Video Transcript

What This Detail Is – And What You’ll Learn Today

Today we are looking at  a masonry cavity wall foundation junction, with a ground-bearing slab with insulation below the slab. 

By the end you’ll understand how moisture is managed at ground level, how the thermal line is kept continuous at the slab edge , how wall ties actually work , and how the components relate to each other so the detail performs as intended.

Glossary of Terms

First, let’s run through some of the common elements we will see in this detail. 

• DPC — which stands for damp-proof course, is positioned in the external wall, it is a waterproof barrier installed horizontally within the wall to prevent ground moisture rising up through the masonry by capillary action.
• DPM — which stands for damp-proof membrane is a physical waterproof barrier, preventing moisture from entering the buildings floors and walls. Typically it is a plastic sheet.
• Wall tie — this a metal bar or strap used as a connector between inner and outer leaves of the wall, they provide stability and prevent structural failure. The formed “drip” sheds any water that lands on the tie towards the outer leaf. There are many different types of wall ties depending on the structure.
• Perimeter upstand — is the vertical insulation at the edge of the floor slab which helps to reduce thermal bridging.
  
• Parge coat — is a thin airtight coat of mortar to the inner leaf that reduces air leakage acting as the airtight layer.
• Insulated plasterboard — is the internal lining that adds thermal performance and forms the final finish.

Walk Through the Detail

Starting at the very bottom, the DPM on blinding is your first moisture barrier. It’s only effective if it connects at the perimeter, so you’ll see the turned-up DPM lapping with the DPC line in the inner leaf. 

How the membranes are placed and lapped depends on the ground, construction and other site constraints like the presence of Radon.

The rigid insulation below the slab is continuous and tight to edges;, a separating layer or slip layer helps control moisture movement within the floor build-up and allows differential movement without scuffing the insulation. The in-situ concrete slab provides a strong level base to build non-loadbearing walls and floors off. 

At the edge, the perimeter insulation upstand is important. Without it, you would risk a thermal bridge at the floor-to-wall junction. It runs the full depth of the floor slab and is a minimum of 25mm thick. Across the cavity, which in this detail is 100mm wide fully filled with insulation, wall ties stitch the two leaves together. The small kink or drip in the tie is deliberate: it breaks the path of any water or condensation running along the tie and ensures it sheds to the outer leaf. 

The cavity must extend a minimum of 225mm below the DPC to comply with Building Regs Part C, and the insulation in the cavity must extend a minimum of 215mm below the floor insulation to comply with Part L of the Building Regs. In this detail the insulation below the DPC is moisture resistant in case of water ingress. 

Cavity thickness and insulation type and thickness are designed based on project u-value requirements, exposure zone and other factors. 

The outer facing brickwork protects against weather; the DPC within this leaf blocks rising moisture and aligns with the turned-up DPM so the moisture line is continuous. 

Finally, the parge coat to the inner leaf tightens up airtightness, and the insulated plasterboard brings the internal lining up to the project’s thermal target.

Student Questions

Let’s answer the exact questions raised on the student sheet so nothing is left hanging.

1. How are wall ties placed?
   Wall ties are set out in a staggered pattern as the wall rises, with the drip centred in the cavity. Spacing follows the project specification and exposure category; the key principle is that ties connect the leaves without creating moisture bridges inward. Part A of the building regulations – which is called Structure, provides the spacing requirements for wall ties for cavity walls. Horizontal spacing should be a maximum of 900mm and vertical spacing should be a maximum of 450mm. This image by the LABC demonstrates this really well. 
2. Does the DPM go through the cavity insulation?
   No. The DPM stays within the floor build-up and turns up at the perimeter to meet the DPC in the wall. It doesn’t cut through or interrupt the wall insulation. The performance comes from the clean DPM–DPC connection, not from threading a membrane through the cavity. You might see cavity trays in some details that do go through the whole cavity, we will chat more about those in a future video. 
3. Are the layers drilled or mechanically fixed together?
   No. In this detail the system relies on lapped or taped membranes, abutting insulation, and ties bedded in mortar, not on drilling components together. Mechanical fixings are only used where specified (and then it’s about compatible tapes, closures, or fixings that don’t puncture critical barriers).

Key Points for This Detail

Continuity is really important. Moisture barriers (DPMs to DPCs) and thermal layers must read as continuous lines.

Keep the DPC clear of finished ground level which is usually 150mm. Maintain the required clearance so rain splashback and bridging don’t defeat the DPC.

Mind the cavity – Keep it clear of mortar droppings; the ties should not become pathways for moisture inwards.

Show the intent on drawings – A small arrow or note at the perimeter lap, a clear upstand hatch, and a few well-placed labels prevent most site queries.

Project specifics matter. Substrate moisture, exposure, finishes, acoustic and fire requirements, warranties/insurer requirements and manufacturer guidance can all shift the final arrangement. There is always more than one way to design a detail.