Building paper as damp proof membrane at foundation and plinth — two layers in cavity wall, 150 mm above ground level, 20° slope, and how the membrane also plays a role in radon protection per BR18 §332.
Introduction
The damp-proof course (building paper or membrane) at the foundation is the mason's most important moisture barrier: without it, rising ground moisture draws up into the brickwork via capillary suction. BR18 sets performance requirements for moisture protection, and SBi guidance 233 + Mur & Tag provide specific guidance.
Placement and width
- 01At least 150 mm above surrounding terrain — industry guidance (BR18 specifies functional requirements, not fixed level).
- 02Width adapted to wall thickness: supplier widths are 5 / 11 / 15 / 20 / 30 cm.
- 03In cavity walls: two layers — lower layer in the foundation's full width/length, upper layer from the front of the outer wall to the back of the inner wall.
Upper layer in cavity wall
- 01At least two shifts are carried up the back wall.
- 02Pour at least 20° towards formwork — recommended not mandatory.
- 03Joints: overlap at least 100 mm, glued with asphalt adhesive.
- 04Airtight at all penetrations.
Åbne studsfuger = drænhuller
Open studs (drain holes) are placed directly on the membrane in the front wall so any condensation in the cavity can drain out. The drain holes must not be blocked by mortar residue from laying.
Radon function (BR18 §332)
The bigger picture: moisture protection below ground
The plinth membrane in the cavity wall is only one link. Underground, moisture, radon and water must be managed in a continuous chain — from the perimeter drain on the outside, through the capillary break layer, to the membrane that ties the foundation's damp-proof course together with the floor slab's radon membrane. If one link breaks, moisture or radon finds a way anyway.
Capillary break layer under ground slab
Under the floor a capillary break layer of coarse, washed materials is laid — e.g. 16/32 mm gravel (often approx. 150 mm) or lightweight clinker. The coarse grains have such large pores that water cannot be drawn up capillary. The layer is also part of radon protection because it is so air-open that radon suction can be established under the slab if needed.
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