Concrete blocks do more structural and thermal work in a modern UK cavity wall than almost any other single component. Choose the wrong type and you either pay over the odds for thermal performance you do not need, or you specify a block too weak for the loading — both errors that cost money to correct after the fact. This guide covers the three block families you will encounter most often in domestic and light-commercial work: aircrete (autoclaved aerated concrete), dense aggregate, and hollow dense aggregate.
Standard UK masonry block dimensions are 440 × 215 mm (L × H) in widths from 75 mm to 215 mm, giving a 10 blocks/m² coverage (with a standard 10 mm mortar joint). Unlike bricks, blocks are commonly laid in a single stretcher course; the wide face is 440 × 215 mm.
Aircrete (Autoclaved Aerated Concrete) Blocks
Aircrete — sold in the UK under brand names such as Thermalite, Celcon and Durox — is made by aerating a Portland cement and PFA (pulverised fuel ash) slurry before autoclave curing. The result is a block with a cellular microstructure: low density (typically 450–800 kg/m³), low thermal conductivity and excellent workability (it can be cut with a handsaw).
Key performance parameters:
- Compressive strength: 2.8 N/mm² to 8.7 N/mm² (declared mean values per BS EN 771-3)
- Thermal conductivity (lambda): 0.11–0.19 W/mK (density-dependent)
- Density: 450–750 kg/m³ (varies by grade: Shield, Turbo, Hi-Strength)
- Fire resistance: Excellent (non-combustible, contributes to compartmentation)
- Vapour resistance: Low — does not trap moisture
The inner leaf of a modern cavity wall is almost always aircrete because the thermal contribution is significant: a 100 mm aircrete block at 0.11 W/mK gives a wall element R-value of 0.91 m²K/W, compared with just 0.07 for a 100 mm dense block. This reduces the cavity insulation specification needed to achieve Part L compliance.
Grades by strength:
| Aircrete Grade | Typical Product | Compressive Strength | Lambda (W/mK) | Density (kg/m³) |
|---|---|---|---|---|
| Standard / Shield | General inner leaf | 2.8–3.6 N/mm² | 0.11–0.13 | 450–475 |
| Turbo / Party Wall | Party walls, ground floor | 3.6–4.4 N/mm² | 0.15 | 600 |
| Hi-Strength / Solar | Loadbearing inner leaf | 7.3–8.7 N/mm² | 0.19 | 730–750 |
For two-storey domestic loadbearing inner leaves, a 3.6 N/mm² block is typically adequate; three storeys or concentrated loads (steel beam padstones) require 7.3 N/mm² or a dense block padstone insert.
Dense Aggregate Concrete Blocks
Dense aggregate blocks are cast from Portland cement, crushed aggregate (limestone, granite or recycled concrete) and sand. Density ranges from 1,800 to 2,200 kg/m³ — roughly four times denser than aircrete. This density gives them high compressive strength and good sound insulation, but poor thermal performance.
Key parameters:
- Compressive strength: 7.3 N/mm² to 35 N/mm² (higher grades for columns and piers)
- Thermal conductivity: 0.85–1.33 W/mK
- Density: 1,800–2,200 kg/m³
- Sound reduction (100 mm leaf): ~45 dB Rw (mass law)
Dense blocks dominate wherever thermal performance is secondary:
- Below DPC and in foundations — dense blocks resist moisture and ground pressure better than aircrete
- Retaining walls and basement inner skins
- Party walls — mass is needed for sound insulation under Part E of the Building Regulations; a 215 mm dense block solid wall (at ~440 kg/m²) meets the mass requirement for the robust detail E-WM-1
- Padstones under steel beams — a standard 7.3 N/mm² aircrete block can crush under concentrated loads; a 20–35 N/mm² dense block or precast padstone is specified instead
Hollow Dense Aggregate Blocks
Hollow blocks are dense aggregate blocks with one or more cylindrical or rectangular voids through the face. Compared with solid dense blocks of the same outer dimensions, they are:
- Lighter (10–30 % weight reduction, easier for a single operative to handle)
- Slightly better thermally (air cells reduce effective conductivity to ~0.55–0.75 W/mK)
- Reduced sound mass — not suitable where mass law drives the party wall design
- Suitable for reinforced masonry — voids can be filled with vertical rebar and grout for retaining wall or basement panel construction
Hollow blocks are specified in BS EN 771-3 under ‘Group 2’ (volume of voids 25–55 %). Strength classes follow the same range as solid dense blocks.
Cost Comparison (2026 UK Trade Prices)
Prices are per block (440 × 215 mm), ex-VAT, builders’ merchant trade:
| Block Type | Width | Strength | Lambda (W/mK) | Cost/Block | Cost/m² |
|---|---|---|---|---|---|
| Aircrete standard | 100 mm | 3.6 N/mm² | 0.11 | £1.20–£1.60 | £12–£16 |
| Aircrete party wall | 100 mm | 4.4 N/mm² | 0.15 | £1.40–£1.80 | £14–£18 |
| Aircrete hi-strength | 140 mm | 7.3 N/mm² | 0.19 | £2.00–£2.50 | £20–£25 |
| Dense solid | 100 mm | 7.3 N/mm² | 1.13 | £1.00–£1.50 | £10–£15 |
| Dense solid | 140 mm | 10.4 N/mm² | 1.13 | £1.40–£2.00 | £14–£20 |
| Dense solid | 215 mm | 7.3 N/mm² | 1.13 | £2.00–£3.00 | £20–£30 |
| Hollow dense | 100 mm | 7.3 N/mm² | 0.65 | £1.50–£2.20 | £15–£22 |
| Hollow dense | 215 mm | 10.4 N/mm² | 0.65 | £2.50–£3.50 | £25–£35 |
Add approximately 10 % for mortar, and delivery/crane offload for upper floors.
Where Each Block Is Used in a Typical UK Cavity Wall
A standard modern domestic cavity wall construction might be:
- Outer leaf: 102.5 mm facing brick (see BS EN 771-1)
- Cavity: 100–150 mm with full-fill mineral wool or partial-fill rigid insulation
- Inner leaf: 100 mm aircrete (3.6 N/mm²) — thermal + structural
- Blockwork below DPC: Dense aggregate or engineering-quality (7.3 N/mm²)
For ground floor party walls in semi-detached or terraced houses (Part E sound requirements), the inner leaf is replaced with a 215 mm solid dense aggregate block, or a 100 mm dense + 100 mm dense twin-leaf construction with an acoustic quilt in the cavity.
Building Regulations Compliance
- Part A (Structure): Block strength must be adequate for the imposed loads. For two-storey domestic inner leaf, 3.6 N/mm² aircrete in a 100 mm leaf is acceptable within the limits of Approved Document A Table 1 (maximum wall height 2.7 m, floor spans ≤ 6 m)
- Part E (Sound): Party walls must achieve Dw ≥ 45 dB (airborne); mass and isolation are the main variables
- Part L (Energy): Inner leaf thermal conductivity feeds the whole-wall U-value calculation; aircrete significantly reduces the cavity insulation depth required
- Part C (Moisture): Blocks below DPC must be of low water absorption or adequately protected; dense aggregate blocks rated W2 to BS EN 771-3 are preferred
Key Standards
- BS EN 771-3:2011+A1:2015 — Specification for masonry units: aggregate concrete masonry units
- BS EN 771-4:2011+A1:2015 — Autoclaved aerated concrete masonry units
- BS EN 1996-1-1 (Eurocode 6) — Design of masonry structures
- BS 8000-3:2001 — Workmanship on building sites: masonry
- Building Regulations Approved Documents A, C, E and L
[!tip] Handling aircrete efficiently Aircrete is soft enough to saw, drill and chase without specialist equipment — a standard wood saw, a wood bit and a bolster will do. However, the surface is dusty and friable; always seal before plastering with a PVA bond coat to prevent suction from pulling water out of the plaster coat before it sets.
