ATLA News On-line
Newsletter Issue 7 No 4  November 1998

Build Your House with Water? 

by Andrew Alcorn

How much water does it take to make a house?

Or ... how many barrels of oil; or how much gas, coal or steam?

FrontPage

Tararua Wind Farm
Open Weekend

Embodied Energy Content
in NZ Materials

Compost Bins from
Demolition Materials

The GMO Controversy

Notice Board

Join ATLA

Home

All of these are energy sources used in New Zealand for manufacturing building materials (or other products), when they are either used directly or turned into electricity.

Every time we make or do something, we have an effect on the environment, and we use energy to do it. If we know which materials use less energy we can make choices about which ones to use in a building. But there might be only a small amount of a high-energy material in a house, so will it matter?

One way of assessing the environmental impact of a house, is to consider the total amount of energy used to make it.

The energy used to make any material or product is called its embodied energy, energy coefficient or energy intensity. Embodied energy coefficients of building materials can be difficult for practitioners to interpret and apply to a projected design.

Earth, especially if it comes from the site, has a low embodied energy per kg. It is a fairly heavy material, however, and quite a lot of it is used in making walls of, say, 300 mm thickness. The total energy for a house may thus be higher than first anticipated. Since no two houses are the same, it is necessary to add up all the energy intensities of the materials to know how much the whole house has used.

Baled straw is another low energy material. Because straw is a light material, even when it is used to make walls 500 mm thick, the total amount of embodied energy is low. The energy involved in the plaster and mesh needs to be remembered, as does the colouring agent, if used. This is one advantage of Earth in the energy stakes - you don't need to paint it.

The real question, of course, is how much energy will the house have used by the end of its life? It's no use making a house with little embodied energy if it then requires lots of energy to make it habitable (heating, lighting, hot water, appliances, etc. Even water requires energy to be collected, treated and delivered to your house.)

In New Zealand, the typical house takes 1 to 10 years to use as much energy running the house as it did to build it. After 50 years, then, it will have used between 5 and 50 times as much energy running the house as it took to build it. This does not include how much embodied energy goes into repairs and maintenance, such as painting. That could raise the amount of energy used on materials for the house to a much higher proportion - it takes quite a bit of energy to renew that steel roof every 30 years. Paint, too, is quite high in embodied energy.

What does this mean if we want to build a house that uses the least energy over its lifetime, and by implication has a minimum environmental impact?

a. The first aim is a house that uses little energy to run it. Since water heating is usually the largest single consumer of energy in a house, it helps to have solar heated hot water, or a wet-back off a fire that you'll be using for heating or cooking, or both. (Wood for a fire must be considered as an energy source, but if you are growing your own timber, there is the comfort that you are absorbing as much CO2 on your property as you are releasing in the burning of the wood). Probably better than burning natural gas at Huntly to make electricity to warm you at Wellington.

b. The next biggest consumer of energy in a house is space heating. So let the sun do it via good solar design, and use LOTS of insulation. If you use enough insulation, you can get away with needing NO extra heating apart from cooking, appliance heat and the people themselves. This can be done in really cold climates, so it should be easy in New Zealand. This is something to remember when designing an Earth house. Earth is not a very good insulator, so unless you can get lots of sun in to warm up the large thermal masses of the Earth walls, you'll need to think about adding insulation; especially in the roof.

c. Firstly, a house that uses little energy to run it. Secondly a house that is going to last a long time with relatively little maintenance in terms of replacement materials. Roofs are a real problem here because they take most of the brunt of the weather - wet, cold, hot, sun. This means they're inclined to wear out quite quickly. If you can afford the expense of a long lasting roof, it will mean less energy used in the long term. Earth is really good on longevity. Many hundreds of years if you do it right.

First - little energy to run it, second - long-lasting. Thirdly, use materials that have a low embodied energy. Look at the list and make your choice, but remember to add up for the whole house. (How much of that material will the house use?)

In practice this means it may be necessary to change our thinking about some materials. Copper has 2.5 times as much energy embodied in it as steel. If a copper roof will last 3 times as long (?) as a steel one then over the life of a building less energy will have been used.

We also need to get used to designing for long life. If you're not designing for at least 100 years, maybe you're doing something wrong (In Europe there are millions of buildings that are hundreds of years old). That means thinking hard about things like fasteners. What use is it, if you have good windows, but the hinges or screws rust out after 40 years...or 80 years? If you look closely at old houses it is generally the fasteners that fail first, causing more extensive damage.

Reference:

Alcorn, J. A. (1996) Embodied Energy Coefficients of Building Materials.

Centre for Building Performance Research, Victoria University of Wellington.

Ed note: an interesting comparison, for the less technically orientated .... ?

If you have a 1.5 litre jug of Water, to heat it from 20ºC to 100ºC you will need 504,000 Joules of Energy (or 0.56 MJ). The Power (kW or Watts) available will dictate how quickly you can do so. (1W = 1Joule/sec)

Table: Embodied Energy in NZ Materials