Eco Challenge: "Help Us Build a Greener Home"

ABOVE: Oak framing is not a natural fit for someone wanting to build a green (and therefore airtight) house, but the industry is developing good solutions.

The Brief

“We want a sustainable house that is self-sufficient in energy,” says Peter Hilliard. “We want to do our bit for the environment, which is why we have gone for oak frame. But it seems that our site has got sun and wind and we would like to use it. We are willing to invest, but the investment has to make financial sense.”

The house, as designed, is of traditional post and beam construction. The infill wall of the oak frame will be insulated with 150mm polyurethane (PUR), with 120mm PUR between roof rafters, 150mm PUR under the floor and 50mm PUR to intermediate floors. Windows will be double glazed with a U-value (how well a building element conducts heat) of 1.8W/m² but airtightness has not been specified and is assumed at 10m³/hr (how fast air escapes the house — 10m³/hr is the minimum standard required). Overall this represents an improvement of perhaps 20% over current Building Regulations Part L requirements.

But there are two problems. Firstly, oak frame is inherently extremely difficult to get airtight. Green oak dries over time and as a consequence tends to shrink and crack. Structurally this has no impact but it usually opens up gaps that let the air out. The Hilliards’ house, as designed, will have a peak heat load of around 11kW and the brutal truth is that we cannot significantly reduce that figure without covering up the oak frame and thereby the gaps that will appear in it. And covering up the oak frame is not something most people want to do.

Secondly, Jane has set her heart on a four-oven electric Aga, significantly increasing the electricity demand (SEE BELOW).

The Background

This is Peter and Jane Hilliard’s third selfbuild in ten years. Peter has an advantage in that he is a bricklayer by training and runs a small contracting company. “I am always juggling,” says Peter. “If I am working on my own house, I am not earning money — and I need the money to pay for work on my house.” Jane works as a teaching assistant and between them they project manage.

This project meant selling their very comfortable house and moving the family, including three children, a mile up the road to a log cabin on the new site in Powys. The three bedroom ‘cabin’ is spacious, warm, very comfortable and fully equipped. As well as providing goodquality accommodation, the log cabin also has the potential to become a holiday let, once the couple’s build is finished.

The project they have taken on is a traditional-style oak frame house of some 200m² with a detached double garage. Five acres of land surrounds the house, which sits on the flank of a hill with breathtaking views in all directions. The plot is approximately two miles from the nearest village and the only grid services available are water and electricity.

The Analysis

The energy demands of the household are:

The annual average wind speed here is 5.7m/s (metres per second). At that speed the wind turbine will produce 9,600kWh per year, giving a Feed-in Tariff (FiT) income of £2,716 (find out more about FiTs). In addition, it is likely the solar array could produce a Renewable Heat Incentive (RHI) payment of £360 p.a. and the heat pump £630. Combined, this is an annual income of £3,706.

Of the 9,600kW of electricity produced on site, some 4,500kWh will be available to be used on site, the remainder being exported to the grid. This means that a further 20,500kWh must be purchased from the grid. At current prices this will cost around £2,940, meaning that the net return from FiT’s and RHI is around £766 p.a.

Tim's Recommendation

• Wet underfloor heating to both floors, with high-efficiency radiators in the bathrooms.

• Ecocat hot water tank, supplied by Chelmer Heating (01245 471111, chelmerheating.co.uk). The Ecocat is one of the few tanks that takes multiple heat sources efficiently.

• Flat-plate 4m² solar thermal array to the south-facing (rear) roof elevation — supplied by Llani Solar (01686 412552, llanisolar.co.uk). This local supplier is one of the best UK solar installers.

• 12kW ground-source heat pump. The Hilliards have plenty of land adjacent to the house, so installing this type of heat pump will be no problem. In this situation the COP (Coefficient of Performance — i.e. the ratio of the change in heat at the output) will be around 4.0, about as good as we can get.

• Evance Iskra r9000 – 5kW wind turbine – supplied by local firm Dorrell Renewables (01834 819238, dorrellrenewables.co.uk).

Peter and Jane's Response

The Hilliards need to find perhaps £35,000 extra to invest in renewable energy. If they do that they will have an all-electric house with a degree of sustainability – and the Aga that Jane wants – that is free to run and provides a small annual income.

“Overall it looks good to me,” says Peter. “The underfloor heating we already had in mind, but using heat pumps and solar panels makes sense now you’ve explained it to us.” The wind turbine might be a different matter. “It’s a lot of money,” he adds. “To be honest, we don’t know if we will have it available at the end of the build. But if we do, we will definitely go for it.”

This is a common problem and at this stage the couple are committed to the project as it is designed. There is no real opportunity to change the design or build programme to make the funds available, and without the wind turbine they will have a big annual electricity bill. They are the sort of people who might just make it happen.

Further reading:

• Read more from Tim Pullen in his blog
• Self-Sufficiency: A Green Dream or Reality?
• How to Specify Your Eco Home

Simply Sustainable Homes - A No-nonsense Guide to Green Building Written by eco expert Tim Pullen, this book offers simple, but not simplistic, advice and information for anyone wishing to build their own home or upgrade an existing house in a sustainable way.