Floor construction and moisture control

Questions behind this page

How does a floor change with the foundation? What sits above a slab? What does a vapor barrier do? Can a floor go directly on earth or gravel?

Think in functions

A working floor needs support, a usable surface, control of moisture, and the required thermal performance. Different layers can serve more than one function.

Assembly	Typical functional sequence
Slab on grade	Prepared support, specified ground-moisture control, concrete, compatible finish
Raised framed floor	Foundation supports, beams/joists, subfloor, compatible finish
Slab with added subfloor	Concrete, specified moisture-control system, approved subfloor assembly, finish
Prefabricated floor cassette	Transportable framed assembly connected to a designed support system

The ordering and placement of insulation, air barriers, and moisture-control layers depend on the complete design.

Vapor control is not drainage

A capillary break limits liquid movement through small pores; a vapor retarder limits vapor transmission. Neither substitutes for drainage or fixes water entering at a wall or plumbing leak.

NRMCA’s CIP 29 recommends a durable ASTM E1745-compliant retarder and discusses placement directly beneath concrete receiving moisture-sensitive finishes. It also distinguishes a capillary break from vapor control.

Above a slab

Direct finish installation minimizes buildup when the slab and finish are compatible. A floating or raised subfloor may add insulation or provide a wood nailing surface, but introduces height, moisture, and detailing questions. Check thresholds, stairs, doors, and appliances before adding layers.

Flooring readiness depends on the selected system’s moisture and substrate limits; concrete strength and drying are different milestones (NRMCA CIP 28).

Earth and gravel

An outdoor gravel surface is a different use case from a habitable indoor floor. Earthen floors are a specialized assembly, not ordinary flooring laid on bare soil. Feasibility requires a complete structural, moisture, sanitation, and approval strategy for the intended occupancy.