Starting in Spring 2010, we will be building a 1.5 – 2 storey family home approx 200sqm with straw bale walls. Our aim is to build a low impact, energy efficient, healthy home, with a garden designed around the production of fruit and vegetables.  Once complete, we will measure the building’s performance. We aim to combine traditional materials like lime and clay; waste materials, such as recycled newspaper; natural materials, like straw; and modern technology, such as solar panels and a heat pump. In addition, we want to design the building to breathe, thus avoiding the use of moisture barriers, and minimise the use of materials with high embodied energy, such as concrete. Hopefully, the result will be a super insulated breathing building, that takes advantage of solar gains, encourages natural ventilation, and has a small environmental footprint. Follow progress over the next 12-18months on our blog and feel free to add comments as we go!

The house has been designed by us and below we have tried to summerise some of the key bits along with a site plan!

  • Breathable construction
  • Airtight construction
  • High level of insulation
  • Passive solar gain
  • Thermal mass: limecrete floor
  • Good connection between the house and garden
  • Highly productive garden
  • Good indoor air quality: minimising the use of VOCs

  • Emphasis on local, natural materials
  • Strawbale walls
  • Natural insulations: hemp and lime, straw, sheeps wool, recycled newspaper
  • No concrete
  • High performance windows and doors
  • Clay plaster
  • Natural paints
  • Untreated timber
  • Cedar cladding left natural, not finished with sealers / oils / varnish

  • 6sqm solar thermal panels (hot water)
  • Ground source heat pump: feeds under floor heating downstairs
  • Whole house heat recovery: controlled ventilation, redistributes heat gains around the house
  • Wood burning stove for supplementary heat

  • Energy efficient lighting
  • High level of daylighting
  • Energy efficient appliances

  • 9000L rainwater recycling: to be used for drinking water / bathing etc (phase 2)
  • Timber frame shed with cedar cladding and green roof (phase 2)
  • Rammed earth vegetable storage shed (phase 3)

The site is located just outside Newmarket.  It was purchased in two parts, from different owners - a run down bungalow and a piece of land adjoining it.  The bungalow is to be lived in during the construction of the main house.  Phase 2 of the project involves demolishing the bungalow, and constructing the main shed and rainwater harvesting, which are to be located where the bungalow is situated.  

The aim was to create a well insulated, airtight house that was built without the use of concrete.  It was important the the building was designed to breathe, and that materials with different moisture properties were put together in such a way that they would complement each other and work together to protect the fabric of the building.  The house aimed to take advantage of passive solar gains and use natural, local materials wherever possible.  We wanted the garden to be very productive, and in order to be as self-sufficient as possible, there should be adequate storage for produce.

From early on the desire was to build the external walls out of straw bales.  We liked the concept that these could be grown in the field next door, with virtually no energy required to transport them to site.  Straw is an annually renewable natural product, it uses very little energy to produce it (other than that provided by nature - sun and rain!) and there is estimated to be a large surplus each year.  Its carbon credentials are great, in fact it has a negative carbon footprint: it takes in carbon dioxide while it grows, it then stores the carbon in its fabric for the lifetime of the building, and it it can be composted after use!  It is highly thermally insulating, acoustically insulating, healthy (ie it does not give off harmful fumes and does not cause hay fever) and affordable.  In addition it is a very accessible building material and is enjoyable to work with.  After taking part in a straw building course run by Amazon nails (a company who have pioneered straw building in this country), we were sold on the idea.  

Initially we investigated with Cambridge Architectural Research making the straw walls loadbearing.  The straw has enough strength, but in the end we concluded that the straw was too 'springy'.  Whilst this flexibility wasn't a problem in itself, we wanted to run services through the first floor and would have had difficulty making them tolerant enough to accommodate this!  It also makes detailing around things like windows more difficult.  In the end we decided to use the straw bales as insulation behind a light-weight timber frame, constructed of basic 2x4 softwood.  The attraction of this was that it is relatively cheap and easily accessible. Often softwood is treated to preserve it, however by its nature a preservative is inhospitable to life, it creates an environment in which insects and fungus are unable to survive. As a result this means it is also not particularly good for human life - just read the warnings for the different timber treatments.  We decided this was against the concept for a healthy building and so took the decision to not treat any of the timber.  Instead, we have worked hard to prolong the life of the timber through good building design - protecting it from water, allowing it to dry out etc.

The straw bales need to be rendered with a natural breathable material - usually either clay or lime render / plaster.  We are lucky enough to have a fantastic brickworks in the next village, Cambridgeshire Brick and Tile, who were willing to sell us clay to use.  We loved the fact that the clay was dug so locally, and that it is possible to apply with your hands (lime burns in contact with skin).

Straw bales have a very good insulation value but not very high thermal mass.  The advantage of a material with a high thermal mass (usually bricks, concrete, masonry etc) is that it helps to level out peaks and troughs in internal termperature, and store heat.  It was difficult for us to introduce any thermal mass into the walls, so we decided that we should choose a floor which would work in this way.  As we didn't want a concrete floor, we decided to look into doing a limecrete slab and lime screed.

A lot of straw bale buildings tend to have a 'home-build', wavy appearance.  We wanted to show that it was possible to create a crisp look, to hopefully would appeal to a wider audience.  To achieve this, we decided to finish the outside with timber boarding.  We looked mainly at two types of timber, larch and cedar, both of which are durable softwoods, which can be left untreated (ie without any preservative, oil, varnish etc).  Initally we thought we would use British larch, however on further investigation we learned that because it is grown faster here than in Siberia (where a lot of larch comes from) it has a shorter predicted life.  The cedar we have used, on the other hand, comes from Canada (admittedly not very local!) but has a life expectancy of 60 years untreated.  There are different opinions about how long the larch would have lasted but at best it probably would have been about half of this.

When you build with straw bales everyone says that you should have a 'good hat, and a good pair of boots' - ie, the straw should be well protected at the top and bottom.  To achieve this straw bale buildings often have a large roof overhang.  Whilst it was less important for us, because the external timber cladding was acting as a rain screen, protecting the straw, we decided to stick with this advice.  We also wanted to include a covered external area for eating outside, by extending the roof.  To achieve this, we decided to use a lightweight roof covering, the most obvious of which is a metal roof.  We ruled out zinc and copper as corrosion contaminates the water run off (which we want to recycle).  Coated steel is galvanised with zinc and stainless steel can contain heavy  metals chromium and nickel.  So we decided to use aluminium.  The disadvantage is that it uses a high amount of energy to process from a raw material.  However, as we understand it can be recycled an unlimited number of times and uses approx 80% less energy to recycle than process from its natural raw state.

As we wanted to avoid the use of concrete we did not use conventional foundations.  We are lucky enough to be on chalk, which was not too far below the ground level.  We excavated a large pit, which was a minimum 650mm below ground level (into the chalk).  From here we built up masonry perimeter walls.  We filled the centre with LECA (lightweight expanded clay aggregate).  This is the insulation under the ground floor limecrete slab, and it is also the damp proof course.  It allows water to drain through it, but prevents water from being wicked up.  There is no conventional polythene damp proof memebrane.

The timber frame was built in frames and fitted to dwarf brick walls a min 250mm above ground level.  The straw bales were then fitted behind the frame.  The bales are designed to brace the timber frame.  The straw bales were built on a horizontal "timber ladder", and tied to additional timber ladders every second course.  The bales were compressed with ratchet straps and strapped down, then the ladder was fixed back to the timber frame.

The roof is supported on timber trusses at 1/3 intervals along the length of the house.  As it is not supported by the straw bales, the roof was put on before the first floor bales were fitted - this helped to keep the straw dry during installation (particularly with the large overhangs).

The house has been heavily insulated and is designed to be very airtight to reduced heat loss and hence requires less heating.  Once the energy requirement has been reduced then renewable energy sources can be introduced.  The ground floor is to be heated with a ground source heat pump.  The first floor has no heating (other than towel rails in the bathrooms).  There is a mechanical ventilation heat recovery system, which will redistribute the heat around the house via the supply air (in additional to the fact that heat rises).  Solar thermal panels will supply hot water.

There is a high level of natural daylighting and when this is not sufficient primarily low energy lighting will be used.

Buildings require different types of ventilation: background ventilation, extract ventilation (from bathrooms etc) and purge ventilation.  Uncontrolled infiltration is undesirable.  Background ventilation levels are set by Building Regulations and they are requried to provide fresh air and dilute and disperse residual water vapour and pollutants.  As we are focusing on reducing pollutants and improving indoor air quality theoretically we probably could get away with a lower level of ventilation. While building regulations acknowledge this, they admit that at the moment they do not take this into account (paragraph 0.29).  Background ventilation in airtight buildings is usually provided by either trickle vents or ventilation systems.  As we are spending a lot of time trying to make the building very airtight it seemed the wrong decision to add trickle vents to the windows.  Instead we are installing a mechanical ventilation heat recovery system which will extract heat from the extract air and use it to pre-heat the supply air.

The landscape forms an important part of reducing the energy and carbon footprint of the inhabitants.  Food production and transport requires a huge amount of energy (particularly when buying out of season products).  It is not enough to make all homes carbon neutral if we are still buying apples from New Zealand.  The garden here has been dedicated largely to food production.  There will be space for both perennial and annual vegetables.  Soft fruit, a small orchard and herbs.

The most important lesson we have learnt to date is that natural materials have specific times of year when they should be used!  It is very risky to move outside of these times as the British weather is very unpredictable!  Weather windows need to be taken advantage of when they are there - therefore not all materials can be installed on a DIY basis.

When building with straw it is important to put the roof up first before putting any straw in.

Clay render should not be undertaken after October.

Do not under estimate how long hemp and lime insulation takes to dry.