SIP Stacking Specificity
Structural Insulated Panels (SIPs) consist of an insulating foam core, which is frequently but not exclusively a Polyurethane (PUR), fabricated in a factory between two structural facing boards, which are very often Oriented Strand Boards (OSB), though other board products are sometimes used for specific reasons. SIPs of OSB with a PUR core were manufactured by SIP Building Systems Ltd (SBS) for the Moorfield Hotel, Brae, Shetland.
The SBS Structural Insulated Panels were fabricated into more than 100 room sized volumetric "Modules", fully finished, fitted and furnished. Quality checked to a high Standard they were shipped from Warrenpoint Harbour, County Down, Northern Ireland in an impressive logistical operation by McAleer and Rushe, to be stacked on the prepared Hotel site from March 2013. Before the weather worsened the external envelope was completed while Building Services were connected and commissioned. This is explained well in a You Tube video by McAleer and Rushe, promoted on the SBS website.
The Hotel designers were ICA Architects, though perhaps not for the construction.
On 27 July 2020 the Moorfield Hotel burned to the ground from one end to the other, but thankfully with no loss of life or injury. The Modular Hotel had been built to serve the Oil and Gas Industry, but two months earlier block bookings had been cancelled with Shetland's industrial economy in decline. There is an investigation into the cause of the fire.
However, there is something strange about the low-rise Modular construction.
Why was a Modular building constructed of Structural Insulated Panels?
SBS has a BBA Certificate BBA Certificate 06/4312. That was reissued on 6 September 2013 and amended on 18 December 2013. The original was issued on 20 April 2006. Surely the BBA Certificate available online should be Issue 02. Not Issue 01.
To know exactly what the BBA had Certified for SBS in 2006 it would be necessary to go back to that original, which is presumably the Certificate McAleer and Rushe had referred to when manufacturing the Modules. However it would be amazing if BBA Certificate 06/4312 of 20 April 2006 was for Modular rather than Panel construction. SIPs are usually Panel Systems to be assembled on site as external and internal walls and roofs. Floors are often Engineered Softwood Web Joists with Decking. That structure is then finished on site. Just as the SBS BBA Certificate 06/4312 of 6 September 2013 shows.
According to the Structural Timber Association there are two ways to build with SIPs as either non-loadbearing "Infill" within a Primary Structural Frame of Concrete, Steel or Timber, or as a loadbearing "Wrap" of Panels supporting floors and roof. The SBS BBA Certificate is talking about a loadbearing Panel System. The STA say that "... currently five storeys are the maximum practical height for loadbearing SIP buildings. Above this, an additional structure needs to be incorporated into the building to carry the imposed load."
Of course since the Building (Amendment) Regulations 2018, amending Regulation 7 of the 2010 Building Regulations, the use of combustible materials like OSB and PUR in an external wall made from a SIPs Panel is no longer acceptable in high-rise residential. The internal walls may be SIPs. For all buildings not included in Regulation 7(4) SIPs may be used at any height, which includes any residential building with the top storey below 18m above Ground. The Moorfield Hotel would be unaffected by Regulation 7 (2018).
The structure requires Stability in a Fire. As 10.1 of the current BBA Certificate 06/4312 says, when loaded and tested to BS 476: Part 21 the loadbearing Panel System achieved a 30 Minute and 60 Minute structural Fire Resistance. There appears no mention of "Infill". That is a singular SIP thickness in a loadbearing Panel System, not as a pair of SIPs across a horizontal Module-to-Module joint in a Modular stack. As 10.2 further explains:
"Assessment of test results and design details shows that panels are suitable for use in external walls, not less than one metre from a relevant boundary, and in separating walls that require fire resistance periods not less than:
- external walls 30 or 60 minutes (from inside)
- separating walls 60 minutes (from either side)."
30 Minutes is a ‘Short’ duration, while 60 Minutes is a ‘Medium’ duration in Scotland. Not enough for a high-rise building, of course, but this was a low-rise residential development with good Escape. But the BBA also point out that the single SIP forming the external wall of the Panel System has a structural Fire Resistance from INSIDE-OUT only, while the single SIP forming the separating walls of the Panel System has a structural Fire Resistance from BOTH SIDES. Just keep that single SIP in mind when thinking about Modules.
When Modules are made to be lifted and stacked as rooms the construction forming the volumes necessarily results in Cavities between Modules because they are paired.
Some SIPs manufacturers have BBA Certificates with "Party Walls" between Domestic dwellings as a pair of SIPs with a Cavity between. You could imagine a terrace of "Party Walls" with a Cavity in each pair of SIPs. But it is going further to take that Cavity construction and repeat it between every room in a building like an Hotel.
SIPs used to make Modules form pairs in the separating walls between rooms, with vertical Cavities. The top of the lower Module forms a pair with the floor of the upper Module, with horizontal Cavities. Additional horizontal Cavities occur at the concrete base structure to Ground, and at the Roof. There are also vertical Cavities between Modules that might contain sections of Escape Corridors with the site built Stair and Lift Core. Then the vertical Building Services Risers off the corridors penetrate the horizontal cavities. Parts of Risers or Corridor walls will be in different modules, with joints between.
These vertical and horizontal Cavities in a Modular stack would be a three dimensional matrix of extensive Cavities if they were not Fire Stopped to create Compartmentation.
That is different to a Panel System building in which the SIPs are singular, not paired.
In 10.3 of the BBA Certificate it is stated that "... the OSB/3 panel linings have a Class 3 surface spread of flame designation." Class 3 to BS 476: Part 7 is a ‘High Risk’ lining for spread of flame in Scotland. Within the Modules the rooms are lined with Plasterboard to reduce the spread of flame. But no such lining is applied to the combustible OSB in the Cavities between Modules made of SIPs.
Moreover, the Modules for Moorfield Hotel were wrapped in a plastic "Marine" protection when shipped, which also had to survive several months of Shetland weather on site before the external envelope was complete. That protection is retained in the Cavity as the Modules are stacked and closed against the rain. Water down Cavities would destroy the finishes, fittings and furnishings. The OSB and the temporary weather protection membrane means the Cavity contains a fuel load that is unprotected by any lining of Plasterboard. In a Panel System using single SIPs that is not the case.
BBA Certificate 10.4 says "... junctions between the panels in external and separating walls will adequately maintain the fire resistance of the separating wall". But there are many different kinds of joints with Cavities in a Modular stack in which SIPs are paired.
Referencing the Scottish Technical Handbook the BBA Certificate continues to make specific observations, for a Panel System. But the BBA are looking at the "Domestic" Technical Handbook in 10.5., when it is stated:
"The panels can form part of a separating wall between dwellings in Scotland in accordance with the exceptions permitted by Mandatory Standard 2.2, with reference to clause 2.2.7."
BBA Certificate 06/4312 is expressly limited to "... structural insulated panels for use above the damp-proof course in domestic applications up to two storeys high (plus room-in-roof) as the loadbearing inner leaf of an external cavity wall or as part of separating walls, internal loadbearing walls and pitched roofs." The Moorfield Hotel project was outside the scope of BBA Certificate 06/4312 in 2013.
The Non-Domestic Technical Handbook should be referred to for the Moorfield Hotel development. Fortunately the Mandatory Standards have the same numbering. The archive of Scottish Technical Handbooks is here:
The Modular System was factory made and ready for shipping to site for stacking from March 2013. So the 2011 Non-Domestic Technical Handbook applies. Although the BBA Certificate was updated in 2013 and could refer to the 2013 Technical Handbook. The Mandatory Standards were unchanged between 2011 and 2013, at least for Section 2. Fire:
Mandatory Functional Standard 2.2 - Separation
“Every building, which is divided into more than one area of different occupation, must be designed and constructed in such a way that in the event of an outbreak of fire within the building, fire and smoke are inhibited from spreading beyond the area of occupation where the fire originated.”
The BBA rather oddly reference 2.2.7 - Junctions in 10.5, while seeming to disregard the prior discussion of combustibility in the Non-Domestic Handbook:
2.2.4 - Combustibility
"Exclusions from non-combustibility - every part of a separating wall or separating floor (other than a floor finish such as laminate flooring) should be of materials that are non-combustible. However this is not necessary where the building does not have any storey at a height of more than 18m and the separation is between:
- a shop or office and a dwelling above the shop or office in the same occupation where there is no other dwelling above the shop or office, and the area of the shop or office is not more than 1½ times the area of the separating floor or
- units of shared residential accommodation or
- a dwelling and a unit of shared residential accommodation.
To reduce the risk of a fire starting within a combustible separating wall or a fire spreading rapidly on or within the wall construction:
- insulation material exposed in a cavity should be of low risk or non-combustible materials (see annex 2.E) and
- the internal wall lining should be constructed from material which is low risk or non-combustible and
- the wall should contain no pipes, wires or other services."
All very sensible, but not discussed by BBA Certificate 06/4312. Neither does the BBA point to Annex 2.E Reaction to Fire, which in Scotland has a "Low Risk" definition:
British Standards
"The surface material (or where it is bonded throughout to a substrate, the surface material combined with the substrate) has a surface of Class 1 and, when tested in accordance with BS 476: Part 6: 1981 or BS 476: Part 6: 1989 has an index of performance (I) not more than 12 and a sub-index (i1) not more than 6."
That, of course, was the Paragraph 13b. definition of Class 0 in Approved Document Part B Volume 2 (2013) for England and Wales:
The alternative in Scotland to testing to BS 476: Parts 6 and 7 is explicitly given in Annex 2.E Reaction to Fire as:
European Standards
"The material has achieved a classification of B-s3, d2 or better when tested in accordance with BS EN: 13823: 2002 and BS EN ISO: 11925-2: 2002."
It is not possible to transpose National Class 0 (Using criteria in BS 476: Parts 6 and 7 testing) to European Class B-s3, d2 (Using tests in BS 13501-1) but even in Scotland in 2013 that was being done. In England and Wales that transposition was done through Diagram 40 of Approved Document Part B Volume 2 (2013) until Grenfell Tower fire. But those classifications are certainly not achieved by more combustible OSB and weather protection membrane in the Cavity construction of a Modular System.
Beyond the Reaction to Fire of materials, the BBA sensibly concentrate on:
2.2.7 Junctions
“Where a separating wall or separating floor meets an external wall, another separating wall, a compartment wall or any other wall or screen used to protect routes of escape, ... the junction should maintain the fire resistance duration of the separating wall or separating floor. Where a separating wall forms a junction with a roof or a fire resisting ceiling (see clause 2.1.16), the junction should be constructed in accordance with the recommendations in clause 2.1.15.”
This turns into a longer reading of the Non-Domestic Technical Handbook , which stresses the importance of Compartmentation in the protection of Escape Routes:
Mandatory Functional Standard 2.1 - Compartmentation
“Every building must be designed and constructed in such a way that in the event of an outbreak of fire within the building, fire and smoke are inhibited from spreading beyond the compartment of origin until any occupants have had the time to leave that compartment and any fire containment measures have been initiated.”
2.1.15 Junctions
“The basic principle is that junctions between compartment walls and compartment floors (including fire resisting ceilings, see clause 2.1.16) and other parts of the building should be designed and constructed to prevent a fire in one compartment flanking the wall, floor or ceiling and entering another compartment at the junctions including any solum space or roof space”
2.1.16 Fire resisting ceilings
"Where a fire resisting ceiling, including a suspended ceiling, contributes to the fire resistance duration of a compartment floor or is part of a construction as described in clause 2.1.15, the ceiling should be constructed in accordance with the following recommendations:
- the ceiling should not be easily demountable;
- openings and service penetrations in the ceiling should be protected in accordance with clause 2.1.14;
- junctions with the ceiling should follow the guidance in clause 2.1.15;
- the ceiling lining should follow the guidance to standard 2.5;
- the ceiling may contain an access hatch which, when closed, will maintain the fire resistance duration of the ceiling.
However the guidance in this clause should not be applied to fire resisting ceilings protecting the load-bearing capacity of a floor only. In such cases, the compartment wall should be taken through the ceiling and floor to form a continuous and complete barrier to fire between the relevant compartments.”
2.1.14 Openings and service penetrations
“Compartment walls and compartment floors (including a fire resisting ceiling) are intended to prevent fire passing from one compartment to another. Openings and service penetrations through these walls or floors can compromise their effectiveness and should be kept to a minimum. The solum and roof space should not be forgotten. Openings and service penetrations should be carefully detailed and constructed to resist fire.”
Mandatory Functional Standard 2.5 - Internal linings
“Every building must be designed and constructed in such a way that in the event of an outbreak of fire within the building, the development of fire and smoke from the surfaces of walls and ceilings within the area of origin is inhibited.”
The Modular System made from SIPs at Moorfield Hotel will have to be looked at closely to see how the Cavity construction was built. That will not be easy as it burned to the ground from one end to the other. The significance of Fire Stopping to limit the extent of Cavities which contain a fire load of combustible materials, may be underestimated. But even for a single loadbearing Panel System, the BBA could state in 10.6:
"Constructions incorporating the wall and roof panels must include suitable provision for cavity barriers and for fire stopping at junctions with other elements. The maximum distance between cavity barriers, both vertically and horizontally must be accordance with the Mandatory Standard 2.4."
Mandatory Functional Standard 2.4 - Cavities
“Every building must be designed and constructed in such a way that in the event of an outbreak of fire within the building, the unseen spread of fire and smoke within concealed spaces in its structure and fabric is inhibited.”
Potentially extensive internal Cavity construction requires a lot of attention to linear Fire Stopping. Fire Resistance against fire and smoke is required to last longer than the Cavity Barriers that would be required in a Rainscreen System, on Compartment lines or around Windows and Doors. In a Modular System this Fire Stopping has to be installed while the Module is suspended from the crane. It has to be installed into gaps that have Movements and Tolerances. Fire Engineered Fire Stopping requires a Service Life equal to the Design Life of the building. Access for maintenance or replacement is impossible. Cavity Fire Stopping becomes critical, even in low-rise development:
2.4.0 Introduction
“Fire and smoke spread in concealed spaces is particularly hazardous because fire can spread quickly throughout a building and remain undetected by the occupants of the building or by fire and rescue service personnel. Ventilated cavities generally promote more rapid fire spread around the building than unventilated cavities due to the plentiful supply of replacement air. Buildings containing sleeping accommodation pose an even greater risk to life safety and demand a higher level of fire precautions. For these reasons, it is important to control the size of cavities and the type of material in the cavity.”
The BBA make the correct point in 10.7, when they caution:
"... where any other form of wall construction incorporating the panels (including any service penetrations) is subject to fire-resistance requirements, an appropriate assessment or test must be carried out by a UKAS (United Kingdom Accreditation Service) approved testing laboratory."
Service Penetrations can breach the Plasterboard and OSB linings from inside the room. The combustible Polyurethane insulation could be exposed. For Cable distribution and Electrical Socket Back Boxes the Plasterboard linings are often fixed on battens with an internal Cavity to the OSB. These lining Cavities must have limited extent. For penetrating Services reinstatement of the Fire Resistance must be achieved using tested Passive Fire Stopping products. If the Services penetrate through into the extensive Cavity and there is inadequate Fire Stopping at Compartment lines, rapid fire spread could ensue. In 10.8 the BBA note:
"Alternative build ups of plasterboard can achieve higher fire performance levels but have not been assessed by the BBA. Technical advice should be sought from plasterboard manufacturers."
But the BBA were only talking about enhancing a single Panel System based on SIPs using Plasterboard. Not a Modular System made from SIPs that are paired at Cavities.
Moorfield Hotel fire could prove to be very significant, if the issues with Cavity construction are going to be better understood. The first lesson to learn is not to rely on a BBA Certificate for a specific Panel System when making a different Modular System.
An innovative "Modern Method of Construction" might need more Fire Engineering attention in specific Cavities created for practical reasons.
Maybe Moorfield Hotel had no structural warranty. The emerging structural warranty providers for MMC will take notice, as Modular is more widely promoted by Central and Local Government in urban areas, and increasingly for high-rise.
There may even be an investigation by CIRIA, looking hard at the Service Life of critical Fire Stopping in inaccessible Cavities within Modular Systems, since they are involved with the Buildoffsite Property Assurance Scheme for the Durability of MMC. That investigation might be expected by BLP Insurance and Lloyds Register who underwrite BOPAS.
Photograph by Georgia Smith, Shetland News
Photograph by Shetland Fire and Rescue Service
Technical Designer
4yDoes anyone know if the Scottish Fire and Rescue Service have finished their investigation? I have tried looking for a Report but to no avail to date.
Façade Cavity Protection System designer retrospectively fitted protection for existing Façade provisions. THE ONE SOLUTION TO THE CLADDING CRISIS. Smoke Control Consultant Surveyor Engineered solutions
4yCavities are generally the route for Secret Smoke and heat transfers making protection of any penetrations of the utmost importance. Our "Twin Therm Closers" are A1 fire rated by design and composition 0779 2628510 0r paul@ashfire.co.uk for details. Easily retrospectively fitted or built in on or offsite constructions.
Retired former building inspector. Proud dad to Chris and Dan Walling of CGM Brickwork. 2nd Dan Black Belt in Wado Ryu Karate. Rugby Union fan. Motorcyclist with a passion for 1950s and’60s British bikes.
4yI'd like to know how this happened and if there was a latent design fault with the modular system. Unfortunately design issues have been found on some systems and due to the nature of "pod" building they are not often easily picked up on site.
I worked out of an office in one of these “Modular” structures for fourteen years at the fire service college. We often used “the bouncy castle”, as we called it, as an exercise for our students in fire safety inspection procedures because the structures were full of gaps and voids. We used to joke that we would be able to re-use the fire doors after a fire as we reckoned the walls and floors were probably good for about ten minutes FR, maybe twenty and the doors were all decent 30 minute doors ... set in ordinary frames.
Retired (well almost)
4yIan I make a professional assumption that these modular units have been subject to fire testing including all aspects of fire integrity. If the tests concluded that the system met the standards, there are only 2 possible failures I would suggest. Either the build was sub standard, or the testing was not adequate. Thoughts please. #fire #construction #standards #whataboutsprinklers #keepthemsafeoutthere