Fire Performance of Sandwich Panel Systems

This Technical Briefing is written specifically to assist insurance surveyors and under-writers in understanding the fire issues relating to sandwich panels. It may also be of use to building occupiers and risk managers. For the purposes of this guidance document, sandwich panels (also known as insulated panels and sometimes as composite panels) are defined as follows: Sandwich panels are a building product consisting of two metal faces positioned on either side of a core of a thermally insulating material, which are firmly bonded together so that the three components act compositely when under load (wind-loading, access loads etc). Sandwich panel systems comprise the panels, their jointing methods and the type of support provided. The purpose of this Technical Briefing is to give a basis on which to assess the use of sandwich panels in respect to property protection. It recommends a risk-based approach to the various relevant parameters including amongst other things use of the building, location of panels, type of panel and standard of fire safety management. This document deals with property protection issues and complements Appendix F of Approved Document B to the Building Regulations, 2000, which is limited to life safety aspects. Composite panels with facings other than metal are not covered in this document. This definition also does not apply to built up external cladding systems assembled on site. These typically use lightweight non-combustible (Euroclass A1 or A2) mineral wool rolls (rock fibres or glass fibres). The spread of fire is influenced by: • fire safety management • compartmentation • sandwich panel system construction • combustibility and fire performance of panel core material • application of sandwich panel system Not all sandwich panels contain a combustible core. Even sandwich panels containing a combustible core do not themselves create the fire risk; they require a significant size of ignition source to propagate the fire. Not all combustible materials used in the construction of sandwich panels have poor fire performance. Good standards of fire safety management can substantially reduce the fire risk. Building regulations do not necessarily provide sufficient fire protection for insurance purposes. For example, unlimited floor areas are a major contribution to losses because of the lack of fire resisting compartment walls. 1 Technical Briefing Fire performance of sandwich panel systems 1 Introduction and background 2 Technical Briefing Fire performance of sandwich panel systems 1.3 Importance of fire safety management 1.4 Lack of compartmentation 1.5 Importance of good joints and support systems 1.6 Internal use of sandwich panels 1.7 External claddings Insurers should satisfy themselves that the building operator maintains a high standard of fire safety management as this can reduce the possibility of accidental ignition. A detailed risk assessment should be undertaken to establish the likelihood of ignition, before a decision is made on the suitability or otherwise of a particular sandwich panel system. Many factory buildings do not incorporate fire resisting compartment walls which would make a significant contribution to reducing spread of fire since this is not required by Building Regulations and their supporting documents. Active intervention by fire fighters is less likely in the absence of compartmentation. The introduction of an adequate level of compartmentation may be a sensible alternative to replacing panels. Poor joint detailing and inadequate support for the panels lead to rapid delamination of the facings, exposing any core directly to the fire condition. Depending on the type of core, premature collapse of such panels allows the ensuing fire to spread rapidly thus making effective fire fighting almost impossible and extremely hazardous. In such cases, the complete loss of the building is almost inevitable. Panel systems that prevent, or restrict delamination should be viewed as presenting less of a risk than those that do not. The majority of fires where sandwich panels have been identified as being the main mechanism in fire spread have concerned their use internally in the food industry (see 5.1 for typical causes). The dominant panel type, used to enclose food production areas has been metal faced expanded polystyrene. Recent unpublished investigations by BRE have indicated that poor levels of fire safety management were also a major influence. Fire spread via the external envelope is not so common and therefore does not have the same level of risk. Fire statistics confirm this and replacement of such panels is probably not justified. Some reasonable control on fire performance may be needed, particularly for higher risks. It may not be necessary to replace all external envelopes (claddings) installed in existing buildings provided there are other compensatory risk improvements. Sandwich panels systems used for external claddings typically have a polyurethane core and are supported inside the building by purlins and mid rails at frequent spacing. Polyurethane is a thermo-setting plastic which, whilst it burns, also forms a char layer. The end use requirements for external cladding panels make it important that they are assembled with weather-tight joints and this may also help to improve fire performance. In addition, the internal framework tends to act to some extent as a barrier to internal flame spread. External cladding may be the subject of an arson attack, so adequate precautions need to be taken to minimise this. External cladding may be involved later in the fire when the contents of the building have become fully involved in the fire. 2 2.1 Non combustible 2.2 Limited combustibility A material that when tested to BS 476: Part 4:1970, non-combustibility test for materials, meets the requirements for non-combustibility. Materials of limited combustibility a any non-combustible material; b any material of density 300kg/m2 or more which, when tested to BS 476: Part 11:1982, method for assessing the heat emission from building products, does not flame and the rise in temperature on the furnace thermocouple is not more than 20°C; c any material with a non-combustible core at least 8mm thick having combustible facings (on one or both sides) not more than 0.5mm thick (where a flame spread rating is specified, these materials should also meet the appropriate test requirements); d any material of density less than 300kg/m2 which, when tested to BS 476: Part 11, does not flame for more than 10s and the rise in temperature on the centre (specimen) thermocouple is not more than 35°C and on the furnace thermocouple is not more than 25°C. 3 Technical Briefing Fire performance of sandwich panel systems 2 Definitions 3.1 Importance of risk assessment 3.2 Fire Safety Management In reaching a decision on the suitability of a sandwich panel system for a particular function or application, due regard should be taken of the likely chance of ignition as well as the nature of any inception risk, fire load in the building and the potential to involve the panels. Also the financial exposure needs to be considered. Experience from fires in food factories has indicated that fires start often because of poor standards of fire safety management. Whilst better performing sandwich panel systems will slow down or prevent the spread of fire, an adequate standard of fire safety management is still needed as discussed in the following section. As the risk gets higher, due to a combination of these factors, there will be a need to put more emphasis on non-combustible sandwich panels. Conversely, for low risk applications (eg secure, stand-alone cold stores) it may be possible to accept sandwich panels that do not meet the LPS 1181 requirements. This is illustrated in Appendix 1. The risk of combustible sandwich panels contributing to the spread of fire may be significantly reduced by the maintenance of a good standard of fire safety management. With respect to combustible sandwich panels, the following factors are of particular importance in assessing the hazard that the panel(s) represent: • Processes which are a potential fire hazard should be located well away from sandwich panels. • Combustible materials should not be stacked near to the surface of panels. Timber or plastic pallets should not be stacked close to combustible sandwich panels, a 10m break being widely recommended. • Forklift truck battery charging should be located well away from sandwich panels unless the sandwich panel system can be identified as having at least 60 minutes fire resistance. • Automatic fire suppression systems, appropriate for the process, should be fitted to all heating and cooking equipment. • Flues used to extract hot gases should not pass through combustible sandwich panels unless adequately protected. • As far as possible, services penetrations through sandwich panels should be avoided. If this is not possible, any gaps should be adequately fire stopped. • Electrical cables passing through sandwich panels should always be enclosed in a metal conduit. • Electrical equipment located near sandwich panels should be examined and tested at least annually. • Attaching items to sandwich panels should be avoided. Where this is not possible, care should be taken to ensure that the core is not left exposed or damaged. • The building should be sub-divided into a number of fire resisting compartments wherever practical. • The use of full sprinkler protection to the factory should be encouraged. • Unauthorised access to the external cladding should be prevented to reduce the possibility of an arson attack. 4 Technical Briefing Fire performance of sandwich panel systems 3 General 3.3 What are sandwich panels used for? 3.4 Which insulants are used in sandwich panels? Typical applications for sandwich panel systems are as follows: 3.3.1 External claddings These are used for single and occasionally multi-storey buildings. Polyurethane is the most common core insulation used for this application. Sandwich panels used externally have to withstand wind-loads and also be weather-tight. There are far fewer instances of external envelopes being the cause of severe fire spread compared to insulated internal envelopes used for example to enclose food processing areas in food factories. 3.3.2 Insulated internal envelopes and partitions These are used typically in food factories, cold stores, pharmaceutical industries, other temperature controlled envelopes and high tech clean rooms. Expanded Polystyrene (EPS) is by far the most common core insulation used for these applications. 3.3.3 Fire resisting compartment walls Masonry is not the only solution for compartment walls. Some sandwich panel systems can be used very successfully in this application. High-density rock-fibre mineral wool is commonly used for this application as it can easily provide panels with 90 minutes and up to 240 minutes fire resistance. It is important to ensure that the panel system provides adequate insulation to ensure that combustible materials in direct contact with the unexposed panel side will not ignite. (That is the purpose of the insulation requirement in the fire resistance test) It is also essential that the junction between the compartment wall and the building envelope be designed to prevent fire spread round the perimeter of the fire resisting compartment wall. These are presented in approximate order of frequency of use 3.4.1 External roof and wall applications • Polyurethane (PUR)1 • Polyisocyanurate (PIR)1 • LPCB approved Polyisocyanurate (PIR)(these can be regarded as having better quality and fire performance than the standard PIR)1 • Mineral wool (rock fibre) (MWRF) • Expanded polystyrene EPS, supplied in standard duty (SD) and high duty (HD)2 [Small historic usage, <0.5%, in architectural wall panels] • Mineral wool (glass fibre) (MWGF) (Occasional historic use) 3.4.2 Internal wall, partition and ceiling applications • Expanded polystyrene EPS, supplied in standard duty (SD) and high duty (HD)2,3 • Extruded polystyrene (XPS)4 (behaves similarly to EPS in fire conditions) • Polyurethane (PUR) • Polyisocyanurate (PIR) • Mineral wool (rock fibre) (MWRF) • Modified Phenolic foam (MPHEN) • Cellular glass insulation (CG) (Occasional historic use) 5 Technical Briefing Fire performance of sandwich panel systems 1 Together PUR and PIR account for at least 90% of external applications 2 For many years only FR (Flame Retardant) grades of EPS have been used. (Reaction to Fire Classification Euroclass E) 3 New EN Standards have resulted in a change in nomenclature - SD becomes EPS 70 and HD becomes EPS 100 4 XPS is only supplied in Flame Retardant grades (Reaction to Fire Classification Euroclass E) 3.5 What are the problems associated with sandwich panel systems? 3.6 Why are combustible products used in sandwich panels? Not all sandwich panels present an undue fire risk. Panel systems based on combustible cores vary significantly in their fire performance. Many of the thermal insulating products used in sandwich panel systems are combustible. (e.g. EPS,XPS, PUR, PIR) When openly exposed to a fire they will burn. The potential to ignite is dependant on the size of the ignition source/inception risk and also on the specific type of combustible core. EPS will tend to shrink away from a small ignition source but when exposed to a large heat source will burn and produce molten droplets which have the ability to re-ignite once the ignition source is removed if sufficient heat has been trapped in the panel (any combustible residuals in any material/product may continue to flame after removal of an ignition source for a period of time). PUR will burn before producing a char layer. LPCB approved PIR and Phenolic materials show less tendency to burn. Joint design and method of support can also have a significant effect on fire performance. Cores can become directly exposed to the fire if the facings become delaminated. This can be experienced particularly with self-supporting internal panels which are not secured and where early collapse is possible. Where the ignition source is sufficiently large, or where the contents of the building are already burning, some panel systems may make a significant contribution to the fire. This is particularly true for combustible cored sandwich panels used internally (not part of the external envelope) where the fire load represented by the relatively thick panels may in some cases be higher than the contents of the building. It is important to take into account the fire load represented by the sandwich panels, with allowance being made for how much of the material is likely to be consumed in a fire. Sandwich panel systems approved by LPCB to LPS 1181 will not make a significant contribution to a fire. Unless it can be proven otherwise, it should be assumed that sandwich panels contain a combustible insulation. LPCB approved sandwich panels should be clearly marked (as should all panels to ease identification). Some LPCB approved products are marked with an UV (ultra-violet) identification mark. Combustible insulants are often specified in preference to non-combustible materials when taking into account other properties meeting design needs. The use of roof and wall panels with polyurethane cores in the external envelope has quadrupled over the last ten years. The primary reasons are enhanced and consistent thermal performance, light weight (compared to rock-fibre mineral wool cores) build speed and installation cost. Building Regulations specify thermal insulation requirements. For the internal partitions of food processing factories expanded (or extruded) polystyrene has been specified for the past 20 years as the dominant core insulant in food factories. This is because of low cost, resistance to moisture, hygiene considerations and its light weight, aiding demountability and re-use. More discussion on this may be found in ABI Research Report Number 6, ‘Appraisal of the performance and cost effectiveness of using factory produced metal faced sandwich panels in building applications.’ 6 Technical Briefing Fire performance of sandwich panel systems 3.7 What is the difference between structurally supported external sandwich panel systems and internal panel systems? 3.8 Which types of sandwich panels should be avoided? 3.9 Do sandwich panels create problems when fighting a fire? Sandwich panels (typically with rigid polyurethane cores) used for external envelopes are used with the necessary supporting structure and are therefore less likely to become delaminated in a fire. Purlins and mid-rails may make some contribution in controlling flame spread across the surface. This can be confirmed by observations during numerous tests to LPS 1181. However, the fire performance of such panels is very dependent on the formulation and type of blowing agents used. Consequently, unless the products are certified by LPCB to LPS 1181, their actual fire performance cannot be quantified. Sandwich panels installed inside the building to enclose food processing areas (typically EPS), in the form of partitions, ceilings and wall linings, are often inadequately supported and have inadequate joints. This leads to premature delamination of the metal face, rapid combustion of the cores and premature collapse of the panel. In these circumstances fire fighting becomes at best ineffective (as well as dangerous). In the IACSC guide, some guidance is given to designers to improve joint and support conditions. The majority of food factories probably do not yet incorporate the latest improvements, but many of the recommendations can be installed after the initial construction. This is really dependent on the assessed risk level of the occupancy/occupancy type, and whether the panels are used for the external envelope or inside the building. Care is required when assessing panel specifications. Panels approved for external cladding should not be assumed to be approved for internal applications. Some approved manufacturers market both approved and non-approved sandwich panels. At least two manufacturers use the same reference for the approved panel as for the non-approved panel. Look for the addition of LPC/LPCB within the reference. If in doubt written proof should be obtained from the architect/contractor or panel manufacturer. ABI sponsored research has clearly shown that fire performance can be improved by better joint design and how the sandwich panels are supported to any framework. It is suggested that unless the risk is considered low, sandwich panels that have not been approved are best avoided, particularly in respect to internal applications involving hot food processes. Some improvement in fire performance can be obtained from EPS panels having mineral wool edges. Some manufacturers refer to these as fire stop panels. It is important that combustible materials are not stacked too close to the panel surface because of potential conduction of heat through the panel thickness if a fire occurs on one side, enabling fire to spread. Research has shown that it is more difficult to get good fire performance from sandwich panels having flat surfaces, which are supported from outside (or freestanding), such as those typically used in the food industry. There are concerns that combustible sandwich panels used inside the building (not part of the external fabric) may create an additional problem to fire fighters unless the building is fully sprinkler protected or is subdivided by fire resisting compartment walls. The Fire Services are in general less concerned where sandwich panels are used as external roof and wall claddings, securely fixed to the structural frame of the building, since these do not represent the same danger to life during fire-fighting. 7 Technical Briefing Fire performance of sandwich panel systems 3.10 Fire growth and post-flashover 3.11 What is LPS 1181? Sandwich panels approved by LPCB to LPS 1181 should not create a problem for fire fighting if adequately supported, as any burning is considerably reduced. However, for the more combustible materials, fire-fighting experience has shown that problems do occur, particularly for sandwich panels used internally in the food industry. It is not easy to detect or get to a fire burning within a combustible panel. In such circumstances, not only do the burning building contents have to be dealt with, but the sandwich panels as well. Consequently there is a greater possibility of significant fire damage, as the fire load is greater. In reality, the problems experienced by the fire brigade are as much due to lack of fire resisting compartmentation or fire suppression as the volatile nature of certain types of inadequately specified sandwich panel systems. From initiation to decay, the two main stages of a fire of particular relevance to sandwich panels are fire growth (pre-flashover) and fully developed (post-flashover). This is illustrated in Appendix 2. Fire growth has always been at the forefront of LPC and BRE research and the evolution of the LPS 1181 test addresses one of the main factors that could differentiate between a controllable incident and a total loss. Post flash-over in modern insulated buildings relates more to the developed stage of a fire and thus the requirement for fire resisting compartmentation. This is the basis for the LPS 1208 test. In the context of this document, LPS 1181 is a test to evaluate the performance of sandwich panel systems to assess their contribution to fire growth. Panels satisfying the requirements of LPS 1181 will not make a significant contribution to fire growth. Fire growth is explained in clause 2.2 of the LPC Design Guide for the Fire Protection of Buildings. This represents the earlier stage of a fire before the room or compartment is fully involved in the fire. Sandwich panels that have passed this test are currently given the grade B designation in the LPCB List of Approved Fire and Security Products and Services (grade B designation is explained in Section 4.4). The test comprises building an open-ended enclosure (approximately the size of a large domestic garage) from the sandwich panels with a timber crib located in one corner. It tests not only the panels, but also the jointing methods and supporting system as well. This gives a more precise evaluation of true fire performance than the small-scale reaction to fire tests used by regulators, such as the surface spread of flame and fire propagation tests. Recent developments have led to the standard being split into two parts. Part 1 deals with external claddings. Part 2 with internal enclosures and linings used typically in the food or cold store industry. BRE research has confirmed that if panels are supported from outside the enclosure (as has been the standard practice in the food industry), a worse fire performance is achieved. Consequently a clear differential between the two applications is necessary. In addition, because of the range of risks attributed to internal applications, a more intense heat exposure is specified for high risks such as cooking areas. The need to contain the fire is also important so, for higher risks in the food industry, fire resistance is also specified. (See LPS 1208). 8 Technical Briefing Fire performance of sandwich panel systems 3.12 What is LPS 1208? The following summarises the criteria in LPS 1181 used as a basis for determining performance: Flashover: There should be no flashover at the ceiling (based on a defined temperature limit not being exceeded). Internal Surface Flaming: There should be no sustained surface flaming beyond 1.5m from the perimeter of the crib in both horizontal directions (e.g. outside crib fire area). External Surface Flaming: There should be no flame spread at any location on the external surface of the test building. Concealed Burning/Damage: Should be within defined limits, based on charred/damaged area of core, see LPS 1181 for more information Burning Brands: There should be no fall of burning brands from the ceiling outside the vicinity of the crib fire area Stability: (Internal applications only)This will be satisfied if no part of the test building collapses during the fire test. This will be deemed to have occurred if the deformation exceeds 1/30 of the distance between the ceiling support centres. Delamination of the exposed skin from the core should not be judged under this criterion LPS 1208 is the standard for evaluating the fire resistance of an element of construction with specific application to compartment walls and floors. Fire resistance is related to the fully developed or post-flashover stage of a fire. This is the phase of the fire where all the contents in the room or in the vicinity of where the fire originates are fully involved and are burning. The basic objective is to maintain the structural integrity of the building and to prevent the spread of fire into other compartments. Furnaces are used to test elements of construction for fire resistance. Criteria used for panel assessment are integrity (no gaps that allow fire and smoke to get through) and insulation (combustible materials on the non-fire side cannot become ignited). LPS 1208 uses the methods of test described in BS 476:Part 20/22:1987 or EN equivalent. Test reports express the result in terms of integrity and insulation. An example of which is given below: Integrity: 240 minutes Insulation: 15 minutes* * The insulation value should be noted in particular as this is more indicative of real fire performance. Some sandwich panels may have only been evaluated to LPS 1208 and listed by LPCB because of their specific application in the building. This is particularly true for panels used in compartment walls. However, as most systems having fire resistance above 60 minutes are non-combustible, they therefore meet the requirements without the need for testing to LPS 1181.