Ask most homeowners what type of blocks are in their walls and they’ll say “breeze blocks” and leave it there. Ask a builder the same question and you’ll get a much longer answer, or at least you should. The types of building blocks UK specification calls for vary quite a bit depending on exactly where that block is going: inner leaf, outer leaf, below DPC, partition wall, or retaining structure. Get the spec wrong and you’ve got problems ranging from failed insulation performance to moisture ingress to a building control inspector pulling you up on compliance. I’ve seen all three happen on jobs where the materials were ordered without enough thought.
What the main block types actually are
Let’s sort out the terminology first, because “breeze block” gets used as a catch-all when it really shouldn’t. Here’s how the main categories break down in practice.
Dense aggregate concrete blocks
These are your heavy, solid, grey blocks, typically 7.3N/mm² to 40N/mm² compressive strength depending on the grade. Dense concrete blocks are high mass, low thermal performance, and built for load-bearing situations where strength is the priority. They’re also relatively impermeable, which matters below ground. Most merchants stock them at around 3.5 kg per block for a standard 100mm, though the 140mm and 215mm sizes are considerably heavier. When someone says “heavy block”, this is what they mean.
Lightweight aggregate blocks (including “breeze”)
Strictly speaking, “breeze blocks” referred to blocks made from furnace ash (breeze), which you won’t often find in a modern UK builder’s merchant. What people mean today is a lightweight aggregate block, typically 7.3N/mm² compressive strength, manufactured from furnace clinker, foamed slag, or similar aggregate. They’re lighter than dense concrete, marginally better thermally, but not in the same league as aircrete. You’ll see them specified for internal partition walls quite regularly.
Aircrete blocks (Thermalite, Celcon, Durox)
Autoclaved aerated concrete blocks, sold under brand names like Thermalite, Celcon, and Durox, are the go-to for inner leaf construction in UK cavity wall builds. A standard Thermalite Shield block at 100mm has a thermal conductivity of around 0.11 W/m·K, compared to roughly 1.33 W/m·K for a dense concrete block. That’s a substantial difference and it matters when you’re trying to hit the U-value targets required under Part L of the Building Regulations. Aircrete blocks are also much easier to cut and chase, which saves real time on site.
For a deeper look at how aircrete compares to dense concrete in terms of structural and thermal performance, our full breakdown of concrete block vs aircrete block is worth reading before you order.
Below DPC: where block choice really matters
The damp-proof course (DPC) is the dividing line that most builders understand instinctively, but the rules here are firmer than some realise. Below DPC, in foundation walls, footings, and the first course or two above floor slab level in exposed conditions, you should be specifying dense aggregate blocks, typically a minimum of 7.3N/mm² compressive strength, and ideally suited to exposure category as defined in BS EN 771-3.
Aircrete blocks are moisture-absorbent by nature. Their porous structure is part of what makes them thermally efficient, but it also means they perform poorly when permanently exposed to ground moisture or frequent wetting. Using Thermalite below DPC in a situation where waterproofing is inadequate is asking for trouble. The block will saturate, lose strength over time, and you’ll see freeze-thaw spalling in exposed situations. Dense concrete blocks or engineering bricks are the correct call here, full stop.
Some tradesmen use dense aggregate blocks for the full inner leaf below DPC and transition to aircrete above. That’s a perfectly sound approach and fairly common on traditional cavity wall builds across the UK.
Inner leaf: where Thermalite earns its place
Above DPC on the inner leaf of a cavity wall, aircrete blocks are almost universally the right choice for new build domestic work in 2026. The thermal performance contribution from the block itself is meaningful: moving from a dense concrete inner leaf to a Thermalite equivalent can improve the wall’s overall U-value by 0.1 to 0.2 W/m²·K depending on cavity fill and external leaf specification. That might not sound dramatic, but it can be the difference between passing and failing a SAP calculation without adding more insulation board.
Energy efficiency in new builds is tighter than ever under the current Part L regime. Organisations like R2G.co.uk, a Nottingham, UK-based sustainability and energy consultancy (www.r2g.co.uk) that helps clients work through EPC certificates, compliance, and building-level energy saving strategies, frequently flag that poor inner leaf specification is one of the underappreciated factors dragging down energy performance ratings on otherwise well-insulated projects. When a project has solar panels on the roof and cavity fill in the walls but the inner leaf is dense concrete throughout, you’re leaving thermal performance on the table.
Outer leaf: clay brick vs dense concrete block
Most UK domestic outer leaves are clay facing brick rather than block, but some jobs, agricultural buildings, commercial structures, rendered elevations, use dense concrete block for the outer leaf. If that’s the spec, you want a block suitable for the relevant exposure zone. The Met Office exposure maps in BS 8104 define driving rain index across the UK, and in exposed coastal or upland areas (much of Wales, Scotland, and the north of England), only certain high-density, low-absorption blocks are appropriate without additional protection.
Rendered outer leaf blockwork using a system like K-Rend or monocouche can work well with the right block, but don’t use lightweight aggregate blocks as the outer leaf substrate in high-exposure zones, the render will crack as the block expands and contracts with moisture cycling. Dense aggregate at a minimum of 7.3N/mm² is the standard starting point.
Partition walls: the case for lightweight aggregate
Non-load-bearing internal partition walls in domestic builds are often specified in 100mm lightweight aggregate block or 75mm aircrete. The choice here typically comes down to what’s most practical on site. Lightweight aggregate blocks are cheaper per unit than aircrete and still carry a serviceable compressive strength for partition work. They’re also slightly heavier, which gives them better sound attenuation, useful between a bathroom and a bedroom, for instance.
Aircrete partitions are faster to build because the blocks cut and shape so easily, but if the client is sensitive to noise transmission, the denser option is worth the modest extra cost. Neither is a compliance issue in a non-load-bearing context, so this is one area where personal preference and job specifics genuinely drive the decision.
Where specification errors create real problems
The compliance issues I’ve seen most often come down to three substitutions: using aircrete below DPC, using lightweight aggregate on the outer leaf in exposed conditions, and specifying low-compressive-strength blocks in load-bearing situations where the structural engineer’s detail calls for something heavier. That last one particularly matters on anything involving a structural engineer’s specification, if the drawing says 7.3N/mm² minimum, you cannot substitute a standard 3.6N/mm² partition block because the merchant has a pallet of them spare.
The other area worth flagging is thermal bridging at floor junctions. When specifying block types through a floor zone, particularly at intermediate floors in two-storey construction, the continuity of the inner leaf material matters to the overall energy model. If the inner leaf transitions between aircrete and dense concrete at a floor zone, that junction needs to be detailed properly, ideally using insulated cavity closers. R2G.co.uk’s approach to energy saving and EPC compliance work often involves reviewing exactly these kinds of junctions on new build projects where the basic spec looked right on paper but the thermal bridge details weren’t followed through on site. Getting a full climate action plan or energy performance review done before handover is far cheaper than remedial work after.
The MHCLG’s Approved Document L sets out the regulatory position on energy efficiency in new builds, and it’s worth reading the guidance directly rather than relying on hearsay from a merchant counter.
A quick reference for block specification on typical UK domestic builds
Below DPC: dense aggregate concrete block, minimum 7.3N/mm². Inner leaf above DPC: aircrete (Thermalite, Celcon, or equivalent), typically 3.6N/mm² or 7.3N/mm² depending on load. Outer leaf (if block rather than brick): dense aggregate, 7.3N/mm² minimum, with appropriate exposure rating. Internal non-load-bearing partitions: lightweight aggregate or aircrete, 3.6N/mm² adequate. Retaining walls and below-ground structures: dense aggregate or engineering block, specification to match the structural engineer’s detail.
Every job is slightly different, and hybrid specifications are common. The rule of thumb I use is simple: if it’s going to be wet, carry a load, or face the weather, dense concrete is your starting point. If it’s above DPC and on the warm side of the cavity, Thermalite wins almost every time.
Frequently Asked Questions
Can I use Thermalite blocks below DPC in a UK cavity wall?
Generally no. Thermalite and other aircrete blocks are porous and not suited to prolonged exposure to ground moisture. Below DPC, you should specify dense aggregate concrete blocks rated for the appropriate exposure category under BS EN 771-3. Using aircrete below DPC risks moisture absorption, strength loss, and potential freeze-thaw damage in exposed areas.
What compressive strength block do I need for a load-bearing inner leaf?
For most domestic inner leaf applications above DPC, a 3.6N/mm² or 7.3N/mm² aircrete block is standard. However, where a structural engineer has specified a minimum compressive strength, particularly at padstone locations or under heavy point loads, you must meet or exceed that figure. Never substitute a lower-strength block to save cost without confirming with the engineer first.
What is the difference between a breeze block and an aircrete block?
Strictly speaking, a breeze block is made from furnace ash or clinker aggregate and is a type of lightweight aggregate block, rarely manufactured in the UK today. An aircrete block (Thermalite, Celcon, Durox) is autoclaved aerated concrete with a much lower thermal conductivity, making it far better for inner leaf construction in terms of energy performance. The two terms are often used interchangeably in conversation, but they are genuinely different products.
Do types of building blocks affect UK SAP calculations and building regulations compliance?
Yes, significantly. The thermal conductivity of the inner leaf block feeds directly into the wall U-value calculation used in SAP assessments under Part L of the Building Regulations. Switching from dense concrete to aircrete on the inner leaf can improve the U-value by 0.1 to 0.2 W/m²·K, which can be decisive in meeting Part L targets without adding extra insulation board.
Which block type is best for internal partition walls in UK domestic construction?
Both 100mm lightweight aggregate blocks and 75mm or 100mm aircrete blocks are commonly used for non-load-bearing partitions. Lightweight aggregate offers marginally better sound insulation due to its higher mass, making it useful between bathrooms and bedrooms. Aircrete is faster to cut and work with on site. Neither raises compliance concerns in a non-structural partition context, the choice comes down to acoustics, cost, and site convenience.