Post-insulating cavity walls goes mandatory with hydrophobating the façade stones. This obligation is stated in the Bouwbesluit (NL) and the technical prescriptions of the WTCB (B).

Especially on the rain side it is inevitable that rainwater intrudes through the façade stones and the insulation material. In an open cavity wall up the half of the rainwater, with prolonged rainfall, can penetrate.

The warmth loss after insulating the cavity wall is much bigger than with an open cavity wall. This is clearly visible infrared. The loss of warmth with saturated walls and insulation is 28 times more than with air.

This doesn’t mean that you don’t have to insulate, on the contrary. Insulate first the cavity walls, but make the façade immediately water repellent, so everything stays dry. By changing the surface tension rainwater in liquid form cannot penetrate into the façade stones but the moisture in gas form can “breath” free through the façade stones.

Often this isn’t recommended to be cheaper. But by hydrophobating you save a lot more on the long term.

With a Karsten tube test you can easily test the water absorption of the façade stones. When in doubt always hydrohobate the façades.

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