What effect do 300mm cavities have on floor area? I’ve just been putting my new benchmark house through its paces and have done a few calculations. As it stands (or rather will — it’s still under construction), it is a four bedroomed detached jobbie on two floors with an internal floor area of 147m2.
It’s basically a brick and block construction with a cavity width of 100mm, designed to be fully filled with DriTherm, designed to give a U value of maybe just under 0.3 (designed to pass Part L 2006 version).
Now if the cavities are widened to 300mm, without any changes being made to the design, the internal floor area falls to just 131m2.
So here it is:
• The external area of the house is 173m2
• The built internal floor area is 147m2 (85%)
• The Passive House version would be 131m2 (75%)
It’s a whole lot of wall, and as such it remains a huge hidden cost of building to very low energy standards.
One conclusion could be that you shouldn’t be designing walls that are 500mm thick, and that it’s time to embrace wall systems which are more efficient.
Another is that you might be able to overcome the space constraints by good design – the Small is Beautiful argument.
Yet another is that when you reduce the process to number crunching metrics (like £x per m2), you inevitably get some strange results and that you shouldn’t take them too seriously.
But next time you read that “Passive Houses only cost slightly more to build than conventional ones”, perhaps you should ask if we are really comparing like with like?