- Robert Pearson is a chartered surveyor and a director of a development company focusing on custom build. He built the Harvest House as a prototype cost effective ‘eco’ house.
- His experience suggests that the most important thing to do is to concentrate on making the fabric of the building as energy efficient as possible.
- The plot was purchased for £110,000 in the Shropshire village of Wistanswick with outline planning for a four-bedroom house.
- Robert worked with the original architect of the house, (Dawn Mitchell of Custom Design Projects), and a services engineer, to adjust the design to improve its energy performance. Key changes were to alter the floor plan to increase the floor-to-wall ratio, essentially making it squarer so that the amount of external wall area through which heat can be lost is minimised for the volume of internal space.
- Although this increased the overall size of the house by 10%, the Planning Committee was happy to support the changes and the revised plans were accepted as a minor amendment.
- The revised planning application also made provision for solar photovoltaic (PV) roof tiles. Rather than specify bolt-on panels, the team specified the Marley Eternit in-line PV, which are hardly detectable on the finished house.
- The team wanted to use a masonry wall construction so that there was more flexibility in contractor selection. The wall consists of a Porotherm block inner skin, a cavity filled with 95mm of Xtratherm insulation, and then a facing brick outer skin achieving a U-value of 0.18.
- The airtightness target was specified at 3. A mechanical ventilation heat recovery unit (MVHR) was also installed. Cost and thermal benefit analysis showed there was little to be gained from specifying a lower airtightness than this – it would not have reduced the size of the heating system or the amount of PV.
- Various elements of the energy efficiency of the building (principally the U-values of floor, walls, windows and roof, and the airtightness) were optimised in the design with the ultimate balancing factor being the amount of PV needed to achieve the SAP rating of 92.
- An Air Source Heat Pump (ASHP) feeds an underfloor heating system to the ground floor, and radiators upstairs. A gas combination boiler was not an option as there was no local gas supply. The heating was zoned for optimum efficiency and with usage information viewed and analysed via a tablet known as the ‘Shimmy’.
- One of the keys to getting the maximum efficiency out of an ASHP is the selection of the right sized unit so that it is operating comfortably in the worst conditions rather than acting at its maximum capacity. This is a bit like operating a car outside the redline zone of the rev counter. In the case of Harvest House it is a 14kw unit. Supplementary heating is provided from a wood burning stove in the lounge.
- Windows are a triple glazed unit from JELD-WEN. U-value of 1.2 was specified.
- Other features of the house include click point taps (these have a brake in them making it easy to only turn them on a bit, and you have to use more force to open the flow more), a shower waste heat recovery system, low-flush toilets and low-flow showers.
- CAT5 high-quality data cabling was specified to enhance homeworking, and low-energy light fittings and appliances were installed throughout.
- A rated Energy Performance Certificate was achieved, with a final SAP rating of 93, and a predicted energy cost of just under £500 a year.
- The total build cost was £230,000, or £1,127/m2 based on a gross internal floor area of 204m2 including the integral garage.
- July 2012 -November 2012 – Detailed design and procurement
- December 2012 – Start on site
- July 2013 – Completion
- Contractors sometimes don’t understand the challenges of building a low-energy house. For instance the builders did an early air test with a result of 4.5. This was the lowest result they had ever obtained, so they thought it was good. In fact they had done it before the internal door to the garage had been fitted. A second air test achieved a result of 2.9.
- It might be helpful to do an air test on completion of the shell, so that the contractor can tackle issues before it is too late. This works well if services voids and ducts are already in the shell and there is no risk of the walls subsequently being compromised. Builders often consider airtightness improvements as a snagging task best done with a mastic gun.
- Be careful that the contractor does not make product specification changes to secure cost savings, or because they want to use their preferred suppliers. They may have given their quote on the assumption that they will make some savings. Such changes could adversely affect the performance of the building.
- Don’t expect radiators run off an ASHP to be ‘hot’ to touch; heat pumps are designed to run at much low water temperatures
- Don’t underestimate the importance of a services consultant for the detailed design of the mechanical and electrical systems. They need to work closely with the M&E subcontractor and the builder to ensure that systems are installed and work as specified.