How the two differ in kind
The two blocks are made of the same family of material and behave nothing alike. AAC is aerated during manufacture and autoclaved, so it is full of small closed pores: light, insulating, easily cut with a hand saw, and weak. Dense concrete block is what it sounds like: heavy, strong, acoustically massive, and thermally poor.
The joint matters as much as the block. Conventional blockwork uses a ten-millimetre mortar bed, and mortar conducts heat far better than an insulating block does — so in a wall of high-performance blocks the joints become a grid of thermal bridges covering a real percentage of the elevation, and they hand back a substantial share of the block's advantage. AAC laid in THIN-BED adhesive at two or three millimetres largely removes that, which is why the system is specified as a system rather than as a block.
The trade is strength and water. AAC's compressive strength is a fraction of a dense block's, so it is not the material for a heavily loaded pier, a padstone bearing, or anywhere a point load lands without a spreader. And it absorbs water readily — it is not a finish, it is not for use below the damp-proof course, and it has to be protected from the weather by render, cladding or an outer leaf.
The factors that actually differ
Show
| AAC (aerated) block, thin-bed | Dense concrete block, conventional mortar | |
|---|---|---|
| Thermal performance | Good on its own, and genuinely delivered because thin-bed joints remove the mortar bridge. | Poor, and made worse in practice by the mortar joints — a wall of dense block is a wall that needs insulation elsewhere. |
| The joint | Two to three millimetres of adhesive. Fast, accurate, and it needs a level first course because there is no bed to adjust in. | Ten millimetres of mortar, which absorbs dimensional variation and tolerates a less perfect start. |
| Compressive strength | A fraction of dense block's. Adequate for ordinary domestic loadbearing walls; not for concentrated loads without spreading them. | High, and available in a range of strengths. The material for piers, padstone bearings and heavily loaded walls. |
| Cutting and handling | Hand saw, and a size a person lifts one-handed all day. Productivity and fatigue both improve. | Cut with a saw or split, and heavy enough that the larger units are a two-handed lift with real manual handling implications. |
| Water | Absorbs readily. Must be protected, must not be used below the damp-proof course, and must not be left exposed on site. | Far more tolerant, and available in grades suitable for below ground. |
| Fixings | Specific fixings designed for the material. An ordinary plug pulls out, and this is the complaint that follows a house built in it. | Ordinary masonry fixings hold well. |
| Sound | Poor. Low mass means low transmission loss, which matters on a party or separating wall. | Good, because mass is what stops sound. This is why separating walls are dense. |
| Dimensional accuracy | Manufactured to close tolerances, which is what makes thin-bed possible at all. | Looser, which the thicker mortar bed absorbs. |
Which one, and when
Choose aac (aerated) block, thin-bed when…
- The wall is part of the thermal envelope and its own performance matters.
- Speed and handling matter — a thin-bed wall goes up quickly and the blocks do not exhaust the gang.
- The loads are ordinary domestic loadbearing, with concentrated loads spread by a padstone or a lintel bearing.
- The wall will be rendered or clad, so its absorption is protected.
Choose dense concrete block, conventional mortar when…
- The wall is heavily loaded, or carries a concentrated load a weaker block would crush under.
- Sound matters — a separating wall, a party wall, anything where mass is the mechanism.
- The work is below the damp-proof course or otherwise in contact with water.
- Fixings will be made into it constantly, and ordinary plugs holding is worth something.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Is thin-bed mortar really better, or is it just faster?
- Both, and the thermal half is the one that is under-appreciated. Mortar conducts heat considerably better than an insulating block, so in a conventionally bedded wall the joints form a continuous grid of thermal bridges across the elevation — and at ten millimetres every course and every perpend, that grid is a real percentage of the wall's area. A block with an excellent declared conductivity laid on ordinary mortar therefore performs meaningfully worse as a wall than its datasheet suggests. Thin-bed adhesive at two or three millimetres reduces that bridge to something close to negligible. The speed is real too — no mixing, no bed to strike, and a course laid as fast as blocks can be placed — but the performance argument is the one that justifies the system.
- What fixings does AAC need?
- Fixings made for it, and this is the practical complaint that follows people into a finished house. AAC's pore structure means an ordinary expanding plug has very little to expand against — it crushes the material and pulls out under load, sometimes immediately and sometimes when a shelf is loaded a year later. The fixings that work are helical or screw-in types that cut a thread into the block over a long engagement, or chemical anchors that bond into it. Heavy items — wall-hung boilers, televisions, kitchen units, radiators — should be planned for, with timber noggins or a spreader plate built in where the position is known. Telling the occupants what the walls are made of is part of handing the building over.
- Can AAC be used below ground or in a foundation?
- No, in the ordinary case, and the reason is water rather than strength. AAC absorbs readily, so a block below the damp-proof course sits in contact with ground moisture and stays saturated — which destroys its thermal performance, adds enormous weight to the wall, and in a freezing climate risks frost damage. Standard practice uses dense concrete blocks of a suitable grade below the damp-proof course and switches to AAC above it. The same logic applies during construction: AAC left stacked uncovered on site in the rain arrives at the wall saturated, and a wall built from soaked blocks takes a long time to dry and will not perform until it has.
- Which is better for sound insulation?
- Dense block, and it is not close. Sound transmission through a single leaf is governed by the mass law — transmission loss rises by roughly six decibels for each doubling of surface mass — and AAC's whole selling point is that it has little mass. That is why separating walls between dwellings are dense blockwork or a designed twin-leaf construction, and why an AAC internal wall between a bedroom and a living room transmits considerably more than people expect. Where AAC is used and sound matters, the answer is a designed assembly — a twin leaf with a cavity, a resilient lining, or added mass — rather than a heavier version of the same block.
- Can AAC carry a loadbearing wall?
- Yes, in the strengths intended for it and for ordinary domestic loads — it is used as loadbearing inner leaf across large parts of Europe. What it will not do is take a concentrated load without help. A beam bearing, a joist hanger under a heavy load, a lintel end or a post landing on an AAC wall needs a padstone or a spreader that distributes the load over enough area, and the block's compressive strength is the number that decides how much area. The other constraint is slenderness: like all masonry, an AAC wall's capacity falls with height-to-thickness, and the lower material strength means the limit arrives sooner. Both are ordinary design checks; neither is a reason against the material.
- Does an AAC wall have to be rendered?
- It has to be protected, and render is the usual way. Exposed AAC absorbs driving rain, which saturates it, destroys its thermal performance while wet and exposes it to frost damage — so it is used as an inner leaf, behind cladding, or rendered. The render system matters as much as the decision to render: it should be vapour-permeable, because a saturated block behind an impermeable coating cannot dry, and it needs movement joints because AAC shrinks on drying more than dense block does. A rigid cement render applied tightly over a young AAC wall is a recognised route to cracking, and the crack is then the path that lets water in behind the render.