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The Observer Article – The two-bed thermal envelope

Eco house built in the Chewton Mendip using Eco-Slab™

A Somerset builder has put up zero carbon-designed homes at a comparable cost to conventional houses. Volume housebuilders seem struck dumb by the achievement.

Things aren’t always quite what they seem in the village of Chewton Mendip, in Somerset. On the outskirts of the village there are six ‘Bronze Age’ barrows, which, closer inspection revealed, turned out to be medieval. And now in the village centre, there’s another little chronological conundrum, one which got the business section of the Observer newspaper in a real flurry last month, when it highlighted three ‘traditional’ looking stone and slate homes which are really built of ultra-modern materials, making them some of the most thermally efficient homes in the country and performing at twice the Passivhaus standard. But the thing that got them into the business rather than the review section was one outstanding fact – all this, it is claimed, has been achieved for the all-in cost of £1,100/m2 – a figure that is the Plimsoll line for the mass housing market, and one below which, most volume housebuilders claim, ‘standard’ construction is unfeasible. Not only that, volume housebuilders have insisted that the new materials and methods required to create a zero carbon home will push this figure way up – meaning they’ll have to increase property prices. Yet a local builder by the name of Arthur Bland in a small Somerset village seems to have proved them wrong. So how come some mock vernacular homes on a village street corner are beating the volume housebuilders to the zero carbon goal?

While they may look like traditional cottages, the three houses are built using cutting-edge materials that put the emphasis on insulation. Walls are constructed of Logix ICF blocks, which arrive on site in 1,200 x 300 x 450mm blocks, consisting of two EPS outer sheets linked by a plastic web that holds them together. Once constructed and shuttered with ply, reinforcement is placed inside the blocks and the whole lot in-situ cast, obviating the need for lintels over any openings. With the thermal mass either side of the foam, the whole produces a U value of 0.14 out of a 353mm depth. Floors are constructed with Eco-slab, an expanded polystyrene flooring system built within the insulated foundation walls. It eliminates thermal bridging to give a U value of 0.13.

Which Code level?

A Unilin Systems SIP panel roof extending from a ridge beam down to a bolted wall plate achieved a roof value of 0.09, and the Samson Wood windows are triple-glazed. Taken together, the total space heating requirement comes to an impressive 7.75kWh/m2/year. Needless to say, there is no need for a central heating system. According to the builder, it meets Code 6 of the Code for Sustainable Homes in construction. It all sounds too good to be true.

Architect Bill Dunster of ZEDfactory is not quite so convinced. ‘These houses are no way Code 6 – they meet only the required heat loss parameters, which is a small part of the overall requirements. They have no solar thermal or renewable electrics, and are built from some of the highest environmental impact materials possible,’ he says. ‘The Styrofoam SIPS formwork also isolates the thermally massive concrete from the room interior – negating the passive cooling potential of a high embodied carbon material.’

Bland would counter this by stating that the energy-saving methods inherent in the build method place the house in carbon credit after only six years. Dunster, however, thinks that at the price, they are not really getting value for money. ‘I suspect that with no biomass heat supply, they are actually Code 3, and the cost for Code 3 is actually quite high – our Code 6 solution costs around £1,395 gross external, including all the solar thermal, solar electric and biomass heat – and with a breathing wall construction to similar levels of thermal performance rather than a sealed plastic shell.’

Senior sustainability engineer Alasdair Young of Buro Happold is less vociferous in his criticism, but remains cautious about the claims, and on the quality of the internal environment being created. ‘The construction approach is certainly novel, but I doubt whether there has been a full post-occupancy evaluation,’ he says. ‘The design does seem quite well placed to cope with climate change and future rises in the mean temperature, but may suffer from the same problems as lightweight build if they are decoupling the thermal mass’ – an aspect that echoes Dunster’s concern about the internal environment quality.

It’s undeniable that increased air tightness will affect air quality, something that is reflected in the fact that Part F, as well as Part L is being consulted on for adoption next year, but Bland has apparently borne this in mind, with the installation of a Genvex mechanical heat recovery ventilation system, providing five air changes an hour, and salvaging the heat to bring 185 litres of water up to a very useable 60°C. Opening windows obviously allow for passive ventilation to be taken up in the summer months. Young’s biggest gripe, however, seems to be more the reticence of the local council to permit any form of renewable technology to be installed on the homes, regardless of whether they are low energy or not.

Mysterious lack of interest

Of course, given the fact that the technology can be dressed up in any form whatsoever, it does beg the question why, at the per square metre costs that the homes came in at, it is not being looked at by the volume housing industry as a means of meeting the government’s zero carbon targets for the UK, without – potentially – any recourse to stylistic change in design.

Bland has his own view on that: ‘I think it is because poured concrete is not usually considered as a medium for building domestic dwellings in the UK, and the building industry will be slow to recognise the advantages that in situ insulated formwork will provide. There are also huge vested interests in the present way of working to the current supply chain. I suspect that producers of building products, beam casters, builders merchants and hauliers will be not too impressed with a system that puts dry and light products together, places steel in them and then casts floors, beams, lintels of any span and structural concrete in one swift operation.’

It is not only this, though, but the fact that like is not being compared with like. ‘When I started this project,’ he continues, ‘Even the SIP roof manufacturers said they couldn’t compete with the cost of a trussed roof. Except a huge amount of work is required to make a trussed roof with a U value of 0.09.’ So even if it were possible, which he very much doubts, Bland says it would be prohibitively expensive, adding: ‘So the system actually isn’t comparable on cost grounds – because it is so much better!’

But if that’s a reason to have the volume housebuilders clamouring to take up the technology, they sure aren’t showing it. RIBAJ approached Redrow Homes, Persimmon, Countryside Properties and Berkeley Homes, all of whom declined to comment on the potential of Bland’s code-busting proposal – a lack of interest or engagement that is worrying purely in its consistency. But as Buro Happold’s Young says: ‘The trouble with volume housebuilders is that ultimately they’re building to a minimum standard, which is a de facto maximum. Entrepreneurial innovations are great, but volume housebuilding is a risk-averse industry – and nobody in it wants to be the guinea pig.’