Making buildings: Agricultural aesthetic overcomes materials challenges on Green House

It seems a very tucked-away site?

Tom van Schelven: The site, in an unmade lane in the Clyde Circus Conservation Area in Haringey, was ideal for us. There was a 1980s building on the plot that had been haphazardly extended. At the end of the lane is woodland and a Victorian coach house. Modern flats had been built further towards the road. 

Nick Hayhurst:  The language of the house emerged from the history of the site. We considered mimicking the later brick development but that would have reinforced the sense of a ‘street’, which this site never was. It was a ‘working’ lane of orchards and animals so our design reflects historic use and aspires to rural rather than urban typologies. That’s also why we inverted the normal house convention with an informal, dynamic frontage and a restrained ‘rear’ facade.

How did you win planning permission for a radical design in a conservation area?

Claire Taggart: The original three storey house was under illegal multiple occupation, which didn’t sit well with neighbours. Six previous permissions for flats were refused as over-development. The planners seemed more at ease with our proposal for a single-family dwelling.

NH: It helped that we proposed lowering the overall built height. It is also set back on the lane side where the original building had projected. From the street it had read obliquely as a crash of different volumes but the new proposal, though made of radically different materials, was more in line with what already existed. So in all, it was ‘lower and less protruding’.

TvS: The local Conservation Areas Committee was in favour of it, though perhaps because it was tucked away down a lane. Even though it looked different, they were intrigued that it stood on its own merits and wasn’t ‘fake contextual’. Even so,  Nick told us we should hunker down for the long run.

NH: We had told Tom and Amandine to expect to go through the process twice, but we got consent in 8 weeks without any changes. Tom liaised with the neighbours about the design early on and when he told the local environmental group supported the idea that it was going to be a low-impact CLT structure. In the end, we didn’t get a single objection and even received letters of support.

Why did you choose to build with CLT? 

CT: We had a limited budget so we chose a simple block form for the house – for its material and constructional efficiencies and a form factor for thermal efficiency. We wanted to reduce embodied carbon, but it was also about the potential speed of construction. The client moved into the original house at the end of 2018, we got consent in 2019 and contractor pricings came back in March 2020 – then everything went on hold due to Covid. But demolition began in autumn 2020 and Eurban’s CLT frame arrived the week before Christmas – critically before the UK left the EU. It took just 10 days to install. The CLT arrived in a protective film which stayed on during construction, which was handy since it rained a lot when it was going up. 

NH: CLT forms almost all of the dividing walls and we used it in detail too, creating consistency with CLT panels for the doors. With no frames or architraves it keeps referencing the agricultural aesthetic we wanted. In the end, the CLT was the project’s most expensive part – about £100,000 with design fees.

The CLT procurement was tricky. Eurban required staged payments well before the start on site as all the work was being done up front. At design sign-off, it was necessary to pay 90% of the fee, with only 10% outstanding by the time the structure was installed. Also, its lead-in time was 20 weeks, but as there wasn’t 20 weeks’ of slab and drainage groundworks managing the programme was critical to avoid site downtime. Compared to a traditional construction programme, it required some leaps of faith. But once the CLT was up, you could have multiple trades in at the same time, so we could place orders for glazing, roofing and cladding with more surety of structural dimensions and openings.

What influenced your other material choices?

CT: Well, obviously the agricultural aesthetic that we were looking for. The design is open-plan at ground level and runs from kitchen and dining space on the south side to the north-facing living space at the rear with the central roof lit space, like a kind of Moroccan riad.

The green wall at the front, on a galvanised steel secondary structure, makes a big difference to the internal temperatures of south facing rooms. Polycarbonate sliding screens filter light and the planters behind act as a green curtain with passive heat mitigation. To ensure fixing bolts for the galvanised frame did not pop through to the internal wall, we put them in the small void above the CLT beams where the services run. 

NH: The rear elevation’s recycled cellulose and black bitumen corrugation is cheap as chips and meets building regulation – and is a bona fide material for agricultural cladding. We specified high-quality Schueco glazing throughout but to keep costs down on for the glass panels and doors at the rear we designed them as big as they could be- 2.2m- while avoiding size premiums. The whole glazing contract, including the roof , came to only £20,000. 

How does the building perform environmentally?

NH: It was fabric first. We wanted to achieve environmental performance through the design itself rather than by adding kit. There’s no MVHR so all heat mitigation is achieved through natural means. That meant giving depth to the elevations, through layering and screening on the south and insulating on the others. Exterior walls have a u-value of 0.16W/m²K. The XXm² central rooflight glazing was pitched to the north to mitigate heat gain with two actuators for cooling through stack effect conditioning. The whole roof has a u-value of 0.13W/m²K. Air source heat pumps on the roof serve the under-floor heating system and heat a 400 litre hot water tank. A battery charged by 24m² of roof-mounted PV keeps the house virtually off-grid in summer and can be charged using off-peak electricity the rest of the year. The whole building’s embodied carbon is 373KgCO2eq/m².

What was the biggest challenge?

CT: There were LABC warranty issues about exposing the CLT; they insisted on a vapour control layer on the inside face, despite a condensation risk analysis from the engineer saying one wasn’t needed. Incorporating it would have meant a plasterboard finish to the CLT to hide the VCL, which would have scuppered our material exposure idea. In the end we secured a warranty with Protek as designed.

TvS: Even with Protek, it involved a lot of protracted and involved conversations. I couldn’t help feeling that while this process must have been done hundreds of times before, that we seemed to be going through it for the first time. It’s almost as if building administration doesn’t keep pace with building technologies. While this was the most painful process for us as clients, we couldn’t be happier. The project came in at under £3000/m².