Is an AAC block lower carbon than a red brick per square metre of wall?

Often, but not always: it depends on how much coal the brick kiln burns and how much cement, lime and boiler fuel the AAC plant uses. Compare one square metre of finished wall of the same thickness: units per m2 times the footprint per unit, plus mortar or adhesive. Use each supplier's own data.

Compare walls, not single units

An AAC block and a brick are different sizes and densities, so a per unit or per kg comparison misleads. Fix a declared unit, such as 1 m2 of 200 mm internal wall, and count what that wall needs: blocks or bricks per m2 from the bond and joint size, and mortar or thin-bed adhesive per m2. For a 600 x 200 mm block face, 1 / (0.6 x 0.2) = 8.33 blocks per m2.

Where each product's emissions come from

• Red brick: coal or other fuel burnt in the kiln is most of it, plus diesel for moulding and haulage. Clay itself adds little.
• AAC block: cement and lime in the mix, coal or other fuel for the autoclave boiler, and electricity for grinding, mixing and cutting. Fly ash is a common ingredient; state how you treated its burden.
• Both: transport to site, breakage and wastage, and the mortar or adhesive.

What can flip the result

A zigzag or vertical shaft kiln burning less coal per brick narrows the gap. An AAC plant with a high cement share or an inefficient boiler widens it the other way. Thinner AAC walls for the same function, plaster thickness, and different wastage rates also move the answer. AAC's lower thermal conductivity can cut cooling energy in use, but that is a use-phase benefit outside a cradle to gate comparison; report it separately.

Making the comparison credible

Use cradle to gate (modules A1 to A3) for both, with the same declared unit and the same data quality. Prefer an EPD or supplier data with the plant and year named over generic database figures. For the brick, ask the kiln for coal per lakh bricks for the season.

Worked example: 1 m2 of 200 mm wall (illustrative)

• Brick: 100 bricks per m2; kiln coal 160 kg per 1,000 bricks = 0.16 kg per brick x 2.42 = 0.387 kg CO2; 100 x 0.387 = 38.7 kg CO2
• Brick wall mortar: 12 kg cement x 0.8 kg CO2e per kg (assumed) = 9.6 kg CO2e; brick wall total 48.3 kg CO2e per m2
• AAC per m3: cement 70 kg x 0.8 = 56; lime 40 kg x 1.2 (assumed) = 48; boiler coal 25 kg x 2.42 = 60.5; electricity 20 kWh x 0.710 = 14.2; total 178.7 kg CO2e
• AAC wall: 8.33 blocks x 0.024 m3 = 0.2 m3 x 178.7 = 35.7 kg CO2e, plus 4 kg adhesive x 0.6 (assumed) = 2.4; total 38.1 kg CO2e per m2
• Result for these inputs: AAC about 21% lower, (48.3 minus 38.1) / 48.3

Cement, lime, adhesive and the plant and kiln inputs are assumptions, not benchmarks. With a more coal-efficient kiln or a cement-heavy AAC mix, the answer can reverse.

What Senseible does here

Senseible reads kiln coal, cement, lime and boiler fuel invoices and electricity bills for brick and block makers and computes Scope 1 and 2 at IPCC 2006 defaults and 0.710 kg CO2 per kWh. A per m2 wall comparison built from these records is a self-declared draft, not an EPD.

• Brick kiln emissions per 1,000 bricks
• What embodied carbon A1 to A3 means
• Embodied carbon from quantities
• Construction carbon accounting

Sources: ISO 21930:2017, Core rules for environmental product declarations of construction products, 2006 IPCC Guidelines, Volume 2 Energy, Chapter 2 Stationary Combustion (coal), CEA CO2 Baseline Database, version 21.0

Last reviewed 2026-10-18.