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The time-honoured engineering technique guesstimation

Professor Michael Kelly

A few weeks ago, an article in the New Statesman criticised a paper by Professor Michael Kelly for using guesstimation to establish a starting point for the cost of decarbonising housing. This is Kelly’s response.

Guesstimation is a well-recognised method for getting a handle on something which is uncertain in magnitude.  There are books about it, e.g.   You guess a figure which is known to be too low and another which is too high, and take the geometric mean.  My initial effort in 2010 came up with a guesstimate of £50,000 to retrofit a house to reduce its carbon dioxide emissions (based on the fact that £5,000 was too little and £500,000 was too much).     

This was just as Lord Drayson, the then Science Minister, acted on my suggestion that a serious pilot programme of whole-house and other retrofit measures should be undertaken to get a handle on costs and materials resources for a nation-wide retrofit programme.  The data from ‘Retrofit for the Future’ was particularly revealing.    See ‘Intent and outcomes from the Retrofit for the Future programme: key lessons’, Building Research & Information, 43(4); 435–451, 2015. See

A set for 45 social houses were subject to a whole-house retrofit, and revealed that, for an average expenditure of £85,000, the average reduction in carbon dioxide emissions was 60%: only 3 of the 45 reached the target of an 80% reduction and another three of the 45 did not get even get to a 30% reduction.   Social houses are smaller than the average house size, and to get to a 100% reduction of carbon dioxide emissions the figure per household will be nearer £150,000. 

The ‘Retrofit for the Future’ was a series of one-off exercises, and so one would expect a cost reduction if there was a national programme.  Each house demands a bespoke solution as poorly fitted insulation is worse than no insulation, so the cost reduction is unlikely to me more than 50% with the known technologies of today.   

After 10 years, my guesstimation, or the wild guess according to Richard Black, has been refined from £50,000 to £75,000 by empirical evidence.