Airtight buildings

Materials World magazine
3 Aug 2015

Cliff Fudge, Technical Director at H+H, argues the case for the thin-joint system and associated methods of building. 

In the UK, the vast majority of new-build houses are still constructed using masonry materials, usually brick and block, with mortar. When it comes to laying the blocks there are two systems that can be employed – traditional and thin-joint.  

Traditional mortar is what is most commonly used on building sites across the UK. It is practised by all bricklayers and has been tried and tested over decades, even centuries. Traditional mortar is widely known and understood and, therefore, it is not difficult to find a bricklayer with the appropriate skills. 

Thin-joint is a modern method of construction that has been used on mainland Europe for around 50 years. However, even after five decades, it is still considered to be innovative, particularly in the UK, despite being the go-to method of building in many European countries.

The thin-joint system combines the use of large format, accurately dimensioned aircrete blocks and quick-setting thin-layer mortar to create a highly productive and cost-effective building system. The name refers to the thickness of the mortar joint – traditionally, mortar joints are 10mm but the thin-joint system reduces it to 2mm. Instead of using traditional sand:cement mortar, the thin-joint system uses a cement-based mortar that is supplied as a dry, pre-mixed powder. 

Recent Government initiatives have focused on speeding up the supply of housing while improving the quality of build and reducing its environmental impact – targets that can be reached with the help of such a system, as it allows for construction times to be significantly reduced. 

Speeding up

The speed at which a building can become weather tight is the main onsite benefit of using thin-joint. Historically, block layers have only been able to build six courses per day, but this fast-setting mortar allows for continuous laying until storey height is reached. The approach enables the inner and outer leaves of a cavity wall to be constructed independently of each other, allowing for a weather-tight shell to be constructed at a much quicker pace. There are other practical benefits that help increase the build rate, such as being able to cut the blocks on site using a saw and move them around with greater ease, as they are lighter.

The main components of aircrete blocks used in the thin-joint system are pulverised fuel ash, cement, lime, and a very small amount of aluminium powder. It is the chemical reaction caused by the aluminium powder that creates thousands of non-interconnecting cells. It is these cells that make the aircrete blocks much lighter in weight than the denser concrete block alternatives. 

Keeping warm

Aircrete has the advantage of an inherent thermal efficiency, which, combined with thin-joint mortar, helps reduce the air permeability of a wall, decreasing the amount of heat lost through air infiltration. 

The benefits of aircrete are not simply confined to helping keep homes warm in winter – its high thermal mass also helps to keep buildings cool in the summer. 

One of the consequences of successive governments’ drive to improve the fabric energy efficiency is that it can sometimes lead to lightweight buildings overheating in summer. This is because a highly insulated fabric that retains heat in winter is less beneficial on a warm summer’s day, particularly once the heat from occupants, large numbers of electrical appliances, and solar gains are included. As a result of these heat gains, new homes that are constructed from lightweight materials can become uncomfortably hot in summer. 

In this case, the advantage of an aircrete-based construction is that it has a relatively high thermal mass, which allows the walls to absorb heat during the day, limiting the rise in internal temperature. The heat is stored until internal temperatures fall, when the heat is released back into the building. The cycle repeats on the next day. For this to work most effectively, it is usually necessary to flush away the released heat at night, by opening the windows to encourage ventilation. This solution works well in the UK, where night-time temperatures are significantly lower than the day-time peak.

Building regulations are likely to become increasingly tighter when it comes to energy efficiency. This system of using traditional masonry materials with an innovative mortar could help meet targets, reduce construction times, and improve thermal management of homes.

Case study

Speed of build is crucial on many projects, which is why H+H used the thin-joint system to develop the ‘Rå Build’ method of building, where the internal leaf of the external walls and any required partition walls can be built without delays and the workability of aircrete blocks allows for any opening to be easily constructed for windows and doors. 

A housing association project in Kingsland Suffolk is a prime example of how the Rå Build method of building can help to quickly deliver a project. Wellington Construction built a pair of semi-detached bungalows on a restricted site for Orwell Housing Association and was able to reach roof plate height in only four and a half days, from the damp proof course to the final block being laid. 

Paul Ollington, construction director at Wellington Construction, said, ‘For these bungalows time and quality were of utmost importance. The thin-joint method definitely adds to the speed at which builds
can happen.’