Geoff Wilkinson looks at the potential for using straw in housebuilding, and how it complies with the Regs
With scare stories about shortages of bricks and other building materials emerging almost daily, our focus this month turns to the use of straw as an alternative option. Straw is, technically, a waste product from wheat production and is therefore perfect for projects where sustainability is a key consideration. Nearly 4 million tonnes of straw is produced annually in the UK and, since approximately 7 tonnes is required for the construction of a three-bedroom house, about half a million homes a year could, theoretically, be built in this way.
Straw dates back thousands of years worldwide as a construction material but, in the West it has acquired an undeserved notoriety as a poor choice, not least on account of the Three Little Pigs fable. But is its bad reputation justified? Readers may be surprised to learn that there are tens of thousands of buildings made of straw cob (straw mixed with a clay-based subsoil) in the south-west of England alone, and that these have survived quite happily for hundreds of years, proving that the method is resilient.
However, straw construction was effectively outlawed in the UK following the Great Fire of London. The first sets of National Building Regulations, introduced between 1800 and 1900, required external walls to be built of brick or stone. It wasn’t until the 1984 recast of the regulations that straw once again became a viable option as a construction material.
Here are the key regulations, problems you will encounter getting approval, and how straw can be made to comply:
Part A Unlike most other forms of construction, there are no structural Eurocodes to follow with straw. So, while bales can be used for load-bearing, their strength varies depending on compaction and the type of earth used to bond them. It is likely that you will need to incorporate a structural timber frame to have this approved. This also helps to reduce the required thickness of the wall, which is often seen as a barrier to the commercial use of straw bale because of its impact on plot density.
Part B As mentioned, the 1666 Fire of London effectively put paid to the use of straw as a construction material and building inspectors are likely to be wary of cob for this reason. But this is not entirely justified, since straw bales perform surprising well during fire tests, and up to two hours stability is possible. Yet it is likely that building control will require they be encased with non-flammable facings, such as plaster internally and lime based render externally.
Part C One of the biggest problems with straw is that it will degrade when wet, so it is unlikely to be acceptable within a flood-risk area. Even in low-risk areas it is essential that the walls are raised on a stone or brick plinth with a suitable damp-proof course. Any renders or plasters should be breathable, while bathrooms or wet areas should be tiled on a marine ply background. You may be asked to produce an interstitial condensation model to prove there is no risk of condensation being formed within the make-up of the wall.
Part E Straw bale construction is most common in detached buildings, but can potentially perform well in reducing sound transmission and typically offer 48-50dB sound reduction.
Part J Selection of heat-producing appliances (open fires and log burners in particular) requires care to ensure flues passing through the straw are adequately separated.
Part L The thermal insulation properties of straw incorporated within timber frame can achieve U-values of between 0.19 and 0.11W/m2K, and have been accepted towards Passivhaus accreditation. However, there have been problems in getting approval of unmodified cob without undue reliance on active measures, such as heat pumps.
Part P Electrical wiring should be housed within proprietary plastic conduits and sheathing (metal should be avoided within cob), and consumer units should be mounted on firm, non-combustible backings.
Part Q Recently introduced requirements for security mean that lightweight framed walls need to incorporate a resilient layer around external doors to prevent unauthorised entry
Geoff Wilkinson is managing director of Wilkinson Construction Consultants www.thebuildinginspector.org