Ceramic (hollow clay) block

What is a ceramic block and how is a wall built from it?

A ceramic block is a fired, hollow-core clay unit, and in Slovakia it is what the majority of load-bearing house walls are built from. The internal webs and cavities cut the weight of the unit and lengthen the path heat has to travel through it, so thermal grades reach a declared thermal conductivity of roughly 0.09 to 0.14 W/(m·K), and grades with a mineral wool or perlite filling reach about 0.07 to 0.11 W/(m·K). Those figures belong to the block alone. Mortar, plaster and every junction still have their say, which is why the number in the product sheet and the number in the finished wall are never quite the same.

The current default is the ground block, whose bed faces are machined to an exact height so units can be laid in a thin bed of mortar or in mortar foam only a few millimetres thick. Conventional laying in a full bed of ordinary mortar is still possible and still common in renovation, but an ordinary mortar joint conducts heat far better than the block around it, so it degrades the wall thermally and carries more water into it. Perpend joints are interlocked rather than mortared, which is why a chipped or badly cut unit at a reveal matters more than it looks.

Should the wall be a single leaf, or a structural block with external insulation?

Wall thermal performance in Slovakia is governed by STN 73 0540, whose Part 2 sets a recommended value of 0.22 W/(m²·K) for the external wall of a new dwelling and a target value of 0.15 W/(m²·K) applying to new buildings from 1 January 2021. Those two numbers are where the build-up gets decided. A single leaf of thermal blocks with no added insulation reaches the recommended value comfortably. Reaching the target value with a single leaf is tight, and in practice needs a filled block, a generous wall thickness and faultless detailing. A structural block wrapped in ETICS clears both levels with room to spare, and the masonry itself costs less.

The question is therefore not which wall is better but which package you are buying. The single leaf is one trade, one layer and a facade with no board on it, so it is simpler to programme and simpler to repair. The insulated wall is two trades, but it gives a more robust thermal balance and forgives the ordinary imperfections of masonry.

Where does fired clay masonry win?

Most of the advantages follow from the material being heavy, and from the fact that Slovakia has been building with it for generations.

• Thermal mass: a heavy wall shifts and flattens the daily temperature peak, which in a Slovak summer is worth more than any cooling switched on in the afternoon.
• Acoustics: areal mass is the most reliable tool against airborne noise, and clay brings it without any extra measure.
• Fire: fired clay is reaction to fire class A1 to EN 13501-1, non-combustible and contributing nothing to a fire.
• Durability: fired clay does not age chemically and tolerates rough handling on site.
• Trades: a mason who works with the system daily exists in every district, which lowers both risk and price.

What does a ceramic block wall cost you in programme and detailing?

Masonry is a wet trade. Water arrives with the mortar, with the concrete of ring beams and floor slabs, and above all with screeds and plaster. It has to leave before floor finishes and final surfaces go on, which means a drying pause that has to be planned and ventilated through rather than skipped. Houses closed up and heated too quickly show damp corners and peeling paint in their first winter, and that is a fault of the programme, not of the material.

The second cost is thermal bridging. Window reveals, lintels, reinforced-concrete ring beams and the junction with the floor slab are made of materials whose conductivity has nothing in common with the block. Unless they are resolved, with an insulated upstand at the ring beam, insulated reveals and the window set in the correct plane, the U-value in the drawings and the U-value in the wall diverge. The third cost is weather dependence: wet trades have temperature limits and the winter break is real.

How is a ceramic block different from an aerated concrete block?

Both are non-combustible masonry units laid in thin-bed mortar, and both are sold as complete systems with lintels and accessories, so the difference is not in how the wall is built but in what the material does once it is standing.

If the programme matters more than the mass, the comparison worth making is not with another block at all but with timber frame construction, which removes the wet trades entirely and reaches the same U-values in a thinner wall.

What do clients most often get wrong about ceramic blocks?

The first misconception is that the conductivity printed on the pallet is the performance of the wall. It is not: it is the performance of one unit in a laboratory, before mortar, plaster and junctions are added. The second is that a thicker wall compensates for a poor reveal. It does not, because heat leaves through the shortest available path, and an uninsulated reveal is exactly that. The third is that a masonry wall breathes and therefore ventilates the house. Vapour diffusion through a wall moves a tiny fraction of the moisture a household produces daily; ventilation, whether by window or by unit, is what removes it.