The £45,000 low-energy home

The recent Self-Build on a Shoestring competition demonstrated how one can build a two bedroom house with exceptionally low energy consumption for less than £45,000. That you can build a truly affordable, really low energy house should surely be more widely understood. You will, however, have to do most of the building work yourself, says Jon Broome, and the house will be simple and compact... The short listed design by Jon Broome Architects is based on simple tried and tested timber frame construction which has been shown to be straightforward to build by people without previous building experience. The design of the foundations and the frame are refined to reduce costs, improve performance and increase the ease of construction.

Low costs make the house affordable but also enable quality and standards to be increased to Passivhaus levels appropriate for the 21st century, radically reducing emissions and residents' fuel bills whilst ensuring comfort and preventing overheating in an increasingly unpredictable climate. The house can be heated for around £80 per annum and a solar hot water system will provide around half the yearly hot water demand reducing energy consumption still further.

The house is designed so that it can be built in a terraced or semi-detached form as well as a detached house. It is conventional in layout and appearance with a pitched roof and choice of finishes to make it generally acceptable to potential self-builders, neighbours, local councillors and funders. Homes which are properly sustainable have to accommodate changing needs and so the front and back walls and internal partitions are not structural so that the house can be easily adapted and extended. All services are incorporated within the intermediate floor and service voids in the external walls so that they are easily accessible and capable of modification and upgrading when necessary.

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1 (Medium)

The plan

The house offers a number of possible layouts within a small footprint; open plan or separate living room, two double bedrooms or one double and two singles, kitchen/ family room facing the street with living room opening off the back garden or kitchen/family room leading out into the back garden.

The first floor rooms are open to the underside of the roof and this creates a storage or sleeping mezzanine level accessible by ladder. This increases the usable area of the house without adding significantly to the cost. It also creates a spacious feel to the rooms and makes it much easier to achieve an airtight envelope. The eaves' level is lowered to 1.8m2 above floor level to save money by reducing the built volume and this lends an intimate feel to the bedrooms.

The house is equipped with a ground floor shower and utility room usually only found in larger houses. This has space for a washing machine and waste bins and is provided with a sink.

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2 (Medium)

The construction

The approach is to refine timber frame construction to improve buildability and energy performance whilst, reducing costs. The four concrete pad foundations, 600mm in diameter and 900 deep, are cast in holes made with a post-hole auger. This has proved quick, easily self-buildable and inexpensive. The timber ground floor is supported on 150mm high concrete blocks. On a sloping site, the floor is supported on a concrete filled drainpipe with reinforcement bars which has proved easy for self-builders to construct. In this way you can build on a sloping site at little extra cost making difficult to develop sites feasible for self-build development.

The superstructure is a 'stick built' balloon frame with full height studs from ground to eaves built on site in the North American manner; prefabrication is too expensive and requires a crane.

What is a 'balloon frame'?

The superstructure of the £45,000 low energy house is a balloon frame as invented in the US in the 1830's which uses light timber studs nailed together instead of heavy timbers with carpentered joints. The two-storey studs are continuous from ground floor to eaves unlike the 'platform frame' variant which is in general use today in which single storey frames support the intermediate floor 'platform', on top of which the upper single storey frames are erected. The inside of the wall frames are clad in oriented strand board, OSB, to give stiffness against horizontal wind forces and to provide a vapour control and airtight layer to prevent condensation in the construction and improve energy efficiency. The advantage of a balloon frame is that the OSB sheathing is continuous at the intermediate floor, which is supported on a horizontal timber fixed to the face of the two storey wall panels. This makes it much easier to achieve an airtight envelope. The balloon frame is 'stick-built' on site as is common in North America, rather than prefabricated, as this is more economical.

The first floor is supported on a horizontal timber fixed to the face of the structural sheathing on the inside of the wall, which provides racking resistance to wind loads, as well as creating an airtight layer and a vapour control layer to prevent condensation in the construction. This sheathing is continuous at floor level and eliminates the complicated junction, which is difficult to make airtight in the more usual platform frame timber construction used in the UK.

The intermediate floor is constructed of open web joists which accommodate the ducts required for the heat recovery ventilation system. This floor void is linked to service voids on the inside of external walls and this allows pipes and wires to be run without any penetrations of the airtight sheathing. The wall voids are formed by horizontal battens and are insulated and in this way, with a layer of fibreboard on the outside of the wall, thermal bridging is almost eliminated at little extra cost.

The single ply roof membrane is heat welded and has proved to be fully self-buildable. The membrane is protected by soil seeded with grass and wild flowers at low cost. The stairs are supported on hangars from above and this enables the whole staircase to be constructed by simply screwing timbers together at right angles. In a similar fashion, the kitchen is constructed of open plywood carcasses fitted with prefabricated drawer kits so that the whole kitchen can be assembled by self-builders.

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4 (Medium)

Sustainability

The house envelope is sufficiently well insulated and airtight having triple glazed windows with insulated frames to help achieve Passivhaus standard. The green roof moderates surface water runoff and creates new wildlife habitats. Timber construction has an inherently low environmental impact and low cost materials tend to be natural materials used in their original state so reducing costs and also environmental impacts.

Appearance

The pitched roof provides additional floor space with a reduced building footprint and also has a conventional appearance that will be preferred in many situations. A conventional tiled roof can be used if the context demands it but at additional cost. An alternative flat roof is simpler to build and extend and allows rooms to be re-planned below. A full range of lightweight 'dry' cladding materials can be used to suit the budget and the context. Timber in a number of configurations, horizontal, vertical, board-on-board etc. is the cheapest but may require periodic maintenance depending on the finish. Cladding panels, brick slips or tile hanging are more expensive but require less maintenance and may be appropriate in some situations.

Tried and tested techniques

The design does not seek innovation for its own sake but rather demonstrates what can be achieved by refining and developing tried and tested techniques to produce straight­forward houses with high performance and conventional appearance.

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3 (Medium)

Jon Broome, with thanks to Gordon Hutchinson, the QS, who kindly did the cost plan.

Broome, J. (2014) ‘The £45,000 low-energy home’, Green Building Magazine, vol. 24, Autumn 2014, pp. 43-44.