Two recent questions one on choosing the right material for maximum sustainability, the other asking how many anomalies there are in the Code for Sustainable Homes have sparked off some heated debate…

Concrete, timber, steel or something else?


Andrew wrote: It's a bit like saying apples or bananas which is the better fruit? Timber for instance might have a low embodied energy, but if it is illegally logged or unsustainably managed is that better than say concrete or steel? From an energy in use perspective, materials with a higher thermal mass like concrete should be better. We should choose materials that are best suited to the purpose and have a low, long lasting and capable of being easily dissassembled for reuse as opposed to requiring them to be smashed to bits for recycling or landfill.

Yelkcub69 replied: This statement is a clear contradiction. The best suited materials for a particular building may not be long lasting nor be capable of being easily disassembled for reuse. What takes precedence? How suitable, how long lasting or how easy to disassemble it? Again it depends on what your motivation is. For example the following shows indicative motivations:

Developer - most economic material that makes him the most profit
Contractor - whatever he can make the most money out of that complies with spec
Architect - how pretty it looks
Engineer - which one achieves his required factor of safety
Ecco warrior aka sustainability consultant - which everyone ticks the most boxes without any care about cost to client

Andrew's reply: Perhaps I wasn't being terribly clear. Its not an either/or, its about a compromise and finding what fits best in a given situation. Where I sit I would disagree with your generalised statements about what various parties are after. As a sustainability manager with a developer/contractor we have sometimes favoured materials that may be slightly cheaper, or more techically suitable but with a higher impact than some other materials, and on other occassions spent more on materials based on their sustainability credentials. This is for various reasons - including preference (ours, our clients or to suit end users, planners and other stakeholders). Deliverability and quality are also important factors, and all too often there are a confusing array of competing products.

In some cases on similar projects we have used different solutions - in situ steel frame on one and modular build on another. It is still difficult to compare materials like for like even with tools like the bre's green guide.

Yelkcub69 wrote in response: I stated that the example showed indicative motivations. It was not generalised but was purely showing that different individual have different motivations to decide what is most appropiate. I say again horse for course!

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How many anomalies are there in the Code?

The recent article in Building Sometimes things aren't as green as they seem reports on the loopholes in the Code for Sustainable Homes that means it's easier to reach levels three and four using electrical heating rather than gas even though electric emits more CO2. Are there any other similar anomalies in the Code?

Iain Fraser wrote: The anomaly described above was due to a fudge in Part L 2006 and how Energy credits are awarded under the Code. The fudge came about when the Electric Heating and Ventilation Association understandably lobbied legislators to save their industry. Of course, if you build a house well (to passiv-haus standards) an electrically heated building isn't necessarily worse than a gas heated one. The problem is when you don't build to those high standards and install electric heating as a cost saving measure.

As for an anomaly... how about installing water saving devices in a rural area with a low population density which receives over 3,000mm of rainwater a year?

Matt of Natural Building Technologies wrote: You also need to look at heatloss - the more heat you can 'loose' without dropping insulation values, the easier it is to make a % the greater your outside wall area, the more heat you can loose. So, it is easier to pass a detached house or end terrace through the code than a flat in the centre of a block (so having minimal heat loss area, harder to make your savings...)

Then there is the issue of cold bridges - we (And the code) focus on U-values, but the importance of Y/psi values is only covered vaguely in BR and CSH - yet in a highly insulated shell, real world cold bridge losses can be as much as the rest of the wall. So to compare two 'identical' U-values for two different wall construction methods is not the whole story - one complete, built wall may have an overall heat loss far greater than the other. Few people seem clued up on this as an issue. The answer surely is to go down the Good Homes Alliance / AECB route and suggest maximum energy use and carbon use, per m2 per year rates, broken down to electric and gas, and to monitor energy use post occupancy for 2 years?

Mike replied: As a country ever more reliant on oil and gas imports, in a world where discoveries of the stuff has more or less fallen since the 80's and in which more countries are becoming increasingly protective of their own reserves, we need to be flexible. Electricity is more flexible insofar as there are more ways to generate it. The trick is to find a way to generate enough of it in a "green" way.

Matt wrote in response: Actually, the really clever trick is to reduce how much energy we need BEFORE we look at any expensive, complex and difficult green energy projects. There was a fascinating article in Green Building Magazine, suggesting we only NEED about 20% of the energy that we could possibly generate. There are also challenges of how we persuade huge globalised energy companies to think local and regional. Until we can do this, they will pay lipservice to the simple, fast and cost effective measures such as insulating and draft proofing the existing building stock- and focus on mega-sized energy generation.

Mike wrote back: I agree, though I don't think it necessarily requires a "really clever trick" when history only shows that the more plentiful and accessible fossil-based fuels has become, the more we have consumed.Our whole model depends on "growth", as though it's the secret to immortality. "Sustainable growth" is the new buzzword; imagine your Doctor prescribing the sustainable growth of cancer; the sustainable growth of traffic accidents; or the sustainable growth in alcoholism? The two words are almost oxymoronic. It's really easy to reduce energy consumption: ration its supply. Not so politically easy to do in a democracy, however.

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