2D Panelised Systems

Panelised systems are not one thing; they run their own sub-spectrum from 'open' to 'closed', and naming where a panel sits is the first skill here. An open panel is a structural frame only — a timber stud or light-gauge-steel frame, perhaps with sheathing board on one face — made flat in the factory and delivered as a skeleton. Everything else happens on site: insulation stuffed in, services run through, boards fixed, the panel lined and the outside clad. An open panel is light, cheap to make and transport, and forgiving, because site trades still have access to the inside of the wall; but it leaves a great deal of site work and site-quality risk.

A closed panel arrives far more finished: the frame comes insulated, with a vapour control layer and breather membrane, services (conduits, sometimes pipes) already run, boards fixed both sides, windows and doors installed, and sometimes the external cladding already on. It is a wall you stand up and, more or less, leave alone. A closed panel concentrates quality and speed in the factory — the insulation is installed in controlled conditions, the airtightness layer is continuous and inspected, the services are set out accurately — and minimises site work, but it is heavier, more delicate to transport and handle, and commits more decisions earlier (once services are buried in a closed panel, changing them is a factory operation, not a site one).

Between the two sits every gradation: a panel closed on one side, a panel insulated but not serviced, a panel clad but open internally for site services. The designer's judgement is to choose the degree of closure that fits the project: how much site labour is available and skilled, how tight the programme is, how repetitive the building is, how much the design might still change, and what the transport and craneage allow. This is the same 'how far along the slider' question as the whole spectrum, nested one level down — and getting it right is most of designing a good panelised building. Open panels suit flexible, labour-rich, still-evolving projects; closed panels suit fast, repetitive, decided ones.

Panels are built from a handful of structural families, and each carries a different character. Timber frame panels — studs sheathed with board — are the most widespread panelised system in the world, light, fast, low in embodied carbon and easy to cut and fix; they dominate low- and medium-rise housing in many markets and are increasingly used with engineered wood. Light-gauge steel (cold-formed steel) panels use thin, roll-formed steel studs instead of timber: dimensionally very stable, non-combustible, immune to rot and termites (a real advantage in parts of India), and made to tight tolerances on automated lines, though they need care on thermal bridging and acoustics. Precast concrete panels are a heavier, more structural family — solid or sandwich wall panels, hollow-core or solid floor planks — cast flat in a factory and craned into place; they bring mass, durability, fire resistance and finish in one element and are a mature, strong capability in India for repetitive structures, at the cost of weight and craneage.

A distinct and important type is the structural insulated panel (SIP): a thick core of rigid insulation sandwiched between two structural boards, forming a panel that is simultaneously structure and insulation. SIPs make highly insulated, airtight envelopes quickly and are popular for energy-efficient low-rise buildings; their discipline is that openings and services must be planned precisely, because you cannot casually cut a structural-insulated sandwich on site.

There is also a vocabulary point about floors and roofs: panelised floors and roofs are often called cassettes — factory-made flat structural units (timber or steel joists with decking, or precast planks) that drop onto the walls to form each level. A panelised building is really a kit of wall panels plus floor and roof cassettes.

The material choice is not free: it sets the panel's weight (and so transport and craneage), its fire and acoustic behaviour, its thermal performance, its tolerance and its embodied carbon, and it interacts with the local supply chain — timber where there is a timber industry, precast where there is precast capacity, light-gauge steel where rot, termites and fire argue against timber. As always, the binding structural, fire and acoustic performance of any panel is the engineer's and the manufacturer's to establish, not the sketch's; the designer chooses the family and designs to its discipline.

Watch a panelised building go up and you see why the system sits where it does on the spectrum. On a prepared slab with accurate setting-out, a small crew and a crane locate the wall panels against a marked grid, plumb and brace them, and connect them to each other and to the slab; then floor cassettes drop onto the walls to form the next level, the next lift of walls goes up, and finally the roof panels or trusses close the top. A weathertight shell that would take weeks of wet, sequential trades on a conventional site can be stood up in days, because the heavy, skilled, weather-sensitive making already happened in the factory and the site task is reduced to accurate assembly.

But the shell is not the finished building, and this is the honest heart of the trade-off. How much remains depends on how closed the panels were. With open panels, the standing shell is only a frame: insulation, services, linings, finishes and cladding are all still to come, which is a lot of site work — essentially the whole interior and a good deal of the envelope. With closed panels, far less remains: perhaps jointing, taping, the services that cross between panels, and final finishes. Either way, two things are always left to site and deserve design attention: the panel-to-panel joints (which must carry structure, resist weather, and meet fire and acoustic requirements across what is now a line of discontinuity) and the on-site services distribution that stitches the panels into working systems.

The erection logic also explains panelised construction's signature advantages over volumetric. Because panels ship flat, transport is dramatically more efficient — you are not paying to move finished air, and a single lorry carries the panels for many rooms. Because you assemble planes rather than place fixed boxes, the system is flexible: the same panel kit makes many different plan shapes and sizes, so panelised construction suits varied, non-repetitive buildings far better than volumetric does. The cost of that flexibility and transport efficiency is precisely the site finishing work that volumetric would have eliminated in the factory. Panelised is the middle of the spectrum because it splits the difference — and designing it well means deciding, plane by plane and joint by joint, how to split it.

Panelised construction rewards a specific set of design disciplines, and they are worth holding explicitly. The first is the joint. A panelised building is a field of panels meeting at lines, and every one of those lines is a structural, weather, fire and acoustic event. The panel faces are made accurately in the factory, but they meet each other and the site structure with real-world variation, so each joint must be designed to transfer load, keep water out, maintain fire compartmentation and limit sound transmission, *while absorbing tolerance*. More panels mean more joints mean more of the building's performance living in junctions — which is why joint design, not panel design, is often where panelised projects succeed or fail.

The second discipline is the grid and dimensional coordination. Panels want to be a manageable kit of repeating sizes that suit the factory line, the lorry and the crane, and that set out cleanly on the slab. A design that works to a sensible panel grid gets cheap, repeatable panels and simple setting-out; a design that ignores the grid gets a zoo of bespoke panels and awkward junctions. This is the modular-coordination thinking of Module 5, applied to planes.

The third is tolerance and the slab. Panels are precise; the foundation and slab they land on are not, unless you make them so. The interface between accurate panels and a less-accurate substructure is a classic source of trouble, which is why panelised (and all off-site) construction pushes accuracy down into the groundworks and setting-out far more than conventional building does.

The fourth discipline is honesty about the balance. Panelised construction is genuinely the flexible middle: lighter and cheaper to transport than volumetric, more adaptable in plan, lower in craneage demand — but it leaves more site work, more joints to get right, and more site-quality risk than a finished module. Whether that balance favours panels over components (less site work, more commitment) or over volumetric (more site work, more flexibility, cheaper transport) is a project judgement about repetition, transport distance, plan variety, programme and local labour. In India, panelised systems — especially precast for repetitive structures and light-gauge steel where termites, rot and fire rule out timber — are among the most practical off-site options, precisely because they demand less craneage and transport than volumetric while still pulling the heavy making off-site. The binding structural, fire and acoustic performance of the panels and their joints, and the craneage and transport limits, belong to the engineers, the manufacturer's tested system, the lifting specialists and NBC India. The design judgement — degree of closure, panel grid, joint strategy, where on the slider to sit — is yours.