What is porous brick?
Porous brick is brick in which the fired clay body itself is riddled with small voids. This is not the same as the large vertical cavities of a perforated or hollow block — it is the clay between those cavities that has been made porous. The result is a unit that is lighter and insulates noticeably better than a dense solid brick.
Two ways to make the pores
- Through the clay itself. Some clays naturally contain diatomite (kieselguhr), or it is added. Firing leaves a fine porous skeleton behind.
- Through a burn-out additive. Sawdust, cork, paper sludge, coal dust, fly ash or polystyrene beads are blended into the clay. During firing that material burns away completely and leaves a pore at every point. This is by far the most common method.
The choice of additive determines how coarse the pores become and how far the density drops. An ordinary solid brick sits at roughly 1,800-2,000 kg/m³; porous bricks sit well below that, somewhere between about 600 and 1,400 kg/m³ depending on type.
What you gain and what you give up
- Gain: thermal insulation. Still air in small pores conducts heat poorly. The thermal conductivity of a porous body is far lower than that of dense brick, so less insulation thickness is needed.
- Gain: weight. Lighter units are easier to lay and put less load on the foundation.
- Loss: compressive strength. Pores cost load-bearing capacity. For a loadbearing wall, always check the declared compressive strength class against the load.
- Loss: mass, and therefore sound insulation. Airborne sound insulation comes mainly from mass. A light porous unit used as a single-leaf separating wall between dwellings will not perform; that needs a heavy or an acoustically isolated construction.
- Watch out: moisture. Porous material absorbs water. Most porous bricks are intended for internal work or the inner leaf; use them externally only where the manufacturer explicitly declares them frost resistant.
Practical points
- Wetting the units before laying is covered by the manufacturer’s instructions. A strongly absorbent unit draws the mixing water out of the mortar, so it never cures properly.
- Fixings hold less well in a porous body. Use anchors tested for porous and hollow masonry — for heavier loads, resin injection anchors with a sleeve.
- Drill without hammer action. The material cuts easily but also breaks out easily.
Related terms
- Brick
- Perforated brick
- Porous
- Poroton
- Aerated concrete
- Thermal conductivity
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