What is a mortar bridge (mortar snot)?
Mortar snots are bedding mortar that is squeezed out of the joints into the cavity during bricklaying and sticks out there. In Dutch this is called a speciebaard (literally “mortar beard”), also cementbaard or mortelbaard. As long as the mortar only protrudes, it is a snot. If the mortar touches the insulation or the inner leaf, or if mortar droppings fall into the cavity and form a connection, a mortar bridge is created: a bridge of mortar between the outer leaf and the inner side of the cavity wall.
Why is it a problem?
- Moisture getting in. The cavity exists precisely to carry rainwater that penetrates the outer leaf downwards. A mortar bridge conducts water to the insulation or the inner leaf, resulting in damp patches on the inside.
- Thermal bridge. Mortar is a poor insulator. Heat escapes via the bridge and condensation can form on the inner wall.
- Poor fit of insulation. Mortar snots on the inner leaf push insulation boards away. This creates a false cavity behind the insulation and gaps between the boards, where cold air can circulate. That costs a great deal of insulation value.
- Blocked cavity tray. Mortar droppings that fall to the bottom of the cavity can block the open perpends (weep holes) and the cavity tray.
Application: how to prevent it
- Strike off the joint on the cavity side. With each course, strike off protruding mortar with the trowel, including on the cavity side of the leaf.
- Insulation tight against the inner leaf. The cavity face of the inner leaf must also be flat, without snots, so the boards fit well. Fix the boards with insulation clips on the wall ties.
- Keep the cavity clean. While laying, consider placing a batten or strip on the ties to catch falling mortar, and lift it out regularly.
- Check at the bottom. Keep the open perpends above the cavity tray clear, so water and moisture can escape.
- Inspect if in doubt. With an endoscope through an open perpend you can look into an existing cavity to see whether there are mortar bridges.
Common mistake: “pressing” insulation boards against an inner leaf with mortar snots without removing the snots first. The board then sits hollow and the insulation performs much worse than calculated.
Related terms
- Cavity wall
- Outer leaf
- Inner leaf
- Thermal bridge
- Cavity wall insulation
- Mortar
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