Crawl space: definition and effect on the screed

What is a crawl space?

A crawl space is an empty, generally ventilated void of shallow height, arranged between the natural ground and the lowest floor of a building. It is not intended to be occupied, unlike a cellar.

Its principle is simple: by removing direct contact between the soil and the floor, it cuts the shortest path for moisture to travel indoors.

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What is it for?

Three functions combine. It distances the floor from ground moisture, it allows services to run and be inspected without breaking up the finished floor, and it moderates the effects of ground movement on the structure.

Its ventilation is essential. A sealed crawl space becomes a volume saturated with moisture, which defeats its whole purpose and even reverses its effect: moisture accumulates against the underside of the floor.

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What does it change for insulation?

The floor above a crawl space is a floor over an unheated space. It faces a ventilated void, therefore cold in winter, and it forms part of the thermal envelope of the building just as an external wall does.

Insulating it is therefore not optional. Two routes exist: insulating from above, under the screed, or insulating on the underside from within the crawl space where its height allows. The first consumes interior height, the second preserves it.

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What does it change for the screed?

Less than for a floor laid on the ground, since direct capillary rise is already cut by the void. The subject does not disappear for all that: a poorly ventilated crawl space remains a source of moisture from below.

The main point of attention concerns the structure of the floor itself, often made of beams and infill blocks. Its permissible load governs the screed thickness that is achievable, and its flatness governs the average thickness to allow for. See our thermal insulation solutions.