Technical Guidance Note 17: Use of Mineral Fibre Insulation

Unless walls are shown to be well protected by trees or buildings, or do not face prevailing winds, mineral fibre cavity wall insulation should only be used in the following methods of construction:

Single storey with face brickwork:  75mm cavity, or greater (full fill, or partial fill with a 50mm residual cavity and approved insulation retaining fixings).  With weathered, or bucket handled joints.

Multi-storey with face brickwork:  minimum 50mm residual cavity (thermal insulation thickness as required) with approved insulation retaining fixings.  With weathered, or bucket handled joints.

Single or multi-storey with rendering or cladding:  minimum 50mm cavity (full fill, or partial fill (with a 50mm residual cavity) and approved insulation retaining fixings).  Rendering should be in two coats with a total thickness of at least 20mm (nominal mix 1:1:6, or 1:1/2:4 for dense concrete blockwork).

Mineral fibre insulation slabs consist of layers of resin bonded (phenol formaldehyde resin binder), water repellent treated glass wool, or crushed mineral fibre.

Calculations in accordance with BS 8104: 1992 can be provided to determine the exposure of a proposed building.  The construction method adopted, may then be shown to be appropriate for the sheltered, moderately or severely exposed location.

Typical Agrément Certificate/manufacturer’s conditions are:

1. In full fill, it should be ensured that the slabs fill the cavity and are built into the wall as construction proceeds.
   
   
2. The insulation thickness should remain constant where possible.  Should any changes in thickness occur a horizontal (vertical) damp proof cavity tray should separate each thickness change.
   
   
3. A minimum thickness of 50mm should be maintained where possible.  Where, for structural reasons, the insulation thickness is reduced by the intrusion of ring beams, etc, a minimum thickness of 25mm of insulation should be maintained and the manufacturer’s advice on fixing and weather proofing should be specifically sought.
   
   
4. Raked or recessed mortar joints should be avoided in high exposure areas.
   
   
5. Wall ties and fixings are installed correctly and are thoroughly clean.
   
   
6. Excess mortar is cleaned from the cavity face of the leading leaf and any debris is removed from the cavity.
   
   
7. Mortar droppings are cleaned from the exposed edges of insulation slabs.

Guidance taken from BS5628: Part 3: 1985, BS8104: 1992, BRE Report: Thermal Insulation: Avoiding the Risks, BBA Agrément Certificate No 95/3212.

Building Control Group
Planning Section
Portland House
Richmond Road
Worthing
West Sussex
BN11 lLF

Tel: 01903 221344

E-mail: building.control@worthing.gov.uk

Minicom: 01903 204500

Fax: 01903 207365

Answerphone: 01903 214034

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