inner leaf make a significant contribution to achieving these targets. Block thermal conductivity values for use in U-value calculations must be manufacturer-certified values from test data. BS EN 771-4 covers the specification for AAC masonry units including compressive strength classes and dimensional tolerances.

Full Building Regulations guidance

AAC Block

Just so you know...