CROSS Safety Report
The risk of swapping insulation behind cladding
This report is over 2 years old
A reporter is concerned that insulation behind cladding is being swapped without a full appreciation of the implications.
Two materials are commonly used for insulation, polyisocyanurate (PIR) insulation and mineral wool.
Whilst industry is increasingly turning to mineral wool for its fire resisting property, other issues including moisture retention and material density must be appreciated.
Key Learning Outcomes
For designers and specifiers:
Cladding must be viewed as a whole system, the interaction between all components should be considered and any change should be approved by a person who is competent to do so
Be aware that while mineral wool has inherent high fire resistance, to achieve the same insulation levels as other products it may need to be applied in thicker layers. This can result in an increase in weight.
Mineral wool may also retain moisture more than other products, this should be considered especially in the attention to fixing and sealing details
Give attention to the design of cladding systems and the safety-critical aspects of their fixings and anchors
Find out more about the Full Report
Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others. If you would like to know more, please visit the reporting to CROSS-UK page.
A reporter is concerned that after the Grenfell Tower fire, the industry has ‘knee-jerked’ into swapping PIR insulation for mineral wool insulation behind cladding. The reporter worries that the consequences have not been fully considered.
Any material change requires careful consideration
The reporter notes the following about mineral wool insulation:
It is approximately 70% heavier than PIR insulation for a given thickness
The U-value is approximately twice that of PIR insulation. This will mean using thicker insulation when using mineral wool (the lower the U-value, the better insulated the building element).
The reporter notes the following concerns that relate to roof panels:
Footfall will break the fibre bond in mineral wool insulated composite panels. This will lead to a loss of composite action and sudden failure without warning. This will happen at spans much shorter than quoted in load span tables.
Walkways for maintenance must be provided independent of the roof panels. This is not necessarily required for PIR insulated composite panels.
Composite walk-on-ceilings are a particular risk due to the flat surface of the steel which connects to the mineral wool. For reference see CROSS report 54 Walkable ceilings can deteriorate.
Mineral wool will not adhere to the cladding in the same way as PIR. This means fixing details will need to be amended.
The compression capacity of mineral wool is much lower than PIR. This means the detailing where plant is situated above needs to be carefully considered.
Poor detailing can lead to inadequate systems
The reporter notes a number of concerns that relate to mineral wool being used as wall insulation:
Mineral wool is water and water vapour permeable. This 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.
Where cold formed steel 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.
In the view of the reported, a break, oriented strand board (OSB) or plywood with a breather membrane should be fixed back to steel studs prior to fixing the insulation
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.
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.
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.
Expert Panel Comments
Find out more about the Expert Panels
Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.
This is a 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.
Changing a component can affect the performance of the cladding system
Cladding must be seen as a system, with interactions between all the components that make it up. Changing any 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 taken during construction must also be considered. For example, weather protection of mineral wool is needed in the temporary situation.
This makes 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.
Overlooking the importance of fixings
Manufacturer’s offer guidance on the need for structural justification of fixings and certification bodies highlighting the need for fixings calculations. These are often overlooked.
Cladding, whether new or as a replacement, may not be regarded by clients or contractors as having structural significance. There may not be a structural engineer involved and the importance of the substitution could go unnoticed.
Cladding, whether new or as a replacement, may not be regarded by clients or contractors as having structural significance. 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.
Share your knowledge
Your report will make a difference. It will help to create positive change and improve safety.
Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others.
No feedback has yet been published for this page.