Does aerated concrete contain asbestos or anything harmful?

No. Aerated concrete is made from lime, cement, silica sand and water, expanded with a gas-forming agent and cured in steam. Asbestos was never part of it; the confusion comes from historical asbestos cement corrugated sheeting, which is an entirely different product. Cutting it releases mineral dust as any masonry does, so a respirator belongs with both materials.

Can I combine aerated concrete and clay in one house?

Yes, and it is common: a ceramic external wall with aerated concrete partitions, or an aerated concrete envelope with heavy blocks for the acoustically demanding internal walls. The condition is that each wall is built from one system, mortar, lintels and anchors included, and that the designer specifies how partitions tie into load-bearing walls.

Which material makes the shell more expensive?

The price of the units differs less between the two materials than between grades within one material: a thermal grade always costs substantially more than a structural grade. The cost of the shell is therefore decided more by the chosen build-up (single leaf versus structural block plus insulation), by masons' hours and by plaster thickness than by the logo on the pallet. Have two suppliers price the same bill of quantities.

Do I really need special plugs for aerated concrete?

Yes. An ordinary expansion plug has nothing to expand against in a homogeneous lightweight material. Use anchors made for aerated concrete, and for heavy loads (a balustrade, a wall-hung WC, a large television) ask the designer for a detail, ideally with a spreader plate or a tie into a reinforced concrete element.

Why did the plaster crack on my aerated concrete wall?

Usually one of three causes: the plaster went onto masonry that had not dried, an unsuitable plaster system with a high vapour resistance was used, or reinforcement is missing where materials change and over openings. The material itself cracks rarely. The fix is to respect the drying pauses and use the plaster the block manufacturer specifies.

Is it true that an aerated concrete house is hotter in summer?

At equal wall thickness aerated concrete has less thermal mass, so it damps the daily temperature peak less. In practice, overheating is decided first by the size and orientation of the glazing and by whether external shading exists, then by night ventilation, and only then by the mass of the walls. A clay house with unshaded south windows overheats too.

Do I have to insulate a ceramic block wall?

Not necessarily. A single leaf of thermal or filled units at sufficient thickness meets the recommended value in STN 73 0540 Part 2. If you are aiming at the target value or building a nearly zero-energy house, a structural block plus insulation is the safer and usually cheaper route. The same applies to aerated concrete.

How long should masonry dry before plaster and floors?

It depends on the material, the thickness, the season and how much water arrived with the floor slabs and screeds, so there is no universal number of days. Practically: plan several weeks of intensive ventilation after the roof goes on, rather more with aerated concrete, and let the screed dry by moisture measurement rather than by the calendar. Closing the house up and heating it fast is the commonest cause of damp corners in the first winter.

Aerated Concrete or Ceramic Block?

August 28, 20268 min read

Visualisation of a small mixed-use building: a white rendered masonry lower volume with a pharmacy unit at street level and French windows with slim metal railings, topped by an olive-green upper storey in vertical timber cladding that cantilevers over an open parking bay carried on a slender column.

Aerated concrete or ceramic block: what is actually being decided?

"Ytong or Porotherm" is the most common material question from people who have decided to build in masonry. Ytong and Porfix are brand names for autoclaved aerated concrete; Porotherm and Heluz are brand names for the ceramic block, so the choice is between two materials, not two companies. Whether to build in masonry at all, or in timber, is covered in the pillar article on choosing a structural system; this text is the narrower fight inside masonry.

The differences between the materials are real, but smaller than the difference between good and bad workmanship, and most of the thermal argument is erased by a layer of insulation on the outside.

What is each of these materials?

Aerated concrete is an autoclaved lightweight concrete: lime, cement, finely ground silica sand and water are expanded by a gas-forming agent and cured in steam under pressure. The result is a homogeneous material full of closed pores, the same in every direction and at every point of a cut.

A ceramic block is fired clay with an engineered system of voids. Its thermal resistance comes not from the material (fired clay conducts heat reasonably well) but from geometry: thin webs and rows of cavities lengthen the path heat must travel. The unit is therefore strongly anisotropic, and how it behaves depends on whether the webs are intact.

How does dimensional accuracy change the cost of labour?

Both systems are now laid in their ground versions: bed faces machined to an exact height and a joint of roughly 1 mm of thin-bed mortar or foam. Unground units in conventional mortar need a full bed around 12 mm thick. That is not cosmetic: ordinary mortar conducts heat far better than the block around it, so every full bed joint is a band of thermal bridge through the wall, and carries water into the masonry too.

Aerated concrete does lay faster, but not for the reason the brochures give: it takes substantially fewer pieces per m2 (see the table), so fewer lifts, fewer joints and fewer chances to get something wrong. It does not mean lighter work in the hand: the aerated concrete piece is bigger, so it weighs as much as, or more than, a same-thickness ceramic block. The image of the "lightweight block" belongs to thin partition panels, not to load-bearing masonry.

The other half of the saving is in the plaster: flat masonry tolerates a thinner coat, saving both material and hours of labour. Aerated concrete is more reliable here, because a mason can plane an out-of-true course flat; clay cannot be planed, so flatness has to be achieved while laying, or not at all.

How much does the block's lambda really mean?

The declared thermal conductivity of both materials is very close together (see the table), and it describes the unit in a laboratory, not the wall.

What decides the outcome is what goes on the wall next. With external insulation or a ventilated facade, most of the thermal resistance is carried by the insulant, not by the block: at the usual thicknesses of mineral wool or polystyrene the difference between the blocks disappears behind the decimal places of the final U-value. Reading the block's lambda as a selection criterion is then wasted effort, and a cheaper structural grade with insulation beats an expensive thermal grade without it.

The opposite holds for a single-leaf wall, where the block's lambda is effectively the whole wall. STN 73 0540 Part 2 gives a recommended value of 0.22 W/m2K for the external wall of a dwelling and a target value of 0.15 W/m2K, applying from 1 January 2021. A single leaf in the thermal grade of either material reaches the recommended value at sufficient thickness; the target value is tight and demands the best grade available, a generous thickness and faultless reveals and ring beam. This is where filled clay and the lightest aerated concrete grades earn their price.

Where do the materials genuinely differ?

Property	Aerated concrete	Ceramic block	Decisive?
Conductivity of the unit	about 0.08–0.13 W/mK	about 0.09–0.14, filled 0.07–0.11 W/mK	Only in a single leaf
Compressive strength	order of 2–4 N/mm2 (grades P2 to P4)	order of 8–15 N/mm2	For spans and point loads
Density of the unit	about 400–500 kg/m3 in load-bearing grades	about 650 kg/m3 for a thermal block	Yes
Areal mass of a 300 mm wall	about 120–150 kg/m2	about 240 kg/m2	Yes, through acoustics
Pieces per m2 (300 mm wall)	about 6.7	about 16	Yes, through labour
Flatness and plaster	Can be planed flat, thinner coat	Cannot be corrected	Yes
Cutting and chasing	Hand saw, router, little dust	Saw or grinder, much dust	Yes, for self-builders
Moisture	Strongly capillary-active, dries slowly	Absorbs slower, dries more readily	Yes
Ordinary plugs	Do not hold	Hold in partitions under light loads	Yes, indoors
Reaction to fire	A1 to EN 13501-1	A1 to EN 13501-1	No

Will it carry a two-storey house?

Yes, both will, provided the wall is designed rather than chosen out of habit. The gap in compressive strength (see the table) is large, but must be weighed against what the house actually needs.

A two-storey house with ordinary spans can be built from either, though aerated concrete moves into stronger grades sooner. The difference shows in long spans, large openings and concentrated loads, a job for the structural engineer. In both systems load capacity also depends on the reinforced concrete ring beam and the lintels, designed as one whole with the wall.

How do the two materials behave with water?

This is the most real difference on a Slovak site, and the one discussed least. Aerated concrete is strongly capillary-active: it takes water up readily and dries slowly; clay absorbs more slowly and dries more readily. The unit also arrives from the factory damp, so a large amount of manufacturing moisture is in the wall before the first rain falls.

Keep pallets covered and protect part-built walls from rain, more strictly with aerated concrete. Unfinished masonry should not stand over winter with an uncovered top course, because water standing in the wall freezes. Once the roof is on, plan a ventilated drying period rather than just switching the heating on: a house closed up and warmed quickly shows damp corners and peeling paint that first winter. That is a fault of the programme, not the material, but with aerated concrete it takes longer to put right.

The same subject includes rising damp: the plinth detail and the damp-proofing under the first course must be faultless with aerated concrete, because it draws water from the substructure higher and for longer. Finally, both walls need a plaster that lets the drying happen. Lime and lightweight plasters suit both; sealing coatings with a high vapour resistance suit neither.

Which material is quieter?

At equal wall thickness clay wins, and the reason is physical: the airborne sound insulation of a single-leaf wall rises above all with areal mass, and there clay has a real lead (see the table).

It does not follow that a quiet house cannot be built in aerated concrete, only that internal walls have to be designed rather than derived from the external wall recipe. There are three routes: greater thickness, a different material for the acoustically demanding walls only, or a lining. Both sell products for this: dense acoustic units in clay, calcium silicate blocks from the same maker in aerated concrete. A lining of gypsum fibreboard over mineral wool closes the gap reliably and most cheaply.

Honestly, noise between a bedroom and a hallway is decided more often by the door, by service penetrations and by flanking paths than by what the wall is made of. Walls with unfilled joints and back-to-back sockets perform badly in either material.

How do fixings hold, and how are services run?

Aerated concrete will not hold an ordinary plug; it needs anchors made for the material (see the table). For heavier loads the detail belongs in the drawings. Clay does hold an ordinary plug under light loads, but heavier loads move to injection anchors here too.

For running services, aerated concrete is the more comfortable material (see the table): chases are milled, and there is little dust. In clay thermal blocks, take care the chase does not destroy load-bearing webs. For a self-builder this is the strongest argument on the aerated concrete side.

Does fire resistance decide it?

No, though both systems use it in their advertising. Both materials are reaction to fire class A1 to EN 13501-1 (see the table), and an ordinary house reaches the required resistance at thicknesses that would be used anyway. Anyone raising fire resistance as the main argument is comparing with timber construction, not with the competition inside masonry.

Why not mix the systems within one wall?

The block is only the most visible part of the delivery: with it come the mortar or foam for that unit, lintels, ring beam shells and prescribed corner details, certified as one system, hiding the real differences.

Mixing materials between elements is entirely fine: a ceramic external wall with aerated concrete partitions, or the reverse. Mixing them within one wall is a needless risk: responsibility for the detail moves from the manufacturer to you, and the documentation goes missing for the designer and the insurer. On an external wall every non-standard detail is also a thermal bridge, and non-standard details are what decide the envelope.

So which one should you choose?

Your situation	My recommendation	Why
Insulated wall (structural block plus insulant)	Either; let price and the mason decide	The insulant sets the thermal resistance
Single-leaf wall, chasing the target U-value	Filled clay or the lightest aerated concrete	Here the decimals of lambda and the details decide
Building it yourself	Aerated concrete	Hand cutting, milled chases, fewer pieces, flatness correctable
An experienced firm with its own system	Whatever that firm builds daily	A practised routine beats any catalogue difference
Acoustics are the priority	Clay, with heavy or acoustic units inside	Areal mass is the most reliable tool
Summer stability with large glazed areas	Clay, but shading first	Thermal mass helps, shading helps more
Speed of the shell is the priority	Aerated concrete	Fewer pieces and joints, but longer drying
Long spans and cantilevers	Clay, or reinforced concrete locally	Higher strength means fewer exceptions
The build stands unfinished through a winter	Clay	Less manufacturing moisture, readier drying

One more thing you will not find in the comparisons: the difference between these materials is smaller than the difference between a house whose reveals, ring beam and plinth were resolved on the drawings and one where those details were resolved on site. The choice of block accounts for a few per cent of the cost of the shell. The quality of the details accounts for the heat loss, the mouldy corner and whether the house is quiet.

Frequently asked questions

Does aerated concrete contain asbestos or anything harmful?: No. Aerated concrete is made from lime, cement, silica sand and water, expanded with a gas-forming agent and cured in steam. Asbestos was never part of it; the confusion comes from historical asbestos cement corrugated sheeting, which is an entirely different product. Cutting it releases mineral dust as any masonry does, so a respirator belongs with both materials.
Can I combine aerated concrete and clay in one house?: Yes, and it is common: a ceramic external wall with aerated concrete partitions, or an aerated concrete envelope with heavy blocks for the acoustically demanding internal walls. The condition is that each wall is built from one system, mortar, lintels and anchors included, and that the designer specifies how partitions tie into load-bearing walls.
Which material makes the shell more expensive?: The price of the units differs less between the two materials than between grades within one material: a thermal grade always costs substantially more than a structural grade. The cost of the shell is therefore decided more by the chosen build-up (single leaf versus structural block plus insulation), by masons' hours and by plaster thickness than by the logo on the pallet. Have two suppliers price the same bill of quantities.
Do I really need special plugs for aerated concrete?: Yes. An ordinary expansion plug has nothing to expand against in a homogeneous lightweight material. Use anchors made for aerated concrete, and for heavy loads (a balustrade, a wall-hung WC, a large television) ask the designer for a detail, ideally with a spreader plate or a tie into a reinforced concrete element.
Why did the plaster crack on my aerated concrete wall?: Usually one of three causes: the plaster went onto masonry that had not dried, an unsuitable plaster system with a high vapour resistance was used, or reinforcement is missing where materials change and over openings. The material itself cracks rarely. The fix is to respect the drying pauses and use the plaster the block manufacturer specifies.
Is it true that an aerated concrete house is hotter in summer?: At equal wall thickness aerated concrete has less thermal mass, so it damps the daily temperature peak less. In practice, overheating is decided first by the size and orientation of the glazing and by whether external shading exists, then by night ventilation, and only then by the mass of the walls. A clay house with unshaded south windows overheats too.
Do I have to insulate a ceramic block wall?: Not necessarily. A single leaf of thermal or filled units at sufficient thickness meets the recommended value in STN 73 0540 Part 2. If you are aiming at the target value or building a nearly zero-energy house, a structural block plus insulation is the safer and usually cheaper route. The same applies to aerated concrete.
How long should masonry dry before plaster and floors?: It depends on the material, the thickness, the season and how much water arrived with the floor slabs and screeds, so there is no universal number of days. Practically: plan several weeks of intensive ventilation after the roof goes on, rather more with aerated concrete, and let the screed dry by moisture measurement rather than by the calendar. Closing the house up and heating it fast is the commonest cause of damp corners in the first winter.