Aircrete: A viable choice for building social housing
Timber frame construction has enjoyed a near monopoly with social housing developers in the last decade, possibly as this building method is perceived to be quicker and cheaper than the alternatives. However, contractors are starting to look again at other options and coming up with some surprising conclusions.
Masonry build has been underutilised in the social housing sector in recent years partly because it was felt to be slower than timber frame systems to build, and partly because of a widely held belief that masonry inevitably comes with a hefty price tag. Neither of these assumptions is accurate.
An independent cost comparison carried out by calfordseaden found that making use or an aircrete system from H+H (Rå Build ) resulted in an 8% saving compared with timber frame of SIPS construction. This study, based on a typical 20-unit development of houses, calculated a saving of £59/m2, equivalent to an entire additional house.
This research came to the attention of developer Wates Living Space as the company was working on the design concepts for a new development of 28 houses and flats in Didcot, Oxfordshire for affordable housing provider A2Dominion.
Wates Living Space had not used an aircrete method of construction before, having held the view that such an approach would bring a higher cost. They therefore decided to carry out their own unique cost comparison specifically for this Didcot development. This comparison compared the H+H Rå Build method of construction using thin-jointed aircrete block-work with an equivalent timber frame approach.
The results, based on this Didcot development, showed that not only was the masonry approach no more expensive than timber frame, but in fact some £20,000 could be shaved off the development costs because of the sheer speed of the construction process used with Rå Build.
Rå Build represents the ultimate system build: one suitably qualified contractor is used for the entire process, ensuring quality and speed of build from the start. This contractor forms part of a labour inclusive package that comprises the construction of exterior and inner walls, upper floors and partitions, in fact an entire aircrete inner shell. There are no lead times as all components are available off the peg. An immediate advantage is the elimination of logistical problems that can occur with different contractors and tradesmen onsite, as the aircrete contractor has total control over the initial build phase.
This method of aircrete construction makes use of lightweight large format aircrete blocks and fast setting thin-joint mortar.Full bond strength for the vertical coursing can be achieved within just a couple of hours, in fact quick enough for the first-floor joists to be built in after only 24 hours from starting the first course. Being able to build so quickly up to the roof requires just one visit for full height scaffolding instead of for every level, therefore reducing scaffolding costs. Consequently, entire storey heights can be built by relatively few men in a matter of days.
Wates Living Space was suitably impressed when the inner shells for the 22 houses on the Didcot site took just 16 weeks to finish and be made weather-tight. This meant first-fix trades could start on the inside while external finish work got underway at the same time outside. The Rå Build method of construction requires no waiting time for settlement, unlike other system build approaches, meaning external finishing can commence immediately the shell is completed.
Furthermore, making use of aircrete as a material with thin-joint mortar improves thermal efficiency of the completed structure. This is especially important moving forward as social housing developers are compelled to build to higher levels of the Code for Sustainable Homes and aircrete has already been used to build dwellings to level 6 of the Code. If the fabric of a building can be well insulated to start with then the need for installing expensive micro-renewable technologies will decrease.
In conclusion, it is clear that the main advantages widely associated with timber frame building can also be gained using masonry, especially aircrete and Rå Build. Such a construction method offers a simple, fast and cost effective building fabric that will meet or exceed present and future Building Regulations and ensure a robustness far greater than the competition.