What a 400 millimetre wall costs

The standard external wall in Ghana is a single leaf of sandcrete block at 150 millimetres, uninsulated, with a split unit doing the rest of the work. At Project Topaz the external walls are 400, composite, with the insulation outboard.

Start with what that costs, because it is not only money. Four hundred millimetres of wall around a house is floor area nobody gets to stand in, and the client has paid for it twice: once as land and once as construction.

The area lost is a function of perimeter, not of floor area, so two houses of the same size are not the same decision. A compact plan gives up a small fraction of its floor to a thick wall. A long, thin plan with identical floor area has far more perimeter and gives up more. That is the number worth running before the specification is written, and it takes ten minutes.

WHAT IT BUYS

Outboard is the part that matters. Insulation outside the mass puts the blockwork inside the envelope, so the wall takes on the temperature of the room rather than the temperature of the afternoon. Simulation work on insulated envelopes in tropical climates puts the cooling saving at roughly 20 per cent.

That is a model, not a bill. Nobody is going to build this house twice to check, and we would rather say so than quote the figure as though somebody had.

The thermal mass argument underneath it is also weaker than it sounds. Mass earns its keep where day and night are far apart, and Accra’s swing is about four or five degrees. In that little range a heavy wall can hold yesterday’s heat into tonight as easily as it can even things out. The insulation is doing the work here, not the mass.

And none of it answers a window. Solar gain through glazing is the larger term in a house like this one, where the lounge is glazed on two sides and runs up through both floors. No wall thickness fixes that.