Case study Self-build eco-home
should be employed on an ‘open book’ basis, with Murray retaining the risk on the innovative elements, including the curved glulam oak beams, copper roof and heat pump. The contractor was to compete on the basis of mark-up, attitude to value-based decisions and demonstrable supply chain capability.
Rising cost
According to the RICS guide at the time, re- construction costs can vary between £110 and £125 per sq ft – and Murray aimed to finish the project for around £350,000. However, the initial cost plan was based on a traditional build, and this was pushed to almost £500,000 by value-based decisions such as the curved copper roofs and the low and zero carbon technologies adopted. The out-turn costs were pushed even higher, mainly due to latent defects in the original contractor’s work, says Murray, although he declines to say by how much. Murray found a CIBSE low carbon consultant running a small consulting practice in the next village and employed him to provide SAP analysis before putting the services out to tender. According to John Grazebrook and his team at Good Design Practice, the forecast energy usage was 13,354 kWh/year yielding 44.5kWh/sq m, but 102kWh/sq m in primary energy terms. This compares with a 15kWh/sq m target for
Passivehaus and shows ‘we have a long way to go to get to zero carbon homes’, according to Murray.
If we are going to achieve zero
carbon homes, we need to apply sound engineering principles in smaller-scale projects
He appointed a local contractor to work alongside
the architect to develop the design and cost plan, but a good way into the project the contractor went bust. Around the same time, Murray also lost his ground source heat pump expert, whose company also went into liquidation through no fault of its own. However, he was able to take over much of the project
management himself, including working directly with ground source heat pump (GSHP) manufacturer Ecovision and the system now supplies underfloor heating as well as domestic hot water. ‘The building services design was all about what
was feasible for a home without access to natural gas,’ says Murray. ‘We looked at biomass because we have a good local source of fuel, but ruled that out due to the amount of manual interface you simply can’t avoid with biomass boilers.’ The heavy thermal mass of the home meant it was
ideal for a GSHP, particularly as a greater proportion of the load is for hot water rather than space heating in a thermally designed home. There was plenty of land surrounding the house, so six pipe loops were sunk
www.cibsejournal.com
30
Services
Floor to ceiling
Walls/floors: Made from a combination of local Hornton stone and oak cladding; heavyweight concrete block shell; beam and block floor; galvanised wind posts in some locations
Target U-values:
Windows –1.59; Wall – 0.15; Roof – 0.2
Insulation: Multi-layered Try-iso-super bubble wrap and foil insulation in vaulted copper roof; foil-backed insulation also in cavity walls
Windows: Argon-filled low E double-glazing
Ground source heat
pump: Dimplex Heat Pump – 11kW nominal capacity; 2.34kW motor input
Vaulted roofs add to the sense of interior space.
to a depth of 1.2m in the nearby paddock, benefiting from a relatively high winter-time water table and slight warming from the septic tank soakaway. Murray also runs his domestic hot water at a low
45C to minimise energy use, but the system has an automatic cycling function to guard against legionnella bacteria growth. The cylinder has also been optimised to allow him to add solar thermal at a later stage if it is likely to be of benefit.
Outcome
Technically, Murray has been more than satisfied with the outcome, although his wife is less than impressed with her utility room resembling a centralised plant room, due to the need to accommodate rainwater controls, head pump, MVHR, and a 500-litre capacity hot water storage cylinder. The real problems were contractual, Murray says.
Even before the contractor went bust, Murray had been having problems with them. ‘I always insisted that I did not want a quantity surveyor on my project as they add no value, but I had one even though he pretended to be a project manager.’ At one stage, the contractor packaged up the entire
mechanical and electrical services in an attempt to make their lives easier, but that instantly made it too large for any local firm to handle. ‘When they went bust, I was able to take that back in hand again,’ says Murray. So what were the key lessons learned? ‘The industry is
a shambles,’ says Murray. ‘We are very good at technical solutions and there are some fantastic engineers out there. But if we are going to achieve zero carbon homes, we need to plug the hole in the market for well-trained, excellent conceptual engineers able to apply sound engineering principles in smaller-scale projects.’ l
June 2010 CIBSE Journal
29
Hot water storage:
Dimplex 500-litre hot water storage, set at 45C, but sanitised weekly at 60C; able to increase heat pump temperature to 55C for high occupancy
Mechanical ventilation with heat recovery:
MVHR thermal wheel ventilation to family bathroom and children’s bedrooms; MVHR heat exchanger to master suite, guest suite and utility room
Rainwater recovery:
6,000 litre storage tank; pump in tank and in-house controls; non- potable water serves WCs and washing machine
SAP assessment: Energy
usage – 13,354 kWh/yr yielding 44.5kWh/sq m;
Actual performance: Still
being assessed
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64