The use of Structural Insulated Panel (SIP) systems in construction in the UK has increased over the last decade where they are used as principal loadbearing elements and mainly as internal and external walls.

This Information Paper summarises the results from an experimental programme funded by Communities and Local Government (CLG) to determine the performance of SIP systems exposed to a realistic fire scenario and provides some recommendations for designers, regulators, warranty providers, manufacturers and contractors. Four large-scale fire experiments were carried out on two storey structures incorporating SIP wall panels and a floor system comprising engineered floor joists. More detailed information from the project is available in a number of journal and conference papers[1-4].

INTRODUCTIONStructural Insulated Panels (SIPs) are prefabricated lightweight units that form the principal loadbearing components used predominantly in residential and light industrial buildings[5]. They are a sandwich construction consisting of two structural facings bonded to a lightweight insulating core. In the UK the two face layers are generally formed from Oriented Strand Board (OSB). The insulated core is formed from a polymer-based foam such as polyurethane (PUR), polyisocyanurate (PIR), expanded polystyrene (EPS) or extruded polystyrene (XPS). SIPs are used mainly as internal or external walls and occasionally as roofs or floors.

The use of SIPs in construction in the UK has been increasing in popularity over the last decade. The reasons for this include they are lightweight and strong, the prefabrication of panels results in reduced waste

onsite and increased speed of erection, they are easily adaptable and they provide good thermal efficiency and airtightness when compared with more traditional forms of construction.

As with all other forms of construction, SIPs must be tested to demonstrate their compliance with the requirements of the building regulations. Standard fire resistance tests[6] provide a good indication of the relative performance of elements of building construction subject to a specific scenario based on idealised loading and support conditions and a single thermal exposure corresponding to the standard fire curve.

Tom Lennon and Danny Hopkin

FIRE PERFORMANCE OF STRUCTURAL INSULATED PANEL SYSTEMS

INFORMATION PAPER IP 21/10

Large-scale fire tests of SIP system clad with masonry

2 FIRE PERFORMANCE OF STRUCTURAL INSULATED PANEL SYSTEMS – IP 21/10

However, such testing and related assessments provide little information on the performance of a building system formed of a number of interconnected building elements exposed to a realistic fire scenario. This fact was recognised and the fire performance of SIP buildings was identified as a priority for future research in a project funded by the Department for Communities and Local Government (CLG)[7] in 2008. Subsequently, BRE has completed a research project funded by CLG to undertake an experimental programme to determine the performance of SIP systems exposed to a realistic fire scenario. This Information Paper summarises the results from the experimental programme and provides some recommendations for designers, regulators, warranty providers, manufacturers and contractors.

EXPERIMENTAL PROGRAMMEThe research into the fire performance of SIP structures consisted of a series of laboratory tests on single panels and four large-scale fire tests on two-storey SIP structures incorporating engineered floor joists. The experimental studies were supported by numerical modelling.

Laboratory testingThe laboratory programme comprised a number of tests on single panels with EPS and PUR cores protected with Type A or Type F gypsum plasterboard[8]. The tests included load tests at ambient temperature, unloaded panels subjected to a thermal exposure corresponding to the standard fire curve[6] and panels subject to a combination of applied load and heating. In total 30 experiments were performed on single SIPs of overall dimensions 1200 × 1800 × 150 mm. All panels had two 15 mm-thick OSB skins with an insulated core of 120 mm. The results from the full experimental programme are summarised in Table 1.

Figure 1: Combustion of panel on removal from the furnace

Figure 2: Flush mounted and protruding electrical sockets in panel

3 FIRE PERFORMANCE OF STRUCTURAL INSULATED PANEL SYSTEMS – IP 21/10

Test no. Test reference Results and comments

Ultimate load tests at ambient temperature. Uniaxial compression to determine panel resistance

1 PUR L1 Ultimate load 331 kN. Failure due to sudden brittle crack propagating through the width and thickness of the OSB sheets. De-lamination close to crack site between OSB and insulation.

2 PUR L2 Ultimate load 293 kN. Failure due to sudden brittle crack propagating through the width and thickness of the OSB sheets. De-lamination close to crack site between OSB and insulation.

3 EPS L1 Ultimate load 647 kN due to presence of solid timber frame.

Heat transfer tests. 30 minutes standard fire exposure with no load, 15 mm Type F plasterboard

4 PUR 301 All tests completed without need to terminate. No indication of combustion behind plasterboard for duration of test. Mean temperatures at back of plasterboard were 303°C and 263°C for PUR and EPS respectively. Corresponding temperatures at the interface of the insulation with the back of the OSB on the fire side were 66°C and 84°C. Samples ignited when removed from the furnace once plasterboard removed and sufficient oxygen available for combustion (see Figure 1).

5 PUR 302

6 PUR 303

7 EPS 301

8 EPS 302

9 EPS 303

Heat transfer tests. 60 minutes standard fire exposure with no load, 30 mm Type F plasterboard

10 PUR 601 All tests completed without need to terminate. No indication of combustion behind plasterboard for duration of test. Mean temperatures at back of plasterboard were 139°C and 152°C for PUR and EPS respectively. Corresponding temperatures at the interface of the insulation with the back of the OSB on the fire side were 72°C and 60°C

11 PUR 602

12 PUR 603

13 EPS 601

14 EPS 602

15 EPS 603

Heat transfer tests. 30 minutes standard fire exposure with no load, 12.5 mm Type A plasterboard

16 EPS 301W Tests terminated after approximately 20 minutes due to combustion of the OSB skin. Typical tempera-tures behind plasterboard and at back of OSB were in excess of 300°C

17 EPS 302W

Heat transfer tests. 30 minutes standard fire exposure with no load, 15 mm Type F plasterboard fixed direct

18 PUR 301FD All tests completed without need to terminate. Temperatures at back of plasterboard were 103°C and 376°C for PUR and EPS respectively. Corresponding temperatures at the interface of the insulation with the back of the OSB on the fire side were 68°C and 70°C respectively19 EPS 301FD

Heat transfer tests. 30 minutes standard fire exposure with no load, 15 mm Type F plasterboard. Samples incorporated electrical sockets mounted either flush to plasterboard or protruding (Figure 2)

20 EPS 301P All tests completed without the need to terminate. Measured temperatures through depth of SIP panels very similar to previous test results on panels without service penetrations

21 EPS 302P

Combined heat and load (130 kN) tests. 30 minutes standard fire exposure, 15 mm Type F plasterboard

22 PUR 301HL No load-bearing failure occurred. Temperature distribution similar to heat transfer tests. No discernible difference between PUR and EPS variants

23 PUR 302HL

24 PUR 303HL

25 EPS 301HL

26 EPS 302HL

Combined heat and load (130 kN) tests. 60 minutes standard fire exposure, 30 mm Type F plasterboard

27 PUR 601HL No load-bearing failure occurred. Temperature distribution similar to heat transfer tests28 PUR 602HL

29 PUR 603HL

30 EPS 601HL

Notes:

• The fire exposure tests were carried out in accordance with BS 476-20[6]

• Tests 16 and 17: Standard wall board

• Tests 18 and 19: Lining screwed to OSB otherwise fixed via softwood battens

• Tests 20 and 21: Service penetrations (two double plug sockets).

Table 1: Summary of laboratory tests on single panels

4 FIRE PERFORMANCE OF STRUCTURAL INSULATED PANEL SYSTEMS – IP 21/10

Large-scale fire testsFour large-scale fire tests have been undertaken on SIP structures incorporating engineered floor joists and protected from the effects of fire by plasterboard linings to the ceilings and walls. The order and configuration of the tests is shown in Table 2. In each case the overall dimensions of the test compartment were the same, with a floor area of 4 × 3 m and a height from floor to ceiling of 2.4 m. The first floor loading was identical in each case with an imposed load of 0.75 kN/m² spread uniformly over the first floor. The second floor load was varied as in Table 2 to represent either a two- (0.75 kN/m²) or four-storey (2.25 kN/m²) building. Figure 3 shows two of the units in the test facility during the construction stage.

The 30 and 60 minute design solutions were effectively representing a two-storey house and a multi-occupancy apartment dwelling respectively with the appropriate load level in place to simulate realistic conditions. In all cases the fire design was the same and was designed to give an equivalent severity to a 60 minute exposure in a standard furnace test[6]. The results from the large-scale fire tests are summarised in Table 3. Figure 4 shows the fire loading in place before ignition.

Test Design fire resistance period (min)

Core material

2nd floor loading (kN/m²)

F1 60 EPS 2.25

F2 30 EPS 0.75

F3 60 PUR 2.25

F4 30 PUR 0.75

Table 2: Summary of large-scale fire tests

Figure 3: Two of the test compartments during construction of the external masonry cladding

Figure 4: Fire load within compartment prior to ignition

5 FIRE PERFORMANCE OF STRUCTURAL INSULATED PANEL SYSTEMS – IP 21/10

Figure 6: Damage to engineered floor joists. Test F2 Figure 7: Damage to PUR wall panels. Test F3

Figure 5 (above left and right): Limited damage to floor joists and party wall. Test F1

Batten

Residual plasterboard

F4 Maximum atmosphere temperature 1083°C after 49 minutes. Test terminated after approximately 50 minutes due to runaway deflection of the floor caused by combustion of the OSB web of the engineered floor joists once the integrity of the plasterboard lining the ceiling had been compromised (similar to Test F2). Peak temperatures in the floor void approximately 664°C with a corresponding maximum deflection of 120 mm. Although the temperature within the external wall panels continued to increase towards the end of the test, any combustion of the PUR insulation was quickly dealt with by the Fire Service. Temperatures within the core of the party wall remained low throughout the test. The inclusion of electrical sockets did not influence the temperature of the panels

F1 Maximum atmosphere temperature approximately 1075°C after 52 minutes. Test continued up to cooling phase. Peak temperatures in floor void approximately 200°C with a corresponding maximum deflection of approximately 10 mm. Floor joists, resilient bars and party wall remained intact (Figure 5). The polystyrene core material had melted away in localised areas within the external walls. The location of the most significant damage coincided with an unsealed hole used for erection purposes which allowed sufficient air into the system to maintain combustion during the latter stages of the fire

F2 Maximum atmosphere temperature approximately 1078°C after 43 minutes. Test terminated after approximately 50 minutes due to runaway deflection of the floor caused by combustion of the OSB web of the engineered floor joists (Figure 6) once the integrity of the plasterboard lining the ceiling had been compromised. Peak temperatures in the floor void approximately 900°C with a corresponding maximum deflection of 203 mm. Much of the insulation core in the walls had melted away at the end of the test. However, there was no indication of any integrity failure of the wall panels

F3 Maximum atmosphere temperature 1071°C after 51 minutes. Test continued up to cooling phase. Peak temperatures in floor void approximately 200°C with a corresponding maximum deflection of approximately 15 mm. Core temperatures largely unaffected by the fire for the duration of the test but continued to rise in the cooling phase. Some time after the initial fire had been extinguished localised combustion continued within the wall panels with the inner surface of the PUR involved. Although initially there was no evidence of any damage to the wall panels the post-test damage was significant (Figure 7). The inclusion of electrical sockets did not influence the temperature of the panels

Test Results and commentsTest Results and comments

Table 3: Summary of large-scale fire tests

6 FIRE PERFORMANCE OF STRUCTURAL INSULATED PANEL SYSTEMS – IP 21/10

(corresponding to the maximum temperature of the insulation) for both a 60 minute and a 30 minute exposure for a range of different OSB and plasterboard thicknesses (Figures 8 and 9) based on plasterboard fixed to the panels via 25 mm softwood battens. The information should be seen in the light of typical delamination temperatures for EPS (100°C) and PUR (200°C) insulation from the literature[9].

Similar graphs are presented based on the average temperature of the inner layer of OSB in terms of the residual strength of timber at elevated temperature as given in EN 1995-1-2[10] (Figures 10 and 11).

The use of battens for fixing the internal lining reduces heat transfer to the panel and provides a void for services. Where such a service void is present the incorporation of penetrations for electrical sockets will not compromise the integrity of the panel.

DISCUSSIONWithin the scope of the experimental work undertaken and supported by numerical modelling of the performance of SIP buildings in fire a number of important issues have been highlighted. The laboratory tests on individual panels have indicated that the specification of 15 or 30 mm Type F[8] plasterboard for applications where 30 or 60 minutes fire resistance respectively is required is sufficient to restrict the temperature rise within the insulated core to less than 100°C. At such temperatures the performance of the panel is independent of the type of insulation used.

In order to assist in the correct specification of plasterboard linings for specific applications a number of graphs have been developed that are based on the validated heat transfer model produced as part of the research project. The graphs provide predicted temperature data behind the inner layer of OSB

300

400

500

600

700

800

pose

d si

de O

SB

tem

pera

ture

(Deg

C)

(Type F) 8 mm OSB(Type F) 10 mm OSB(Type F) 15 mm OSB(Type A) 8 mm OSB(Type A) 10 mm OSB(Type A) 15 mm OSB

60 minutes BS 476-20 exposure

0

100

200

12 17 22 27 32

Rea

r of e

xp

Plasterboard thickness (mm)

Figure 9: Temperature of rear of inner layer of OSB for 60 minute fire exposure

300

400

500

600

700

800

front

OS

B te

mpe

ratu

re (D

egC

)

(Type F) 8 mm OSB(Type F) 10 mm OSB(Type F) 15 mm OSB(Type A) 8 mm OSB(Type A) 10 mm OSB(Type A) 15 mm OSB

0% of compressive strength

60 minutes BS 476-20 exposure

0

100

200

12 17 22 27 32

Mea

n f

Plasterboard thickness (mm)

100% of compressive strength

25% of compressive strength

100

150

200

250

pose

d si

de O

SB

tem

pera

ture

(Deg

C)

(Type F) 8 mm OSB(Type F) 10 mm OSB(Type F) 15 mm OSB(Type A) 8 mm OSB(Type A) 10 mm OSB(Type A) 15 mm OSB

0

50

12 17 22 27 32

Rea

r of e

xp

Plasterboard thickness (mm)

30 minutes BS 476-20 exposure

Figure 8: Temperature of rear of inner layer of OSB for 30 minute fire exposure

100

150

200

250

300

350

front

OS

B te

mpe

ratu

re (D

egC

)

(Type F) 8 mm OSB(Type F) 10 mm OSB(Type F) 15 mm OSB(Type A) 8 mm OSB(Type A) 10 mm OSB(Type A) 15 mm OSB

25% of compressive t th

0% of compressive strength

30 minutes BS 476-20 exposure

0

50

100

12 17 22 27 32

Mea

n f

Plasterboard thickness (mm)

100% of compressive strength

strength

Figure 10: Temperature of mean inner layer of OSB for 30 minute fire exposure

Figure 11: Temperature of mean inner layer of OSB for 60 minute fire exposure

7 PAGE HEADER RIGHT – PAGE HEADER LEAFLET NUMBER

The results and observations from the large-scale fire tests have confirmed the ability of a structure incorporating SIP wall panels and engineered floor joists to survive a real fire scenario with an equivalent severity of 60 minutes exposure to the standard fire curve. As with other forms of construction, the performance of the structure is very much dependent on the correct specification and installation of the internal linings. The mode of failure of such a structure has been shown to be runaway deflection of the floor plate due to ignition and rapid combustion of the web member of the engineered floor joists. The rate of deflection increases very rapidly as the floor system approaches collapse. Such a scenario is not influenced by the SIP system and would be the same for other panelised, framed or traditional masonry construction systems.

RECOMMENDATIONSAs mentioned above, the correct specification and installation of the internal linings to both the ceiling and the floor are critical to the performance of the system in a real fire situation. Based on the results and observations of both the tests on the individual panels and the large-scale natural fire tests, supported by numerical studies, recommendations for achieving design fire resistance periods of 30 minutes and 60 minutes are given in Table 4.

The graphs in Figures 8 to 11 provide information on the relationship between panel temperature and specification of OSB face layers and plasterboard linings and can be

used by manufacturers to select a suitable combination to achieve the required performance. Installation should comply with the instructions and detailed guidance produced by the plasterboard supplier. Compliance with recommendations for minimum lengths of fixings and minimum centres between fixings is particularly important.

REFERENCES [1] Lennon T, Hopkin D, El-Rimawi J and Silberschmidt V. Large scale natural fire tests on protected engineered timber floor systems, Fire Safety Journal 2010. [2] Hopkin D, Lennon T, Silberschmidt V and El-Rimawi J, A. laboratory study into the fire resistance performance of structural insulated panels (SIPs). Proceedings of 6th International Conference on Structures in Fire, Michigan 2–4 June, 2010. [3] Hopkin D, Lennon T, Silberschmidt V and El-Rimawai J. Full scale fire tests on structural insulated panel and engineered floor joist assemblies. Proceedings of 6th International Conference on Structures in Fire, Michigan 2–4 June 2010. [4] Hopkin D, El-Rimawi J, Silberschmidt V and Lennon T. Modelling the fire resistance of structural insulated panels: heat transfer. Proceedings of 12th International Conference on Fire Science and Engineering (INTERFLAM), 5–7 July 2010. [5] Bregulla J and Enjily V. An introduction to building with Structural Insulated Panels (SIPs), BRE Information Paper IP 13/04, BRE, Watford, 2004. [6] BSI. BS 476-20: 1987. Fire tests on building materials and structures – Part 20: Method for the determination of the fire resistance of elements of construction (general principles), BSI, London, 1987. [7] Communities and Local Government. Innovative Construction Products and Techniques BD 2503, Communities and Local Government, London, January 2008. [8] BSI. BS EN 520: 2004, Gypsum plasterboards – Definitions, requirements and test methods, BSI, London, 2004. [9] Ashby MF and Gibson LJ. Cellular solids: structure and properties. Cambridge University Press. Second edn 1997.[10] BSI. BS EN 1995-1-2: 2004, Eurocode 5: Design of timber structures – Part 1-2: General – Structural fire design, BSI, London, 2004.

FURTHER INFORMATIONA full report on the test programme carried out by BRE will be published by CLG in due course.

Further information on SIP technology is available at www.uksips.org.

ACKNOWLEDGEMENTSThe information presented in publication is based on work carried out by BRE under a contract placed by CLG. The views expressed are not necessarily those of CLG. The active participation of the UK SIP Association is gratefully acknowledged as is the significant contribution to the experimental programme made by Lafarge Plasterboard.

FIRE PERFORMANCE OF STRUCTURAL INSULATED PANEL SYSTEMS – IP 21/10

Recommended specification for fire resistance period of:

30 minutes 60 minutes

Wall lining • 15 mm Type F plasterboard fixed to softwood battens

• All joints taped and sealed

• 30 mm Type F plasterboard fixed to softwood battens

• All joints between layers staggered

• Exposed joints taped and sealed

Ceiling lining

• 15 mm Type F plasterboard fixed to resilient bars

• All joints taped and sealed

• 30 mm Type F plasterboard fixed to resilient bars

• All joints between layers staggered

• Exposed joints taped and sealed

Services • Incorporated within service void formed by battens

• Incorporated within service void formed by battens

Penetrations in SIP panels

• All penetrations to be adequately fire stopped

• Lifting holes to be sealed* following erection of panels

• All penetrations to be adequately fire stopped

• Lifting holes to be sealed* following erection of panels

Table 4: Specification for specific periods of fireresistance

*Material used to seal the hole should, as a minimum, provide the same level of protection as the material removed.

8 FIRE PERFORMANCE OF STRUCTURAL INSULATED PANEL SYSTEMS – IP 21/10

BRE is the UK’s leading centre of expertise on the built environment, construction, energy use in buildings, fire prevention and control, and risk management. BRE is a part of the BRE Group, a world leading research, consultancy, training, testing and certification organisation, delivering sustainability and innovation across the built environment and beyond. The BRE Group is wholly owned by the BRE Trust, a registered charity aiming to advance knowledge, innovation and communication in all matters concerning the built environment for the benefit of all. All BRE Group profits are passed to the BRE Trust to promote its charitable objectives.BRE is committed to providing impartial and authoritative information on all aspects of the built environment. We make every effort to ensure the accuracy and quality of information and guidance when it is published. However, we can take no responsibility for the subsequent use of this information, nor for any errors or omissions it may contain.BRE, Garston, Watford WD25 9XX Tel: 01923 664000, Email: enquiries@bre.co.uk, www.bre.co.uk

Information Papers summarise recent BRE research findings, and give advice on how to apply this information in practice. Digests, Information Papers, Good Building Guides and Good Repair Guides are available on subscription in hard copy and online through BRE Connect. For more details call 01344 328038. BRE publications are available from www.brebookshop.com, orIHS BRE Press, Willoughby Road, Bracknell RG12 8FB Tel: 01344 328038, Fax: 01344 328005, Email: brepress@ihs.comRequests to copy any part of this publication should be made to:IHS BRE Press, Garston, Watford WD25 9XX Tel: 01923 664761 Email: brepress@ihs.com www.brebookshop.com

IP 21/10© BRE 2010

December 2010ISBN 978-1-84806-161-3

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