Cavity wall insulation is one of the easiest and most cost-effective ways to ensure you have built an energy efficient home for your customers.

There are a few different ways to install cavity wall insulation so make sure you check out our tips below including some examples of how not to do it!

Partial-Fill Cavity Insulation

Partial-fill involves leaving a gap between the outer face of the insulation and the external leaf.

How should it be used?

  • By installing third party accredited insulation slabs.
  • Ensure you leave a minimum residual 50mm clear cavity between partial fill insulation and the external leaf.
  • The correct combination of wall tie and retaining disc must be used and installed in accordance with third party accreditation recommendations.
  • Keep the tops of the insulation batts clean and free from mortar droppings before the next batt is installed.
  • Ensure that outside face of the internal blockwork leaf is kept clean and free from mortar to ensure the insulation can be pinned tight back to the inside wall.
  • The internal blockwork must always be constructed before the external leaf.
  • Additional slots may need to be carefully cut in the partial fill insulation slabs to allow for additional wall ties to be accommodated every 300mm or block course at window and door reveals.

An example of how not to install partial-fill insulation

What’s wrong with this picture?

  • Partial fill insulation batts were poorly installed against the inner leaf
  • The remaining cavity was less than 50mm
  • It is bridged by mortar droppings allowing a path for water to reach the inner leaf.
  • There is evidence of inadequate wall ties. These should be every 300mm or block course at the reveals. (No evidence of slots being cut in insulation slabs to accommodate additional ties).

Full-Fill Cavity Insulation

How should it be used?

  • It may not be appropriate in areas with high or severe weather exposure
  • It should not be used with random sandstone walling or other irregular facing.
  • Recessed mortar joints to the outer leaf are not recommended with full-fill insulation.
  • Only third party accredited insulation products should be used.
  • Insulation batts should be built into the cavity as work proceeds and not pushed in after
  • Insulation slabs should be of the appropriate thickness to fully fill the width of the cavity.
  • Insulation should wrap around corners and not be cut and joined.
  • Do not leave gaps in the insulation. Butt the insulation slabs together at both horizontal and vertical joints and at closures, and install them with staggered vertical joints.
  • Keep joints between insulation slabs clean and free from mortar droppings.
  • Do not place any small offcuts with the cut edge against the wall surface
  • Ensure horizontal joints of insulation correspond with horizontal rows of ties. Where additional wall ties are required, cut insulation slabs neatly to accommodate them.
  • Ensure that fibre insulation is protected from saturation from persistent rain by covering the top of the cavity.  Do not leave insulation exposed to the elements if rain is expected overnight or for any extended periods of time before work recommences.  Saturated insulation does not recover and its performance is severely adversely affected.

An example of how not to install full-fill insulation

What’s wrong in this picture?

  • Insulation batts were found incorrectly pushed into the cavity with cut edges facing the outer leaf.

Blown or Injected Cavity Insulation

This type of insulation is either blown or injected into finished walls through strategically drilled holes. It is usually specified for upgrading existing homes, but can be successfully used on new builds too.

How should it be used?

  • Blown or injected insulation should be carried out by appropriately qualified personnel.
  • The cavity must be inspected prior to the installation (Borescope survey required) to ensure:

- Mortar joints are flush with the cavity and the faces of the masonry are clean.
- The cavities are free from obstructions such as lumps of mortar and parts of bricks.
- Mortar droppings below the horizontal DPC must be minimised.
- All scaffold holes are filled with mortar on removal.
- Blow fibre insulation should be prevented for entering cavity party walls as this may compromise the sound attenuation measures between dwellings.

  • Recessed mortar joints to the outer leaf are not recommended with full-fill cavity insulation.

For more information on how to install cavity wall insulation in an existing building take a look at our article on ‘minimising heat loss in your retrofit project’.

By Olivia Catterall