Chewton Mendip

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)
• Good level of air tightness
• Triple glazed timber windows facing west and south west
• Rain water harvesting for toilet flushing and washing machines

houses from the rear

Although these houses 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.
• 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.

Insulating concrete form by Logix

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. Of course the longer a building exists, the less of a problem this is. 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