Suspended ground floor and crawlspace

Frequently asked questions

Ventilate the void or seal it — why do the markets disagree?

Because they are solving for different climates and different moisture sources, and both positions are coherent. The BRITISH approach ventilates: airbricks through the external wall on at least two opposite sides give a cross-flow that carries away moisture evaporating from the ground, and blocking them is one of the most reliable ways to rot a floor. That works in a mild, damp climate where outdoor air is rarely much wetter than the void. The AMERICAN approach increasingly SEALS: a ground-covering vapour barrier, sealed vents, insulation at the foundation walls, and the space brought inside the building's thermal envelope. That works where summer outdoor air is far more humid than the cool void, so ventilating it CONDENSES moisture into the space rather than removing it. Applying either approach in the other's climate produces the failure it was designed to prevent.

What is a sleeper wall, and why is it built with gaps?

It is a low wall in the void, part-way across the span, carrying a wall plate that the joists sit on — so the joists span from external wall to sleeper wall rather than the full width of the room, which lets a much smaller section be used. It is built HONEYCOMB, with the bricks spaced so there are openings right through it, because a solid wall across the void would block the cross-ventilation the whole floor depends on. Every sleeper wall also sits on its own footing with a damp-proof course under the plate. Two things go wrong with them in practice: the honeycomb gets filled in during later work, which kills the airflow beyond it, and the DPC under the plate is omitted or bridged, which rots the plate the joists are sitting on.

Why does insulating one of these floors go wrong so often?

Because the insulation is usually added from below without changing anything else, and it alters the void's behaviour in two ways at once. It makes the void COLDER, since the heat leaking down from the house was quietly keeping it above the dew point — so the same outdoor air that was harmless before now condenses in it. And it frequently obstructs the airflow the ventilation depends on, because insulation fitted between joists is pushed toward the eaves of the void or tucked into the airbricks. The result is a floor that was dry for a century and rots within a few years of being improved. The fix is not to avoid insulating; it is to treat the void's ventilation, the ground covering and the insulation as one system rather than adding one of the three.