For brick-and-block cavity walls, the most obvious change will be a thickening of insulation to achieve the 0.3 and 0.35 W/m2°C U-values for walls. Some say this will mean the end of the clear cavity in favour of partial-fill, or a move from partial-fill to fully filled cavities. Others, like Huw Evans of insulation firm Rockwool, predict that heavy density blocks will be phased out, in favour of more lightweight aerated types, although this would need careful design to comply with the acoustic regulations. But what has emerged is that there will be many ways to satisfy the new rules.
Mike Hammett, senior architect with the Brick Development Association, says: "We are quite confident that you will be able to build masonry to meet the requirements, but it will probably mean wider cavities." The traditional reluctance to fully fill cavities was based on the possibility of water transmission across the insulation to the inner leaf. But, adds Hammett: "Wider, fully filled cavities will reduce this possibility, especially where the extra width allows the insertion of two thicknesses of insulation."
John Pilkington of insulation manufacturer Owens Corning is slightly bemused by the initial reaction to the proposals. "When people calm down and look at the thermal requirements, they will see that they can be achieved using standard masonry constructions."
A number of manufacturers are looking at ways to make block inner leaves more thermally efficient. Paul Reed, marketing manager of aircrete block maker Thermalite, says the thin joint system can improve the thermal performance of an inner leaf by up to 10%. But that is still not as efficient as timber-frame construction, so block manufacturer H+H Celcon is investing in aerated concrete planks to improve the insulating value of roofs and floors. Technical director Cliff Fudge regards this as an opportunity to address the overall energy efficiency of a dwelling rather than simply concentrating on wall U-values, as he says most people have been doing.
Thermalite's Reed does not see Part L as giving any boost to timber frame, arguing that an aircrete block is a better insulator and needs less added insulation than timber. But he stresses the need to distinguish between the elemental approach to calculating U-values, which he concedes could result in larger footprints, and the target method, which allows trade-offs between roofs, walls and floors.
Under this regime, which Reed says is used for most new houses, the new Part L need not result in wider walls if floors and roofs are more thermally efficient. And he adds: "We don't see a problem with the new regulations. There may be cases where the footprint of a masonry cavity is slightly bigger, but it will not be that common."
Many see the new Part L as boosting the use of timber-frame, now currently accounting for about 8-10% of new house construction. In a post-Egan, energy-conscious industry, its capacity to accommodate large amounts of insulation and its factory-controlled manufacture look attractive – not to mention its suitability for compliance with Part E.
Paul Newman, head of timber technology at the Timber Research and Development Association, thinks that the new rules will necessitate changes in both masonry and timber-frame construction. "But it is vitally important for housebuilders," he explains, "that timber frame can accommodate the new Part L requirements without increasing the footprint of the construction. I think most housebuilders will move to a partially filled cavity if they wish to retain the use of an 89 mm stud." This view assumes that partial insulation of the cavity between the timber frame and external leaf will not change existing walls widths or details.
Newman's comments are echoed by Dave Baker, technical director at the House Builders Federation, who states: "There is no question that if you just want to comply with Part L, timber frame is the easier option as it provides greater potential for insulation. The structural frame is less substantial than an inner leaf of loadbearing masonry. But what we want is the option to build either way without any one party holding a monopoly."
How the timber-frame industry will adapt to Part L is difficult to predict. Some will move to a 140 mm deep stud (also good for three-storey construction) with full mineral wool insulation between the studs; while others may opt to stay with the standard 89 mm stud, maintain the insulation in between and, in addition, opt either for a partially insulated cavity or insulated plasterboard. Both systems could achieve the new U-values.
But some large housebuilders are forming or buying up timber-frame fabrication plants not only to have greater control over the whole construction process, but also to provide greater opportunities for prefabrication. In this, Westbury Homes has been inspired partly by Egan principles, but also by the growing lack of skills in the industry. The timber-frame houses made by its new Space4 division are divided up into panelled sections produced on an automated production. These are delivered sequentially to site and assembled.
Beazer Group is also keen to rationalise its construction process in order to build more timber-framed homes, currently one-third of its output. This may change after its sale to housebuilder Persimmon, but its factories in Edinburgh and Ipswich already supply timber frame kits for Beazer and others.
These use the company's Tee-U-Tec technology, which involves large, closed, "reverse" wall panels that have structural sheathing on the internal side to eliminate troublesome vapour barriers.
The first amendments to Part L are expected this summer. They will include an increase in the insulation value of walls from 0.45 to 0.35 W/m2°C.
The wall value is due to be increased again 18 months later. But the DETR has already said it will extend the period before this second phase to give the industry time to develop new solutions. So perhaps housebuilders can relax. Or can they? The original proposals to amend Part L called for a further rise in insulation values by 2005. So the next panic may not be far away …