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784 Swapping insulation behind cladding without adjusting details

Report ID: 784

Published: Newsletter 55 - July 2019

Report Overview

A reporter is concerned that insulation behind cladding is being swapped without a full appreciation of the implications.

Report Content

A reporter is concerned that after the Grenfell Tower fire, the industry has knee jerked into swapping polyisocyanurate (PIR) insulation to mineral wool insulation behind cladding, without fully considering the consequences.

The reporter states that:

  • Mineral wool insulation is approximately 70% heavier than PIR insulation for a given thickness.
  • The U-value of mineral wool is approximately twice that of PIR insulation, which will tend towards using thicker insulation for mineral wool (the lower the U-value, the better insulated the building element).
  • For roof panels:
  1. Footfall will break the fibre bond in mineral wool insulated composite panels, resulting in a loss of composite action and sudden failure without warning (at spans much shorter than quoted in load span tables). Walkways for maintenance must be provided independent of the roof panels, which is not necessarily required for PIR insulated composite panels.
  2. Composite walk-on-ceilings are a particular risk due to the flat surface of the steel which connects to the mineral wool - see CROSS report 54 Walkable ceilings can deteriorate.
  3. Mineral wool will not adhere to the cladding in the same way as PIR. Therefore, fixing details will need to be amended.
  4. The compression capacity of mineral wool is much lower than PIR. Therefore, the detailing where plant is situated above needs to be carefully considered.
  • For wall insulation:
  1. Mineral wool is water and water vapour permeable, which can be an advantage if detailed well. However, if not detailed well, it will allow water to collect around cladding fixings which can lead to corrosion where the protection is inadequate or broken.
  2. Where cold formed studs are used to support the cladding, a standard detail is to fix the insulation directly to the studs. For mineral wool insulation, this can lead to corrosion of the studs, particularly at the top and bottom cut ends as well as the lip edge. A break, oriented strand board (OSB) or plywood with a breather membrane should be applied in this situation, in the view of the reporter.
  3. Sometimes cladding fixings to studwork are fixed through PIR insulation, allowing for the compressive rigidity of this insulation. Mineral wool insulation does not provide the same resistance. Therefore, cladding fixings should be fixed directly to the studs with structural breaks.
  4. As mineral wool insulation tends to be thicker and heavier than PIR insulation, it results in higher forces and moments in the fixings. Also, with its lower compressive strength, it generally requires more fixings to provide the required restraint.

The reporter feels that if these differences are not considered, the industry could in time start to get structural cladding failures.


Another serious issue and there have been some reported failures due to substitutions of this kind. The problem is that those making such changes may not have the experience or knowledge to appreciate the factors given by the reporter. It is an issue for the cladding industry to address and another case where competency of companies offering the service, and their operatives, is vital.

All changes need to be treated with caution and significant changes must be approved by the designer. Cladding must be seen as a system, with interactions between all the components that make it up. Changing any one of the components must lead to an evaluation of the performance of the whole system with the incorporation of the new component.

When replacing insulation behind cladding, measures during construction must also be considered. For example, weather protection of mineral wool is needed in the temporary situation, making it a more demanding product in terms of temporary works. CROSS is aware of cladding failures in tower blocks due to moisture trapped in mineral wool during construction.

Despite manufacturer’s guidance on the need for structural justification of fixings and certification bodies highlighting the need for fixings calculations, cladding, whether new or as a replacement, may not be regarded by clients or contractors as having structural significance, so there may not be a structural engineer involved and the importance of the substitution could go unnoticed.

When there has been a major failure, there is often a “knee-jerk” reaction and careful evaluation is needed to avoid unintended consequences. The first defence against this is having the experience to identify when a component change warrants a system review. Techniques for keeping water out of buildings, and insulating them, have been developed over years and present complex issues.

The British Board of Agrément (BBA) have recently published a technical case study on avoiding external insulation failures.


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View other CROSS reports published in Newsletter 55

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