Fortunately, it’s been a good, dry week to get the tanking onto the retaining wall. The below-ground (substructure) element of wall needs to continue the waterproof and gasproof characteristic of the floor. We opt for self-adhesive Visqueen Gas Resistant Tanking Membrane. The wall was first treated with a bitumen primer (above left photo) and left to dry and the tanking membrane went on the following day. It was a bit cold, so the self-adhesive element needed a bit of help with a roofer’s torch to get it to stick to the wall, but it went on finally.

The insulation goes on against the tanking. We are using Kingspan Styrozone H350R. We cannot use any mechanical fixings to attach this insulation to the wall (as it will compromise the waterproofing). I speak to Kingspan to see what adhesives they recommend, but they advised that no grab adhesives of any kind (water- or chemical-based) should be used. We therefore opted for an additional ‘sacrificial’ low-cost fibre board to hold the insulation temporarily in place and to protect the insulation during backfilling. The weight of the backfilled stone and earth, once compacted, will hold the insulation firmly in place. The photo (above right) shows a plan view of the insulation against the tanking as the hardcore backfill was going in – absolutely no air gaps between the wall and the insulation.