19th May 2013

A specifiers guide to aircrete - part one by Jenny Smith-Andrews

In part one of our new specifiers guide, Jenny explains away the misconceptions that may exist about our range of aircrete products...

What are the key benefits of H+H products in terms of current housebuilding techniques, particularly high Code levels, and zero carbon in 2016?

Aircrete blocks from H+H have the great advantage of familiarity for site workers combined with a technical performance that suits them for use in the most modern and demanding house designs.

Masonry construction remains far and away the most popular building type for housing (the most recent NHBC figures suggest that over 80% of new houses in the private sector use a masonry fabric).  We believe this is down to a combination of customer preference, cost, flexibility in terms of design and convenience in terms of delivery.

Clearly, as Building Regulations demand ever higher levels of thermal performance all building materials have had to prove their case and aircrete is no exception.   Aircrete makes a positive contribution to the thermal performance of walls because of its inherent thermal efficiency, provided by the air cells that form part of its structure.

A cavity wall construction, using two leaves of aircrete in thin-joint construction and with a cavity width of 125mm can achieve a U-value of 0.19W/m2K or better depending upon the insulation used.

H+H has also worked with some of the most innovative pilot projects experimenting with building methods and materials to find the most effective ways of meeting the higher levels of the Code.

Our first Code 6 (or zero carbon) project was in association with Barratt back in 2008 when we contributed our Vertical Elements to the Barratt Green House built at the Building Research Establishment.    Vertical Elements are 200mm thick storey-high blocks of aircrete that are craned into the site and typically use insulated render on the exterior façade.

These products were used again by Miller Homes in its experimental live site at Miller Zero project, Basingstoke.  For their Code 6 house, the walls were built with 200mm-thick aircrete Vertical Elements with 200mm WeberTherm PHS insulation and a 6mm WeberTherm XM render.  The result was a wall with a designed U-value of 0.09W/m2K.

We have no doubt that aircrete will make a valuable contribution to zero carbon homes of the future, but of more immediate relevance to our customers is the ability to build effectively to current requirements.

Therefore, H+H was pleased to be the only material manufacturer to participate in the AIMC4 project: a Government-backed collaborative project of housebuilders working together to produce standard house designs that achieve values to the enerty section of the Code for Sustainable Homes to Code Level 4, without using any micro-generation technology.

How do H+H’s products fit in to the market in the current challenging circumstance – what are the most lively sectors currently (e.g. housing/commercial refurb/extensions v new build?)

Clearly the market remains challenging for any manufacturer involved in the housebuilding market.   At the moment a sizeable proportion of our product is being supplied through merchant stock to contractors working on refurbishment and extensions.  The ease of handling of the lightweight aircrete blocks will always make them a popular choice for the smaller contractor and the market for extensions does seem to be holding up well.

In terms of new build housing we are finding that the pattern of building has changed since 2008.  Builders are much more likely to build only at the speed at which houses are sold, rather than commencing build on a whole development site at the same time.   For this more piecemeal approach our products are ideal: short lead-times and flexible site deliveries mean that individual building projects can be serviced very quickly.

Is thin joint receiving the take-up expected in the market?  Explain the benefits and the issues if any in terms of changing construction practice.

Thin joint as a construction method has been available in the UK for many years and makes for a very fast and efficient method of construction.   Walls built using the thin 2mm mortar  joint method will also by their nature provide reliable airtightness and low levels of thermal bridging.

There is little additional training required for thin joint, but it is a different method and therefore does require the site workers to understand the principles and application methods.   Thin joint has long been recognised by training colleges and is part of the curriculum for students working on brick-laying and building courses.

However, a depressed market tends to work against innovation and thin joint, while gradually growing in popularity, still does not dominate the market in the way that logic suggests it should.

To help with familiarisation of the technique, H+H has published a number of explanatory documents and also, more recently, launched a series of bite-sized video guides to each stage of a thin joint process. These can be viewed on on our You Tube channel.

Do misconceptions still exist among customers in terms of what aircrete is, and how it differs from traditional concrete blockwork?  Please explain the basic differences and benefits.

We do still get surprised by the some of the questions received by our technical team!  In broad terms specifiers do appreciate the main performance benefits of aircrete: that is to say its inherent thermal insulation as opposed to traditional dense concrete. H+H aircrete products are typically 10 times more thermally efficient than dense aggregate concrete.

However, some of the detailed performance characteristics can be less well understood.   Speed is a good example.  Using the thin joint system a wall can be built to first floor height in just one day as the thin joint mortar does not need the extended curing time of its traditional mortar equivalent.  This allows the watertight shell of a building to be constructed extremely quickly, allowing the interior trades into the build much sooner and significantly speeding up the whole build process.

However, since the inner leaf of aircrete in cavity construction is built independently of the outer skin, the cavity insulation can be inspected before it is covered up.  In addition, the outer skin is taken off the critical path and can be constructed simultaneously as first fix takes place.

What are the blocks’ credentials when it comes to minimising thermal bridging how important is this issue for customers currently?

Aircrete performs particularly well in the area of linear thermal bridging at junctions of construction.   This is an issue that is coming under greater scrutiny, since heat loss from such junctions can account for as much as 30% of the heat lost through the fabric.  Aircrete transmits much less heat than other forms of masonry products, by a factor of up to 10.  As a result, the issue of heat loss at junctions can be significantly reduced.  Aircrete constructions can be assessed and designed such that the loss owing to thermal bridging is typically halved, allowing the designer to have greater flexibility in meeting the Building Regulations.

Aircrete performance values are found in a new set of Handbooks independently published on www.constructivedetails.co.uk/resources.  An important area of reduction thermal bridging is at the foundation of a wall.  H+H Celcon Foundation Blocks are extremely valuable in reducing heat loss through this critical junction.

Come back soon to read part two of the guide