A very interesting terrace of three houses, on a curved road junction, has recently been completed in this Somerset village.
Some of the main features of the design are:
- Very high insulation levels provided by insulating concrete forms (ICF) with added exterior face insulation
- Structural insulated panel (SIP) roofs
- Eco-slab ground floors and upper floors of expanded polystyrene rib and slab formwork with cast reinforced concrete covering. See Eco-slab detail for dealing with Radon gas in this area.
- Mechanical ventilation heat recovery (MVHRMechanical Ventilation with Heat Recovery. This is usually a double fan arrangement which extracts stale air from the house and sucks in fresh air at the same time. As the warm stale air is blown out, heat is extracted from it and passed over to the cool incoming air by means of a heat exchanger. With the latest technology, over 90% of the heat can be recovered. (see Passivhaus standard) )
- Good level of air tightnessA measure of how leaky a building is to air. In other words, how draughty it might be. There are now standard fan pressure tests to check how air tight a house is and the Building Regulations have minimum standards for all new houses (L1A – Conservation of fuel and power in new dwellings (England)). A much higher degree of air tightness is covered by the Passivhaus standard
- Triple glazed timber windows facing west and south west
- Rain water harvesting for toilet flushing and washing machines
Although these houses were not strictly speaking selfbuild (they were built by a professional builder), the use of ICFs for self builders is very attractive for several reasons
- Masonry and carpentry skills are not so important. Assembling the forms is more like building with lego – and very fast. Some of the suppliers do short courses
- In-situ concrete can create almost perfect air tightness – always something difficult to achieve with timber frame, especially if the building shape is complicated.
- In situ-concrete is good on acoustic insulation. There are sometimes complaints with timber houses on this score and it can be difficult to incorporate good sound insulation in some cases.
- Concrete is good for fire separation and this begins to matter with more than two storeys or where it may be advantagious to design flexibility into a building so that certain areas can be isolated from others at a future date. See Lifetime homes"Lifetime Homes make life as easy as possible for as long as possible because they are thoughtfully designed. They provide accessible and adaptable accommodation for everyone, from young families to older people and individuals with a temporary or permanent physical impairment". See the Lifetime Homes page and the Lifetime Homes Foundation web site.
- Concrete can help with thermal massthis is about how much heat something can absorb - so it involves its specific heat capacity and its volume. It can be useful for levelling out the peaks and troughs of temperature within a house. See the page on thermal mass and the inner layer of insulation afforded by ICFs means that a higher internal surface area temperature can be achieved more quickly when the heating is swithched on. See Decrement delayThis relates to the lag time that insulation itself takes to heat up or cool down. It introduces a delay into the effect of the insulation. This can help level out peaks and troughs of temperature. See the section on Decrement Delay.
The problem with all this concrete and polystyrene is one of embodied energythe total amount of energy it takes to make a material (or a building). See more on embodied energy. Both of the materials rate badly in this respect. The pouring of concrete is also a job which requires some experience and expertise.
Using SIPsStructural Insulated Panels - prefabricated (usually in a factory) timber panels often forming part of an integrated building system and aimed at fast site erection. see more on SIPs for roofs is also a very fast way of achieving weather tightness and is good for air tightness. The development was intended to aim for PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. standard and may well have achieved this although they have not been certified as such. (to date – September 2009 – only one house in the UK has actually been certified to the standard although several others have probably achieved it)
These houses were clad in local stone on the front and end elevations to satisfy the planners.
There is an excellent downloadable article from the Green Building Press on this development