Graham Rix

I tend to design houses with a 75mm insulated cavity.  Most architects specify at least 100mm. And it’s not unusual to see, featured in magazines, up to 200mm.

Last year I asked the man who prepares my Energy Performance Certificates to re-enter a 100mm cavity width (in lieu of 75mm) into his computer, – for a new 2200 square feet detached house.   He told me that the saving on the gas bill would be about £11 per year.   However the thicker external walls would have reduced the floor area by twenty two square feet and the house was worth £350 per square foot.  So, excluding the extra cost of the insulation, longer wall ties, extended cavity closers, etc, you end up with the fact that you’ve expended £7700 in order to save £11 a year.  A simple payback period of seven hundred years.  We’re nearer in time to the Battle of Agincourt than to the time the extra insulation will have paid for itself!  The interest alone on the £7700 is likely to be forty times the savings.   If you sponsored an off-shore wind farm you’d recoup your £7700 in perhaps twenty years.  Still not commercially sensible, but thirty five times less daft than 100mm cavity walling.