In our last newsletter, we discussed the many different types of thermal insulation materials available to Contractors and Specifiers.  

This month’s technical bulletin focuses on Stone Wool Insulation and the reasons why it could be considered within flat roof specifications to provide enhanced benefits in terms of Fire and/or Acoustic performance.  

TECHNICAL PRODUCT FOCUS – Stone Wool Insulation for Flat Roofs 

There are many different types of thermal insulation materials available to Contractors and Specifiers.  

These include Polyisocyanurate (PIR), Stone Wool, Expanded Polystyrene (EPS), Extruded Polystyrene (XPS), Cellular Glass and Vacuum Insulated Panels (VIP).  

The main factors influencing the selection of insulation type and thickness of insulation are as follows:  

What is Stone wool and when should it be considered? 

Stone wool boards for flat roofing are produced by a limited number of manufacturers (such as ROCKWOOL). It is important to use the right product. The ROCKWOOL HARDROCK Multi-Fix (DD) board is a high-density Stone wool product which can be used in conjunction with Polyroof systems in flat or Tapered schemes. 

Stone wool insulation is manufactured from minerals such a naturally occurring volcanic rock. It is thicker and heavier than materials such as Polyisocyanurate (PIR) insulation so is not always the first material of choice when considering flat roof insulation.  

However, this article considers two areas where the designer may wish to opt for Stone wool over PIR. 

Fire Performance 

It should be noted that in accordance with Approved Document B (Fire), roof systems may be classed as unrestricted when the tested roof system meets BROOFT4. It is perfectly acceptable to use any thermal insulation type within a flat roof system design providing that is that a BROOF(T4) classification has been achieved for external fire performance. The classification should be in accordance with EN13501-5 (Fire classification of construction products and building elements – part 5).  

Some designers or building owners may however express a preference to choose an entirely non-combustible insulation within the roof system design. Thermal Insulation is categorised under the Euro class system, with 7 classification levels from A1 to F (with A1 being the highest rating). Stone wool products such as ‘ROCKWOOL’ achieve the highest rating in terms of classification – Euro class ‘A1’ or ‘A2-s1,d0’) and are therefore defined as Non-Combustible. See table below. 

Note – ROCKWOOL Stone wool is one of a number of non-combustible thermal insulation products along with FOAMGLAS Cellular Glass that may be used within Polyroof specifications.  

Acoustic Performance 

Sound reduction is another key feature of Stone wool insulation. Due to its composition, it is very effective in absorbing sound waves in comparison to other types of thermal insulation. Stone wool can therefore help to absorb and block out external noise from sources such as air traffic and rain noise. For certain types of buildings there are specific acoustic requirements. For instance, there are minimum performance standards for acoustic design in school buildings. These standards are outlined in Building Bulletin 93 (BB93). The use of Stone wool can assist the designer in meeting these requirements. 


Stone wool Insulation offers designers the following, 

Further information and next steps 

A data sheet for the ROCKWOOL HARDROCK Multi-Fix (DD) Insulation is available by clicking here. 

Polyroof Technical services offer a service to provide bespoke specifications utilising stone wool insulation.  

Tapered designs are also available on request.  

For Further information, please contact Polyroof Technical Services. 

Technical Support

Call On: 0800 801 890

Contact our in house team of experts who will be able to assist you with technical enquiries no matter how big or small.

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