Full-Fill Insulation

Insulation designed to fill the full width of the cavity comes in ‘batts’ of lightweight, flexible insulation material (such as Rockwool), which are usually 450mm high by 1500mm long. Different thicknesses are available to suit different widths of cavities. The height is equivalent to six courses of brickwork (in other words, the vertical spacing of wall ties), so it is usual to construct the external skin of brickwork six courses high, position the insulation at the back of the brickwork, then construct two courses of blockwork for the inner skin, before repeating the process again.

Full-Fill Insulation

Fig. 173 Full-fill cavity-wall insulation.

The notion of full-fill insulation, on the face of it, seems to defeat the whole object of introducing a cavity into an external wall in the first place, with the cavity itself being designed to act as a moisture barrier. Insulation designed for this application is manufactured with the aim of not allowing moisture to pass through, but this can never be completely guaranteed in locations exposed to severe weather conditions.

During the construction of each successive six courses of brickwork it is important to prevent mortar droppings collecting on top of the insulation slabs. If this waste is allowed to build up between the insulation slabs, it can form a ‘bridge’ for moisture to pass across the cavity to the inside of the finished building. The problem can be prevented by placing a timber board on top of the last course of blockwork to catch any mortar droppings that fall during construction of the next six courses of brickwork.

Full-Fill Insulation

Fig. 174 Installation of full-fill cavity insulation.