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Payback time

Cathy Strongman

Wednesday, 23rd April 2008

Cathy Strongman on what makes a green home

‘It’s actually very easy and not expensive to get the roof and wall insulation right and to draught-proof existing windows,’ says Michaelis. The loft is the easiest and the most crucial place to install additional insulation and a 250mm layer will cost around £250 with a payback period of two to three years. Cavity-wall insulation costs roughly £4.70 per metre, or £479 for a three-bedroom semi-detached house. Most houses built before 1930 have solid walls and can be insulated using external render (for which you will need planning permission) or by attaching insulating plasterboard to the inside of the external walls. ‘That’s what we’ve done in Cameron’s house,’ says Michaelis. ‘You might lose a square foot of floor space overall, but it does make a huge difference.’ External wall insulation costs £45 to £65 per square metre, with a typical semi-detached house costing between £3,500 and £5,500. To insulate the same building with internal insulation would cost roughly £650.

Once you’ve wrapped up your house and minimised your energy consumption, the next step is alternative forms of energy. Unfortunately, most micro-generation systems are currently a poor financial investment. ‘Solar thermal panels, which can be installed on the roof to provide a percentage of a household’s hot water, are the best choice,’ says Michaelis. (Michaelis loyally defends Cameron’s much-mocked wind turbine but does concede that they are vastly overpriced.) ‘You can now buy cheap linked-in systems, which are straightforward to install and have payback periods of less than ten years. But with systems like photovoltaic panels, it becomes trickier. The embodied carbon of the product is paid back in two to three years, but the financial payback period can stretch from 25 to 40 years.’ These long payback periods plague most forms of renewable energy and costs often become even greater when dealing with existing buildings. Installing a ground-source heat pump in a new build or a house with a large garden, for example, is relatively straightforward, whereas in a tight urban setting you need to spend £10,000 just to dig the borehole for the underground pipes. The same is true of rainwater harvesting systems, which require a tank to be sunk underneath the house or in the garden. A simple rainwater butt or a grey water system that can be fitted internally are less effective, but more affordable options.

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