Loft Insulation Case Study — Long Loft Insulation Upgrade With Side Wall Detailing, Portsmouth

The problem

The roof space ran the full length of the property, but the floor had no consistent insulation. The low brick side walls and the narrow sections beneath them needed accurate detailing to avoid leaving gaps.

The extended perimeter and long run of exposed timbers created a substantial route for heat loss. The poorest coverage was along the low brick side walls, where the reduced headroom made the bays awkward to reach.

Our assessment

The loft floor measured approximately 47m². We recommended a 300mm Knauf glass mineral wool installation across the full length, using 100mm between the joists and 200mm cross-laid above. Additional care was required along the low brick walls and around the offset hatch.

The installation

The team worked in sections from the furthest end back towards the hatch. A 100mm base layer was fitted between the joists, with close attention to the long edge bays and wall junctions.

The 200mm layer was then cross-laid across the joists to create a continuous blanket. The insulation was trimmed neatly around the hatch and kept clear of the eaves ventilation route. The full depth was checked before completion.

The result

The entire length of the loft was covered with 300mm of insulation, including the awkward sections against the low brick side walls, giving continuous coverage from one end of the terrace to the other.