The case for concrete

A new garden is flowering on our ravaged Clapham plot. Thick grey stems have sprouted in the bare clay, from which rough tendrils rise to the sullen November sky. A vigorous grey mould spreads between them, brittle underfoot. Our great sycamore appears to be shedding its leaves in disgust.

A new garden is flowering on our ravaged Clapham plot. Thick grey stems have sprouted in the bare clay, from which rough tendrils rise to the sullen November sky. A vigorous grey mould spreads between them, brittle underfoot. Our great sycamore appears to be shedding its leaves in disgust.

There's nothing like concrete to inspire fantasies of ecological horror, not least because the manufacture of cement, the key binding ingredient, is a major contributor to climate change. Cement is made by roasting chalk or limestone in very hot kilns, a process that produces carbon dioxide and uses lots of carbon-intensive energy. Up to one kilogram of carbon dioxide is emitted for every kilogram of cement produced.

It doesn't help that concrete is also strongly associated in the British imagination with dismal post-war housing estates, multi-storey car parks and the Hayward Gallery. And it's just such unpleasant stuff: the truck that has been filling our piles and floor slab has no redeeming features; it is a diabolical beast that lifts its stinking tail and evacuates a long stream of toxic diarrhoea into the wheelbarrows waiting below.

But is concrete really so evil? After all, there are lots of eco-builders who think it's the best thing since wholemeal bread. Whatever the downside to the making of cement, concrete's contribution to the performance of a building means it can still be part of a green specification. The case for concrete rests on its strength and its heat-retaining properties.

We are using concrete for strength: although most of Tree House will be built from timber, our roots must be strong enough to sustain the house over a lifetime, potentially beyond the life of the mature sycamore immediately next to it. Longevity should be key to any ecological specification; if a building performs well over a very long life, the ecological costs of construction materials pale into relative insignificance.

We recently visited the site where concrete may have been poured for the very first time in these isles: Richborough, the Roman gateway to Britain on the low Kent coast. Today, after almost 2,000 years, the remarkable cemented walls of Richborough fort still stand tall. Although the stones of the ceremonial arch have long been plundered, the concrete foundations remain. The Romans were adept at exploiting the strength and versatility of concrete, although the survival of their most famous concrete building, the Pantheon in Rome, may say more about its beauty than its structural integrity. In general, ugly buildings do not survive; truly sustainable buildings must be beautiful as well as strong.

Concrete walls and floors are also used in green buildings to smooth out temperature swings by absorbing and slowly releasing heat. This "thermal mass" may become more valuable, because if hot summers become common the last thing we should be doing is rushing home to put our feet up in front of the air-conditioner. When used in combination with big south-facing windows, exposed concrete can also keep a house warm for longer into the evening, potentially reducing heating costs.

There are, however, alternatives to concrete. If you are happy to keep cool in other ways ( see below), timber frame has a far better environmental profile, as long as it is carefully sourced. If you want masonry walls, your best "green" option is to use reclaimed brick or stone with a pure lime mortar, because recycling means the extraction of raw materials and disposal of "waste" are avoided. If you are using new materials, try to source them locally to avoid the emissions involved in shifting very heavy materials. Unfired materials, such as earth, clay and straw, have a long building pedigree and remain practical environmental options. If you do use Portland cement, mix it with as much sand as possible.

In another 2,000 years time, I don't suppose Tree House will be standing, despite its concrete foundations. Hopefully, the house will have been carefully dismantled and recycled, each component becoming a nutrient for a new building or product.

Perhaps the timbers will be re-used in another sustainable building in an era when such design is standard practice. If things don't work out so well, they might yet be used for flood defences or latter-day arks. Perhaps, in 4004, visitors from the balmy new capital of Snowdon will visit the low, mosquito-infested swamp of London and marvel at the longevity of the ancient city's rust-flamed concrete roots.

For more on concrete, see 'Ecoconcrete' by Dr Jacqueline Glass, from www.britishprecast.org.

www.constructionresources.com, 020-7450 2211

www.oldhousestore.co.uk, 0118 969 7711

www.salvo.co.uk

THE DIY GUIDE TO KEEPING COOL

If it's too darn hot to sup with your baby tonight, don't rush to install air-conditioning.

First, try to keep the heat out. Closing the curtains will help, but you want to reflect the light before it gets through the windows. Exterior venetian blinds give greater light control than shutters. These are still hard to source in the UK, but try Lynn Westward on 020-8742 8333 or info@LynnWestward.com. A brise soleil - horizontal slats in front of the window - does the same job with less control.

Second, let the heat blow away. A cross-ventilated room or house, with windows open at both ends, will cool rapidly.

Third, try a low-powered ceiling fan. The feeling of air movement can keep you comfortable even when the air temperature is quite high.

Finally, a concrete, stone or tiled floor will only absorb heat if it is exposed, so don't cover it in shag-pile. In the winter, of course, the same floor may suck the heat from the soles of your feet unless it gets the morning sun - or a burst of underfloor heating - before you put the kettle on.