Walls: Insulating partial fill cavity walls

The thermal performance of cavity walls may be improved by installing Ballytherm Insulation within the cavity. This method is well suited to new build projects. The properties of Ballytherm Insulation match those required for this application: its low thermal conductivity minimises the thickness of insulation required to meet performance standards and so limits the overall cavity width.

Ballytherm is moisture resistant and will not be affected by moisture within the cavity. Ballytherm boards should be fitted to the outer face of the inner leaf. They should be supported on wall ties and restrained against the masonry by clips fitted over the wall ties.

The boards are available with butt, ship lapped or tongue and grooved edges. They should be installed with staggered vertical joints to give a continuous layer of insulation.

The thickness of Ballytherm Insulation required to meet regulations may be determined from table 8.

Design guidance
To prevent thermal bridging Ballytherm boards should be fitted tightly to the edges of door and window openings. Lintels which support the inner and outer leaf should have internal insulation or be packed with Ballytherm Insulation (figure 18).

Where Radon barriers and cavity trays bridge the cavity, the insulation should be extended to the underside of the barrier or tray, then continued immediately above it.

Sitework
Installation sequence

1. Raise the inner leaf 450mm above the outer leaf (two courses of blockwork or six of brickwork).
   
2. Fit the first run of Ballytherm boards against the inner leaf, supported on top of wall ties or the concrete cavity fill.
   
3. Raise outer leaf by 450mm.
   
4. Notch the top of the Ballytherm boards and fit wall ties.
5. Fit clips to hold the boards against the inner leaf.
6. Repeat the sequence to the required height of the wall, staggering the vertical board joints.

Points to watch

• Ensure wall ties slope down from the inner leaf to outer leaf to prevent water travelling to the inner leaf.
• Use a cavity board across the top of the insulation to prevent mortar building up on the insulation or bridging the cavity.
  
• Fit Ballytherm neatly to elements which bridge the cavity such as door and window sub-frames, lintels and cavity closers.