11 Modern Methods of Construction

11.2 Closed Panel Systems

This chapter gives guidance on meeting the Technical Requirements for closed panel systems used to form homes where the height of the top floor above ground does not exceed 18m.

Introduction

This chapter gives guidance on meeting the Technical Requirements for closed panel systems used to form homes where the height of the top floor above ground does not exceed 18m. For the purposes of this chapter, closed panel systems shall include:

timber framed wall and floor panels
light gauge steel framed wall and floor panels
Structural Insulated Panel (SIPs) walls.

The guidance in this chapter does not apply to roof cassette systems or pre-manufactured panels formed from mass engineered timber, hot rolled steel or precast concrete. It also does not apply to use of the systems below ground level.

This chapter should be used in conjunction with Chapter 11.1 MMC systems: general requirements. This chapter seeks to focus on areas of guidance that are unique to the system type and method of construction employed, giving references to complimentary existing guidance provided elsewhere in the Standards where relevant.

There is a strong link to all chapters in Part 6 of the Standards, particularly Chapters 6.2, External time framed walls and 6.10, Light steel framed walls and floors.

The term ‘Modern Methods of Construction (MMC)’ is very broad and covers a wide range of differing offsite manufactured systems and onsite construction techniques.

MMC types have been categorised and defined in the MHCLG Joint Working Group ‘MMC Definition Framework’, see Chapter 11.1 MMC systems: general requirements. The scope of the guidance in this chapter will cover ‘closed’ panel systems that generally fall under Category 2 under the MMC Definition Framework and as defined below.

Definitions of Open and Closed panel

MMC Category 2 light gauge steel and timber frame building systems are typically constructed from panels manufactured offsite under factory-controlled conditions. The extent of prefabrication can vary, with panels being as simple as studs and sheathing, or include external cladding, internal linings, windows and services. Where building systems utilise high levels of offsite prefabrication, traditional site inspection regimes for installation of critical materials, junctions and components are not possible. In addition, the greater the level of factory prefabrication, the greater the potential risks associated with exposure to moisture during construction.

Whilst the extent of offsite prefabrication can vary significantly, light gauge steel and timber frame building systems are typically categorised by NHBC as either ‘open panel’ or ‘closed panel’. For the purpose of Warranty provision, NHBC’s definitions are set out below.

Open Panel

Flat two-dimensional panelised units fabricated offsite in a factory environment used to form wall and floor structures.

Open panel assemblies typically include:

a structural frame of studs and joists
floor decking and sheathing boards as required.

Additionally, open panel assemblies may also include:

a fixed breather membrane to external walls
insulation (between the studs only)
a transparent air and vapour control layer (AVCL) in order that the construction/insulation can be viewed on site (note – if a non-transparent air and vapour control layer is used, the system will be considered by NHBC as being closed panel)
battens forming a service zone
temporary weather protection.

Closed Panel

Closed panel systems are also defined as flat two-dimensional panelised units fabricated offsite in a factory environment used to form wall and floor structures, but in addition to the items set out above for open panel systems, where any one of the following items are factory fitted NHBC will consider the system to be closed panel:

non-transparent air and vapour control layer
sheathing boards to both sides of panels
internal linings (such as plasterboard)
continuous insulation installed to either the inner or outer face of the panels
external cladding or finishes
windows and doors
fixed services such as plumbing pipes and electrical cables
cavity barriers and/or fire stopping.

Guidance for traditional open panel construction formed from timber or light steel frame is given in Chapter 6.2 External timber framed walls and 6.10 Light steel framed walls and floors.

Definitions for this chapter

Cavity	A space enclosed by elements of a building such as the space between the cladding system and the backing wall. The external wall cavity should be adequately drained, and ventilated where required.
Cavity barrier	A construction within a cavity, other than a smoke curtain, to perform either of the following functions: • close a cavity to stop smoke or flame entering • restrict the movement of smoke or flame within a cavity.
Closed panel	Flat two-dimensional panelised units fabricated offsite in a factory environment used to form wall and floor structures, including where any one of the following items are factory fitted: • non-transparent air and vapour control layer • sheathing boards to both sides of panels • internal linings (such as plasterboard) • continuous insulation installed to either the inner or outer face of the panels • external cladding or finishes • windows and doors • fixed services such as plumbing pipes and electrical cables • cavity barriers.
Factory installed	An element of the construction that is placed, positioned, fitted, or secured in, on or as part of the unit as part of the factory construction process.
Fire-stopping	A seal provided to close an imperfection of fit or design tolerance between elements or components, to restrict the spread of fire and smoke.
Floor cassette	Flat two-dimensional panelised units fabricated offsite in a factory environment used to form a load-bearing element of floor construction. Typically includes a series of floor joists joined together with trimmers or end-joists and floor decking.
Installation Manual	Detailed installation guidance and information for the system compiled by the system manufacturer. The Installation Manual describes general instructions for storage and transport of the system, and procedures for on-site assembly and installation.
Interface	Interface either between similar or dissimilar envelope systems (eg at panel joints) or between envelope systems and other elements of the building (eg walls, roof, doors, and windows).
Junction	The point where components, including panels are joined with each other or with other elements of the building (eg supporting structure, roofs and foundations).
Manufacturer	The company which is responsible for the design and assembly of the closed panel system.
Non-standard components	Components that are assembled as part of the system but vary for individual projects. These may be windows, doors or services that may be built into the system.
Primary structural components	Elements of the structure designed to carry and transfer primary loads of the building, including self-weight, dead loads, and live loads.
Secondary structural components	Elements of the structure which do not play a wider role in the structure, but carry loads directly imposed on them (and transfer them to the primary structure) such as self-weight, wind loads, cladding and openings.
Standard components	Components that are assembled as part of the system and are common to all projects.
System	For the purposes of this chapter, this term refers to acceptable forms of closed panel systems.
System Manual	Detailed technical information on the system compiled by the system manufacturer. The System Manual describes the system, the declared performance of the system and evidence to support the declared performance.
Unit	A prefabricated building unit, manufactured offsite as a structural or non-structural assembly in three-dimensional or flat panel format used as part of a system to form the superstructure of a building.

Clauses for 11.2 Closed Panel Systems

Last updated: 2nd January 2025