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Horizontal barrier under foundation wall

The step by step guide to application of horizontal waterproofing inside and under foundation walls.

Crucial to know

28 days old concrete substrate

5 minutes mature time

the second layer after 3 h


Tools needed

a brush & a trowel

a drill with a paddle mixer

protection foil

Step 1

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BT 21 Membrane on the foundation slab

The horizontal waterproofing barrier under and inside the foundation walls
The horizontal waterproofing barrier is always applied before the vertical wall and floor insulation. It protects the building and its components against the capillary rise of moisture. 

If there is no basement in a building and the base plate is located deeply, it is necessary to apply an additional horizontal insulation barrier inside a wall, at least 50 cm above the ground layer. If there is a basement in a building, usually two horizontal waterproof barriers are applied: the first one on the base layer and the second one just under the basement ceiling.

Step 2

Pre wetting the substrate

Substrate preparation

Ceresit CR 166 slurry requires pre-wetting of the substrate before application, avoiding formation of puddles. When waterproofing foundation in the case of rear penetration of moisture, pre-treat the areas with CO 81 Silicifying Fluid.

CR 166 adheres to load-bearing, solid and clean surfaces, free from any substances that decrease adhesion (such as grease, bitumen, dust), namely: concrete (over 28 days old), cement screeds (over 28 days old), rapid-hardening cement screeds Ceresit CN 87 (over 3 days old). Cracks should be widened and then filled with Ceresit CX 5 repair mortar. Uneven surfaces should also be levelled with cement mortar.

Step 3

Preparation & application of ceresit CR 166

Application of CR 166 Slurry

Two-component CR 166 slurry should be stirred until it forms a homogeneous mixture, free of lumps. Wait about 5 minutes and stir the material shortly again.

The first layer of CR 166 should always be applied with a brush on wet substrate, while the next layers can be applied by a trowel or with a brush. The fresh layer should be protected with foil sheets against drying out too quickly. Under average conditions, the next layer of CR 166 can be applied after 3 hours, when the previous layer is hardened but still damp.

Consistency of the mortar should be adjusted to the application method:

application by brush – pour compound B (liquid) into a container, add 2 litres of water and add compound A (powder) whilst stirring,

application by trowel – pour compound B (liquid) into a container and add compound A (powder) whilst stirring.

For horizontal damp-proofing barrier on wall footing, it is recommended to embed a reinforcement fleece (60 g/m2) between layers of CR 166.

Tips & Tricks

For professionals

The Hydroslide Effect with Ceresit CR 166.

The Hydroslide Effect creates an immediate water-repellent effect, providing a stronger reduction of capillary absorption of water vs. other standard slurries. It leads to a stronger reduction of aggressive substances dissolved in water (e.g. chlorides and de-icing salt) ensuring an excellent adhesion of subsequent layers.


products used

  • ${/content/heliux/adhesive/ceresit/com/en/products/waterproofing/central-pdp#/ceresit-co-81/SAP_0201WQC014A5/searchResult.productFields.MainImageAltText}
    For sealing existing brickwork against capillary rise of moisture. Seals capillaries and fine cracks of up. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/waterproofing/central-pdp#/ceresit-cr-166/SAP_0201WJC014B9/searchResult.productFields.MainImageAltText}
    CERESIT CR 166
    2-component flexible slurry, strengthened with fibres and designed for waterproofing and damp-proofing on deformable and non-deformable mineral substrates. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/mortars-and-auxiliaries/central-pdp#/ceresit-cx-5/SAP_0201VBC014O4/searchResult.productFields.MainImageAltText}
    Universal fast-setting mortar for quick, easy and strong fixing and assembling with perfect consistency for horizontal and vertical substrates (indoor & outdoor applications). 

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