What’s the difference between aircrete and lightweight aggregate blocks? Jenny Smith-Andrews, Head of Marketing at H+H explains.
Anyone who works in construction will be familiar with the stacks of grey blocks that are a common sight on building sites. You could be forgiven for thinking these blocks are all the same, but that is not the case.
Building blocks can be dense aggregate blocks, lightweight aggregate blocks or aircrete blocks. Do you know the difference? Use of the term ‘lightweight block’ can be confusing as it can apply to both lightweight aggregate and aircrete blocks, but there are differences and benefits to building with aircrete that don’t apply to lightweight aggregate.
What is aircrete?
Aircrete is a lightweight concrete material that combines strength, durability and thermal efficiency with the ease of use that comes with a lighter material.
Aircrete is made using cement, lime, water and pulverised fuel ash (PFA) with a dash of aluminium. The aluminium causes a chemical reaction that forms millions of bubbles to create a lightweight block.
This light weight makes aircrete quick and easy to work with on site and its material properties make it easy to cut. It can even be cut with a handsaw making it a versatile building material. A standard grade aircrete block weighs 12kg less than an equivalent dense concrete block and 6kg less than a typical lightweight aggregate block.
Aircrete may weigh less but its performance is not reduced. Those bubbles give it better acoustic and thermal insulating properties than dense or lightweight aggregate blocks. Aircrete’s high thermal efficiency enables it to achieve low U-values, easily meeting the standard set out in the 2021 amendment to Approved Document L. Using aircrete can also help to reduce heat loss at thermal bridges, another focus of the updated Part L.
Reducing heat loss at thermal bridges
Linear thermal bridges can account for 20-30% of the heat loss in a typical new build home but building with aircrete can minimise the impact.
There are two types of thermal bridges: non-repeating (or linear) and repeating. Repeating thermal bridges are usually evenly distributed across the building envelope and follow a regular path. Sources of these types of thermal bridge include insulation bridged by cavity wall ties, stud walls and mortar joints.
Non-repeating thermal bridges are caused by discontinuities in the building envelope and occur at junctions between elements such as a wall and a floor or a window and a wall. Where any thermal bridge occurs in a build, heat is more able to transfer through the construction, resulting in greater heat loss from the building.
The relative effects of non-repeating thermal bridging become more significant as homes become better insulated so understanding how to build to minimise this is crucial.
Using aircrete can significantly reduce the thermal bridge effect at junctions, as it has better thermal resistance than dense materials.
Aircrete blocks used in separating walls can achieve added thermal benefits and limit heat loss at junctions with external elements. When used in conjunction with aircrete inner leaves, heat losses at thermal bridges around party walls can be reduced by around 50% when compared to aggregate concrete block party walls.
In fact, as new building regulations increase emphasis on building fabric and thermal bridging, aircrete is a preferred material and this can include its use in foundations.
Strong foundations
The 2021 amendment to Part L recommends using aircrete foundation blocks where aircrete is being used in the construction of external walls to ensure continuity of insulation levels within the building fabric.
There are many reasons to use aircrete below DPC. Firstly, it is relatively faster and easier to lay as it weighs just one third of an equivalent dense aggregate block. In addition, only a single leaf of blockwork needs to be laid to form the foundations, as opposed to cavity construction which requires two leaves of blockwork and concrete infill. This means works can be completed in less time, reducing labour costs.
Aircrete foundation blocks can also help to enhance a property’s energy efficiency by reducing heat lost into the ground while reducing the need for additional insulation.
High strength blocks
Despite being one of the lightest forms of masonry, foundations built with aircrete are fully load-bearing. At H+H, our Foundation Blocks Standard Grade are suitable for low-rise housing, but we also offer High Strength Grade Blocks, with a compressive strength of 7.3N/mm², that are ideal for three storey residences.
Standard grade aircrete blocks are already highly thermally efficient, however the H+H Celcon Solar Grade block prioritises combatting heat loss even more. With a compressive strength of 2.9 N/mm²this highly efficient block has a thermal conductivity of just 0.11 W/mK. It is sufficiently loadbearing for two storey dwellings and can be used below DPC.
So, next time you walk past one of those packs of grey blocks, take a closer look – they are more interesting than they first appear.
For further information of H+H Celcon, click here.
