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Embodied Energy

The embodied energy in a house is the total energy required to build it. That includes the extraction, mining etc. of the raw materials, processing and manufacturing them and transporting them to the site and assembling them.

The embodied energy of materials can vary enormously depending on how you measure it, where it is located etc.

Sawing down a local tree and converting it into usable timber might give a very low value whereas the value for cellulose insulation can vary greatly depending on the extent to which you include the manufacture and distribution of newsprint and where the original timber came from.

As an extremely rough rule of thumb the embodied energy in contemporary houses might be in the region of 10 – 15% of that used during the lifetime of a house (for heating and lighting it etc.) so it is much less of an issue than the ‘in use’ energy, but nevertheless important. With better insulation and reduction of in-use energy the proportion of embodied energy becomes more significant. The energy used to produce building materials can be found on the ICE web site at the  Sustainable Energy Research Team at the University of Bath.

The embodied energy of building materials varies enormously, e.g. aluminium, concrete, glass and most plastics are extremely energy hungry in their manufacture whereas at the opposite extreme, straw bales are extremely low. Timber is also low and has the added bonus of locking up carbon for the duration of the building’s lifetime.

The principle is to use the high energy materials as little as possible and only when nothing else will do. An example of this would be a range of  windows made by Nordan, which are basically very high quality timber but use a small aluminium drip section fixed at the bottom of the glazing unit to shed water away from the lower edge.

Embodied energy is measured in MJ/Kg so it is important to take into account the density of the material. For instance expanded polystyrene has very high embodied energy but is very light so using it for insulation is not as bad as it might initially seem. More information on the values for insulating materials on the Insulation properties page

The challenge with concrete

Probably one of the biggest challenges concerns the use of concrete which is not only very energy intensive in its manufacture but also emits large amounts of carbon dioxide, as limestone (calcium carbonate) is converted to calcium oxide. The outstanding quality of concrete (apart from its structural strength) is its ability to stay in contact with water without rotting or rusting. Hence its almost universal use in foundations. Recently GGBS has become available and the use of this as a substitute for OPC not only saves  on embodied energy and CO2 production but also produces a more durable concrete with a lighter colour. See for instance the Hanson product ‘Regen‘.

There are ways of saving on concrete by alternative designs and by careful digging of foundations….. see more

A great deal of concrete (and money) can often be saved by careful digging of the foundation trenches and laying only the amount of concrete specified in the drawings. It is only too easy to make a mess of the trench digging and dig too deep or allow the trench sides to cave in. The inevitable way out of these problems is to “throw more concrete at it”. The key to getting it right is to have an experienced digger driver and to lay the concrete very soon after digging. Avoid the trench-fill approach as this may be slightly quicker but uses much more concrete.

There may be a slight saving in concrete if you are using a timber post and beam type of construction or the Walter Segal approach because you can simply cast blocks of concrete beneath each corner of the building and the intermediate points where posts occur.

Screw foundations for point loads have recently made an appearance in the housing market and involve large galvanised steel posts with helical screws on their sides like huge augers. These posts are simply screwed into the ground by machine and the frame (timber, steel or concrete) sits on them. They are fast to install and can be reused or recycled at the end of the building’s life. See for example Geologic Foundations Ltd

There are also some slightly more exotic approaches such as when a terrace of houses has foundations only under the party walls and the whole of the rest of the structure spans between the party walls. This may be possible with some SIPs arrangements but is probably of little interest to the average self builder.

How long before someone invents a trapion (or conion?) point load foundation system? This could be ideal for timber post and beam structures such as the Walter Segal approach to construction. It could utilize reclaimed hardcore as a fill material and would be totally recyclable and extremely low in embodied energy. Betafence Gabion Solutions claim 120 years lifetime for their galvanised steel wire bastions (which may not seem a long time for the life of a house). If stainless steel were substituted then the life would be increased enormously.

Another high energy material is glass and there are some issues about how often double or triple glazed units fail (by leaking and fogging up) and how much energy they actually save if you have to replace them regularly.

Glass manufacture is very intensive in terms of embodied energy. Is triple glazing worth it in terms of energy saved when embodied energy is taken into consideration?   see a calculation

This example looks at the energy involved in making the glass and ignores the energy involved in making the heavier frame, installing it etc. However these factors are minimal in terms of energy compared with the glass itself.

Double glazing has a Centre Pane U value of about 1.5 W/m2K (Pilkingtons ‘K'(4/16/4) with argon
Triple glazing has a Centre Pane U value of about 0.8 W/m2K (Pilkingtons ‘K'(4/16/4/16/4) with argon
This gives an improvement of 0.7 W/m2K

so assuming an average temp difference between inside and outside of 10 c. over a heating season of 26 weeks the saving is
0.7 W/m2K x 10 x 26 x 7 x 24
= 30576 Watt hours/year
or 30.576 KWh/yr

volume of glass = 1 m2 x 0.004m = 0.004 m3
density of glass is  2600Kg/m3
weight/m2 = 0.004 x 2600 = 10.4Kg
Embodied energy = 15MJ/kg (from the ICE inventory)
10.4 x 15 MJ
= 156 MJ
= 43.3 KWh

no of years to pay back energy = 43.3/30.6
so it takes about 1.4 years to recoup the manufacturing (embodied) energy.

Note that the actual values for double and triple glazing in window and door frames depend very much on the frames themselves and on the glazing spacer.

Some of the traditional ways of house building are extremely low in embodied energy. An example is cob (using the earth available locally and mixing it with straw) of which there are about 70,000 examples, mainly in the South West of England.

There can be occasional surprises. On first thoughts a living roof, be it turf or sedum, might seem to have less embodied energy than say a clay tile roof where the tiles have been fired in a kiln which uses a great deal of energy. However, when you do the sums you find that the waterproof membrane beneath the living roof may well contain the same embodied energy as the tiles because of the high energy, oil based material it is made of. (This is the reason for Oldroyd’s Xv Green membrane which incorporates a centre layer of recycled plastic, so reducing the embodied energy)

One of the books looking at the subject of embodied energy is ‘Low Impact Building’ which covers natural materials

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