Infill Walls in Autoclaved Aerated Concrete (AAC)
Aerated concrete forgives almost everything except two things: a first course out of level and water. The first cannot be recovered, because a two-millimetre joint leaves no room for correction; the second stalls the site for months.
Overview
TAV. 00AAC is concrete full of bubbles: a fifth to a tenth of the density of an ordinary block, and a conductivity low enough for single-leaf walls to meet requirements with no added insulation. That quality shifts the problem, though: if the block insulates this well, the thermal bridge becomes the mortar joint. From this follows the whole technique — thin-bed laying, with two-millimetre beds instead of twelve, which in turn demands absolute geometric precision from the very first course. The material's second character is absorption: AAC drinks, and drinks a lot. From that follow the rules on site protection, on keeping the joints clean and above all on the render, which cannot be a rigid cement mix spread on a deformable, thirsty background.
The site score
TAV. HThe detail of the craftthe only joint you can adjust
The first course
Waterproofing the base and laying the first course on a mortar bed, brought to level to the millimetre with a laser.
Do the first course with the laser and take your time. Get it wrong here and you're wrong all the way to the roof, and you won't notice until it's too late.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
Two blocks, two philosophies
AAC or clay block: how the choice is really made
These are the two materials competing for the single-leaf wall, and they solve the same problem with opposite strategies. AAC puts the air inside the material, as closed bubbles spread through the whole mass; the porous clay block puts it into the geometry, with thin webs and elongated cavities that force heat to travel a longer path. Different behaviour follows on weight, cutting, fixings and above all water.
A qualitative comparison between two product families. The U-values and strengths to be used in design are those declared by the manufacturer for the specific block, verified on the real wall with its joints and singular points.
The joint worth ten per cent
How much the mortar really costs in performance
This is the calculation that justifies the whole thin-bed technique. An AAC block conducts very little; cement mortar conducts five or six times as much. With twelve-millimetre joints the mortar covers around ten per cent of the wall area, but being far more conductive it weighs on the result by much more than ten per cent. Reduced to two millimetres, the same mortar covers less than two per cent and its contribution becomes negligible. This is not a refinement: it is the difference between a wall that meets the requirements and one that does not.
A schematic representation with indicative proportions. The actual U-value of the wall, including joints and singular points, must come from calculation to the relevant standards and from the manufacturer's declarations.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QHow do you hang something heavy on an AAC wall?
With anchors made for aerated concrete, not ordinary ones. The principle is different: a traditional expansion plug works by pushing against a hard material, and in AAC it simply crushes it and pulls out. Dedicated anchors have a spiral or large-surface geometry that spreads the load over a far wider area. For serious loads — loaded kitchen units, radiators, handrails — the right answer is planned while the wall is being built, by placing an anchoring element or a solid block at the right spot: it costs nothing beforehand and a lot afterwards.
Can AAC be used for load-bearing walls?
Yes, blocks of adequate strength class exist and load-bearing AAC masonry is a well-established technique, but it is a different design chapter from infill work: the block classes change, so do the requirements on joints, the reinforcement of ring beams and the seismic checks. The dangerous confusion on site is the opposite one — using a lightweight infill block where the design called for a structural block, because “it's the same material anyway”. It is not: the class is read off the pack and checked against the design.
What happens if the blocks are left out in the rain?
They drink, and they do not give the water back quickly. A pack of blocks left uncovered for a week can reach the wall with a water content far above the production level, and from there drying takes months. There are three practical consequences: temporarily worse thermal performance, render at risk of being applied to a saturated background, and finishes that blister. The site rule is trivial: packs covered, protective film left intact until use, and walls under construction protected at the top at the end of each day.
Can it be left exposed?
Indoors yes, with a thin skim, and it is a finish with a character of its own: a compact, pale, even surface. Outdoors never without protection: AAC is not made to resist driving rain and freeze-thaw cycles with a bare surface. Blocks with a specific external finish and certified fair-faced systems do exist, but they are dedicated products, not the same block left uncovered. Otherwise the risk is not aesthetic: it is progressive degradation of the surface layer.