Cavity Wall Insulation With Blown Mineral Wool

Cavity wall insulation involves the mechanical injection of a suitable insulant into the cavity, using a specified system designed to ensure a complete fill. Blown mineral wool is one of the recognised materials. Here the mineral wool after passing strict quality control tests in the manufacturer's laboratory is being loaded into the blowing machine hopper.

CIGAThe Cavity Insulation Guarantee Agency provides independent 25 year guarantees for Cavity Wall Insulation fitted by registered installers. The Cavity Insulation Guarantee Agency was established in consultation with the Government to provide householders with an independent, uniform and dependable guarantee covering defects in materials and workmanship. Independent research has confirmed that cavity wall insulation is a very reliable process. Problems are therefore extremely rare, but if they do occur, CIGA has the expertise to ensure that they are swiftly rectified.

Composition & Durability

The mineral wool used for cavity wall insulation consists of mechanically granulated spun glass or rock wool, treated with a binder or water repellent during manufacture. They are produced under strict quality control to ensure compliance with the performance requirements of the appropriate British Board of Agrément Certificate: the equipment for blown mineral wool is also required to conform to, and the installation must be carried out in accordance with, a current BBA Certificate for this process.
Once installed, mineral wool is sufficiently stable to remain an effective insulant for the life of the building.


Thermal Conductivity: For the purpose of U Value calculations to determine if the requirements of the building ( or other statutory ) regulations are met, the thermal conductivity of the insulation may be taken as 0.040 W/MºC.

Density: Typical installed densities range from 18kg/m3 for glass wools to 40kg/m3 for rock wools (depending on individual BBA certificates).

Fire: Blown Mineral Wools are non – combustible, tested to BS 476 : Part 4 : 1970.

Water: Mineral Wool is resistant to water penetration and will not transmit water across the cavity or from below DPC level by capillary action. It does not however act as a water vapour barrier.


Survey: A pre-installation survey is carried out to assess the suitability of the property for the installation of cavity wall insulation.

Site Preparation: Essential ventilation openings such as those providing combustion air or underfloor ventilation, and all flues in the cavity wall must be checked. If adequate sleeving or other cavity closures are not present, installation must not proceed until these openings have been sleeved or otherwise modified to prevent blockage by the insulant.

Installation Procedures:

  • Where a semi–detached or terraced property is to be treated, the insulation is contained by inserting a cavity barrier at the line dividing the properties. This consists of a nylon brush which is retained in the cavity.
  • Injection holes of 22mm or 25mm diameter are drilled at a predetermined pattern/distance (depending on individual BBA certificate).
  • Wool is blown into the cavity via a flexible hose fitted with an injection nozzle, using an approved blowing machine. Filling proceeds from the bottom to the top of the wall and from one end of the elevation to the other.
  • After injection of the insulant, the holes are made good to match the existing finish as closely as possible.

Post Installation Checks: All necessary air vents are checked e.g. those providing underfloor ventilation and combustion air. In all cases flues are carefully checked on completion of the installation by means of an appropriate test to ensure they are not obstructed by the insulant.

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