New build eco-house

This week’s guest post comes from David Searle. David was born in 1938, and was interested in things mechanical from an early age. He was apprenticed to Rover (before the British Leyland debacle), and worked there as a vehicle development engineer. Latterly he acquired considerable computer experience in Product Planning and Timing. He took voluntary redundancy in 1976, foreseeing the decline of the BL empire. He became self employed at the beginning of the microcomputer revolution, and evolved to specialise in process control systems. He retired in 2003, and looked to downsize his accommodation.

This post is based on two talks he gave about creating a new-build eco-house. The talks were given in 2010, and updated in February 2011 to, variously, the Kenilworth Initiative for Climate Change (KICC), the local Renewable Energy Club (REC) and the Heart of England Organic Group (HEOG),  a local group of the Soil Association. This account of a new build complements well the recent postings here on retrofitting insulation, solar panels and other low energy technologies.

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David writes:

I’d like to start with a quotation, in 1931, not long before he died, Thomas Edison told his friends Henry Ford  and Harvey Firestone:

“I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.”

I have had ‘green’ interests for as long as I can remember, going back to a ‘Dig for Victory’ allotment, tended by my father. My father also inspired me by building a bungalow to downsize for his retirement, working well ahead of the building regulations of the time with better insulation and minimised air leaks. So when we decided to downsize, we looked around the local area, but couldn’t find anything suitable. Several years of agonising about the best way of ‘greening’ our old house, led us to believe that knocking it down, and selling off half the land would be the only financial and practical way to arrive at a very low carbon house.

How green should we be?
Basically as green as we could afford! The first thing was to do a lot of research. While browsing the bookshop at the Centre for Alternative Technology, near Machynlleth, I found The New Autonomous House by Brenda and Robert Vale. It instantly struck a chord. Their actual house was completed in November 1993, so there is no really new technology in our house, just more up-to-date versions. The book discusses not only possible solutions to the idea of an autonomous house, with no mains connections, but the reasoning behind each decision.

What do we mean by zero carbon homes?
In an ideal world we would mean a home, which is completely autonomous, i.e. requires no energy inputs, and uses no communal services, unless they were also carbon neutral. A very good approximation is described in The New Autonomous House. Definitely worth a read, even if you don’t decide to build a house based on it, such as ours. The significant difference between their house and ours is that we do have mains water and sewerage. We didn’t feel that a composting toilet and only rainwater supplies were going to be considered acceptable in our location on the edge of a small Warwickshire town. The technology and practicality are entirely proven and practical, but there is the “yuck” factor when our children come eventually to sell the house. Perhaps by then it will be thought of as normal. We can live in hope.

It was similarly important – from the point of view of pleasing the planning department and not leaving our children a problem building – that we built something that looked ‘normal’.

General view of the front of the house from the street.




The roof shows all three renewables in one picture: solar hot water, biomass flue, solar pv.


What are the principles of designing for a zero carbon home?
Get the shell of the house right, then minimise inputs of all kinds! For insulation, you need between 2 and 3 times the thickness of the amounts recommended by the current building regulations. Use a mechanical ventilation heat recovery system, because in a well insulated house, ventilation losses would account for around 75% of the total, assuming current practice, with trickle vents in windows etc.

You need thermal mass, this will stabilise the temperature so that it is cool in summer. In winter it stays warm through the night, after soaking up the daytime temperatures. All these minimise the heat required, and keep it where you want it.

Then you need to evaluate how to supply the reduced amount of energy you need. The big question is heat, because we tend to burn stuff to get it, either at the power station, with around 25% efficiency delivered, or gas, a fossil fuel, or biomass (renewable), or better still passive solar design, supplemented with solar thermal panels for high grade heat. Heat storage is an issue because the sun is intermittent. It has two main forms, structural mass of the building for space heating, and volumes of water for storing high grade heat. If electricity use has been minimised, with low energy lighting, and efficient gadgets, then solar PV panels become an option. They are the last thing to consider, because they have the longest payback.

On the indirect use of energy, rainwater harvesting plays a part, because one of the relatively high carbon consumers is the water industry. On payback generally, we have taken the view that the enhancements will reflect in the sale price of the house anyway. So our criterion for any one item on the menu was 'will the saving exceed the loss of interest on the capital cost?' If so we deemed it to be worthwhile.

How do you do it for a new build?
We did it by following the construction principles outlined by Brenda and Robert Vale in their book, in order to get the shell right, as far as our site and planning issues would allow. What is shocking to me is that their house was built in 1993, and building regulations won’t reach their standards for the shell until 2016!

We then added systems to reduce our resource usage (and hence costs and carbon emissions) in the following priority order.
- Wood burner, later fitted with a flue boiler for domestic hot water
- rainwater harvesting
- solar thermal panels (5sq.m.)
- solar PV panels (15sq.m.).
Interestingly the relative sizes of the two types of solar panel roughly reflect the relative efficiency of solar collection of heat (50%) and electricity (15%) respectively. Our panels harvest almost exactly the same number of kWh. Has it worked? Minor teething and tuning problems apart, emphatically yes!

The wood burner kept us in the range of 20-23 deg C throughout the winter of 2009-2010, and topped up the hot water system to between 50-60 deg C whenever it is lit. The efficiency of the shell is such that, on average non freezing winter days, the temperature drops just over 1 deg C in 24 hours. The lowest temperature we have experienced was on returning from a long weekend away: 17.1 deg C, when we had been running the house between 20 and 22 deg C previously.

There is no other heating except the solar hot water and 2 hours per day of one towel rail, run off the hot water system. At the equinoxes when neither the solar systems, and the woodburner, are producing reliably (the rooms could get too hot!), the immersion heater is used for top up purposes. A key feature is the thermal mass of the building. I haven’t calculated how much concrete is incorporated inside of the insulation, but it is many tonnes. We have no studding partitions - all internal walls and partitions are made of dense concrete blocks. The lifetime calculation, taking into account the embodied carbon in the concrete, still makes this kind of building genuinely low carbon.

Having looked through a textbook of solar design with descriptions of actual solar houses, I was struck that the only one which attempted to be self sufficient in heating through the year, with no other source of heat, used a 63 tonne water tank in the basement as the store. It operated on a temperature range of 15 deg C to 65 deg C using solar collectors on the roof. They were located in Cambridge, Massachusetts at latitude 42 degrees North. We are at 52 degrees North here, and only use the temperature of the indoor atmosphere to charge up the thermal mass.

We have used mains water for flushing toilets for only about ten days in the year, which represents a large saving in water usage. I can’t tell you exactly how much it saves, because we have had a silent water leak for an unknown period our side of the mains water meter. Luckily our water company repaired it at no charge and allowed me to make an arbitrary adjustment to the resulting bill! We have a tank capacity of 3500 litres, and it is sobering to think that we once had a completely empty tank in late spring last year, and overnight it filled to overflowing, i.e. we had received 3½ tonnes of water from our roof overnight.

The solar thermal panels have provided us with a tank full (250L) of 65 deg C hot water on many occasions in the summer, and the pump was active on almost any day with some sunshine in winter. We even had a tank full at 52 deg C in February.

The solar Photo Voltaic panels have reduced our bills (excluding standing charge) from around £750 to £256 giving a payback in around 23 years at current prices. The total payback time will reduce year on year as the cost of fuel goes up, so a payback in 15 years in real terms sounds quite feasible.

Energy Rating on the house
Our house is rated at 101%, and –0.2 Tonnes of CO2 on its Energy Performance Certificate. This is despite the fact that they estimate that we will spend £459 on energy this year, split between £90 for Lighting, £272 for heating, and £97 for hot water. Our actual net expenditure (excluding standing charges) on energy is £256, as we have loads of free wood, and have no gas. Of these, heating and hot water are supplied by solar, and a woodburner with a flue boiler, and an occasional top up from an immersion heater. The immersion heater, the oil for transport of wood and chain saws, are not carbon neutral, although wood is generally regarded as carbon neutral because it is a renewable resource. However there is not enough of it to go round if the entire world were to move over to biomass for energy purposes. Hence we have to go back to solar or geothermal in the not so long run.

Realistically I have no idea how you would rate our house, if you truly wanted to have a zero carbon home, as we buy in electricity, water, and sewage services. We might have been able to use a green electricity tariff, but the buyback of our own generation appeared to be a problem.

Until 1 April 2010, we were paying a normal tariff of 11.7p/kWh imported from the grid, and receiving 28p/unit for those we exported to the grid. Since that date we get 9p per unit for all the electricity we generate, plus 3p per unit exported to the grid [NB: this is an older installation, and so does not receive the newer 43p feed-in tarriff allocated now to new installations]. These feed-in tariffs apply for 20 years.

As of June 2011 (postponed from April) there is a plan for a similar scheme to apply to solar thermal installations (Renewable Heat Incentive), probably on the basis of estimated capacity, as opposed to measured input, as many installations will not be able to measure the thermal input reliably. The proposed rate is 18p/kWh for solar hot water, and 9p/kWh for biomass heating of space and water. The government example of a semi-detached 3 bedroom house, of reasonably upgraded insulation, gives a repayment of £1400/year. I am guessing that our house might qualify for around £1000/year, because our space heating requirements are so low.

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Thanks to David for this post.

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