The A-Z of Green Homes: A to D

A

Airtightness

‘Build it tight, ventilate it right’ is one of the new principles of green homebuilding — based on the idea that air loss from a home is an inefficient use of energy. Under Part L1A of the Building Regulations, all new homes must now have an air-leakage test to assess how much air is being lost from the house through openings in the structure. The Regulations require homes to be built with a maximum of 10 air changes an hour, with a target of five (for comparison, homes built before 1960 could have up to 20 air changes an hour). This is particularly an issue at ‘weak’ areas such as the joining of the roof to the wall structure. Airtightness can be achieved through better building practice, especially in bonding and joints, around sockets and weak points; if achieved successfully, it can reduce fuel costs by up to 80%. In addition, owners of airtight homes report lesser instances of allergy problems.

According to Michael Benfield of self-build and timber frame specialist Benfield ATT, “Greater airtightness is achieved through careful design and scrupulous attention to on-site trade detail.While it is possible to reach present requirements using all masonry construction, it should be noted that, for example, blockmakers have had to devise a ‘thin joint’ system to make this more reachable. Since panelised systems are generally more impermeable than masonry, it follows that it is generally easier for them to attain lower rates of air change. However, even these are subject to possible on-site failures. They need to be properly sealed and caulked to eliminate leakage at all weak points and, like all other systems, can be weakened by careless puncturing of the fabric, e.g. by follow-on trades inserting electrical wiring, plumbing and waste pipes.” Owing to the lack of air movement, mechanical ventilation systems, which automatically extract stale air and replace it with fresh air, must be used when building an airtight home.When combined with a heat-recovery system, which uses the moist warm air extracted from bathrooms (for instance) to produce warm air, the system can also reduce energy consumption.

Air-Source Heat Pumps

The basic idea behind heat pumps is that they take a small amount of heat from a large source and convert it into a much larger amount of heat in a much smaller medium, typically hot water in a cylinder.

Heat pumps are neither a renewable technology nor an example of microgeneration, though they are frequently lumped together with technologies that are, and they are eligible for grant aid.What they do promise is a much greater heat output than energy input: the key to understanding this is in the ratio known as the Coefficient of Performance (CoP), which indicates how much additional power is being produced for any given power input at a range of temperatures. Most heat pumps boast a CoP of 3.0 or higher — this means that they are producing three times as much energy as they are using in operation.

Air-source heat pumps (ASHP) differ from the better-known groundsource heat pumps in that they are much cheaper and simpler to install as they don’t require extensive underground pipework to collect heat. Essentially, you get a box not unlike an air conditioning unit, which can be mounted on or near the house, and this draws heat out of the air. The efficiency of the unit varies with the air temperature and on the coldest days an ASHP may struggle to cope with the demand from a house for space heating and hot water. They also tend to be a little noisy: you need to check the decibel levels and think carefully about where it might be best sited. In contrast, ground-source heat pumps are quieter in operation but require a large area of garden land to be dug up.

B

Biomass

Biomass, or bioenergy, refers to the use of non-fossil fuels to power specialised boilers. Although biomass fuel releases CO², it’s all part of the normal carbon cycle and is, therefore, not adding to atmospheric CO² — provided your fuel is from a sustainable source, where replanting is taking place.

However, unless you happen to own your own wood, or have access to the offcuts from a saw mill, you will have to buy in a supply of biomass, and there are often high transport costs involved in getting biomass fuels to the point of consumption. It would, therefore, seem to suit some sites much better than others. Biomass boilers tend to be aimed at rural houses, which have access to local timber and have sufficient storage space.

In order to address some of these concerns, wood pellet fuels have been developed which are much more readily transportable. They are widely used in Sweden, Denmark and Germany and we now have pellet plants starting up in the UK.Wood pellets are made from timber which is compressed to such a density that it holds the same calorific value as heating oil; better still, it tends to sell for the same price. Pellet fuels are, therefore, being created to trade as an alternative to heating oil.

Biomass and pellet boilers are expensive when compared to fossilfuelled oil or gas boilers, though the best ones offer levels of automation, which take most of the drudgery away from solid-fuel devices of yesteryear. As with many other renewable technologies, grants are available.

More information is available from the Log Pile Website at nef.org.uk.

C

Cob

An ancient building technique using a mixture of earth, straw, sand and other materials. It is similar to adobe and has been used in the UK for many centuries, particularly in the west. It is an exceptionally cheap way of building and while earth is not a particularly great insulator, building with cob does mean creating a very thick solid thermal mass, and it does enjoy high absorption levels of solar energy. It also achieves an exceptionally low carbon footprint.

Code for Sustainable Homes

Introduced in December 2006, this is the key plank of the Government’s policy to move all housing to zero carbon by 2016. It is split into six levels, with each level being progressively more demanding. Level 1 is slightly better than the current Building Regulations standards. Code Level 4 is equivalent to the German PassivHaus standard, an ultra-low-energy house that doesn’t require any on-site power generation. Code Levels 5 and 6 are variously described as net zero carbon and zero carbon. The Code also has requirements for water-efficiency measures and management of waste and resources.

The Government has indicated a timetable for the adoption of the various Code levels into the Building Regulations, specifically Part L in England.Wales, Scotland and N. Ireland are adopting their own timetables but anticipate them being broadly similar, though Wales has broken ranks by insisting that all new buildings there should be zero carbon by 2011.

SEE ALSO ZERO-CARBON HOMES

Combined Heat Power (CHP)

Imagine a boiler that produces electricity as well as hot water. That, in essence, is what combined heat and power (CHP) plants do. Instead of energy just going to waste, it converts most of this waste into usable power, in the form of electricity. It’s been used successfully for many years on large mixed developments where there are shops, offices and homes, but only recently have we seen the advent of singlehouse solutions, known as micro-CHP where it is designed to replace gas boilers.

However, at the time of writing, no one is supplying micro-CHP into the UK market. The leader in this field is Powergen, who has a micro-CHP unit called WhisperGen, which has been installed on a trial basis in a number of homes. But to date they have all been hand-built in New Zealand and until the company can find a mass production facility closer to home, it is not taking any new orders until 2009.

So it’s very much a case of watch this space. Looking a little further into the future, we can anticipate the development of fuel cell-based CHP plants which promise to be even more energy efficient than the gas-powered versions of today. At the moment, domestic fuel cell systems are still some way from commercial reality and it may be ten years or more before we see any hydrogen-based fuel cells in the home. CHP as a technology is still evolving, but it seems it is more likely to be suitable for powering district heating schemes rather than individual households.

More information on WhisperGen is available through Powergen: 0800 096 1160 powergen.co.uk.

Condensing Boilers

Condensing boilers have extra heat exchangers that manage to extract a much greater amount of useable energy from the fuel. The heat exchanger uses the heat in the exhaust gases from the boiler to preheat the water as it enters the boiler, and so recapturing energy that would otherwise be lost. As a result they manage to make use of in excess of 90% of the input fuel — as opposed to just 60-80% in standard boilers (and much less in older models). They can be either combi or standard system boilers, and come in gas or oil models.

Under changes to the Building Regulations in 2004 and 2007, it is now compulsory for all new boilers (in new builds or renovations) to achieve an ‘A’ or ‘B’ rating on the Sedbuk scale of boiler efficiency — which means that boilers must now achieve a minimum 85% efficiency rating. In practice, this means that all new boilers must now be condensing.

Sedbuk boiler efficiency database: sedbuk.com.

D

Dual-Flush Loos

Water Supply Regulations state that a toilet can have a maximum flush of six litres (it used to be 13 litres in the 1960s), but self-builders and renovators can save themselves water (and money, as WCs account for up to 40% of domestic water use) through a dual-flush model — WCs that have two different flush volumes (usually six litres and four litres). In practice, the minimum flush volume is two litres, which is quite common in Scandinavia, although most UK experts believe the practical minimum to be three. It is also possible to retrofit devices in existing cisterns to reduce flush volumes.

Drives

Your driveway – and much of the hard landscaping outside your house – potentially has a large environmental impact, both in its production and in the impact it has on storm sewers in excessive weather conditions. Current practices which prevent rainwater from entering the subsoil (reducing groundwater recharge) mean that infiltration decreases, base flows in streams are decreased and some small streams may dry up — which has significant effects on water supply for everyone. You can reduce it by investing in something like a GrassGrid driveway, which is an interlocking mesh system (similar, in fact, to the look of a dry ski slope) which combines the hard-wearing properties of concrete but allows grass to grow in between — meaning less run-off into the drainage system, a more attractive finish, and a smaller carbon footprint than concrete or asphalt.

Alternatively, it is possible to pick up ‘hard’ materials that allow air and water to pass through. So-called ‘porous asphalt’ and ‘porous concrete’ are at early stages of development but have been used in commercial applications.

The A - Z of Green Homes:

• A-D Airtightness to Dual-flush loos
• E-H Energy-efficient lighting to Hydro power
• I-L Insulation to Low-flow showers
• M-R Micro generation to Reed beds
• S-Z SAP ratings to Zero Carbon homes

Further reading:

• Biomass...A 21st Century Fuel
• The Code for Sustainable Homes
• A Few Facts About The Code for Sustainable Homes
• Heating and Plumbing: Green Options