Sandwich panel

What is a sandwich panel (sendvičový panel) and how is it made?

A sandwich panel is a factory-made composite of two thin outer skins, usually profiled steel sheet, bonded to a central insulation core. The core is most often mineral wool, polyisocyanurate (PIR) foam, or polyurethane (PUR) foam. It is glued to both faces under pressure, so the three layers act as one element that arrives on site in finished lengths.

On site the panel is fixed to steel rails or timber battens with screws or clips, and its interlocking edge profiles form the joints. The panel is the envelope itself, with no separate insulation layer added afterwards.

How does a sandwich panel differ from a structural insulated panel?

Both are factory-made and insulated, but the material and the job differ. A structural insulated panel (SIP) is a timber-building product with oriented strand board (OSB) or plywood faces, and it forms the load-bearing walls and roofs of a house. A sandwich panel has steel faces. It usually spans between the rails or purlins of a separate steel frame, so in most designs it works as cladding or roof covering rather than as the structure of the house.

Feature	Sandwich panel	SIP
Faces	Steel sheet, usually profiled	OSB or plywood
Core	Mineral wool, PIR or PUR foam	EPS, XPS or PUR foam
Typical use	Halls, workshops, cladding, roofs	Load-bearing walls and roofs of houses
Structural role	Spans between a separate frame, usually cladding	Frame, insulation and sheathing in one
Fire behaviour	Set by the core and the full build-up	Set by the foam type and facing

What are the main core types, and how do they behave in a fire?

The core decides most of the fire behaviour. Mineral wool is non-combustible, so panels with a mineral wool core are the usual choice where požiarna odolnosť (fire resistance) is required, such as separating walls. PIR, PUR and EPS cores are combustible. They can give good insulation in a thin profile, but they burn and release smoke, and metal faces do not make a foam core safe on their own.

Fire performance belongs to the whole build-up, not to the product name. Ask the supplier for test documentation covering the exact combination of core, thickness, facings and joints, and check it against the fire requirements the fire authority sets for your project. Never assume a panel is fire-rated because its faces are metal.

Core	Fire behaviour	Insulation per thickness	Typical use
Mineral wool	Non-combustible core	Moderate	Fire-separating walls, halls with fire demands
PIR or PUR foam	Combustible, needs a verified build-up	High	Cold stores, roofs where the profile is limited
EPS	Combustible, needs a verified build-up	Moderate	Light cladding and lower-demand halls

How do joints and airtightness work in sandwich panel buildings?

Joints are where many sandwich panel buildings leak. Panels connect through interlocking side profiles, and each seam is closed with a gasket, sealant or foam tape. A well-sealed face is not enough if the joints, the perimeter at the top and bottom rails, or the openings around doors and roller shutters are left open. Airtightness is a question of detailing and site execution, so the joint system, the perimeter closures and every penetration for ducts, pipes and downpipes must be specified and checked.

If the building has a low air-leakage target, plan a pressure test (blower-door test) on completion and treat the result as a measurement, not a guarantee. The panel system gives a good starting point for airtightness, but the final result depends on the crew that closes each joint.

Where do condensation and thermal bridges form at the fixings?

Metal faces conduct heat well, and every fixing that passes through the outer skin, whether a screw, clip or bracket, creates a point thermal bridge through the insulation. In cold weather the inner face at each fixing runs colder than the panel around it, so condensation forms there, with staining, mould or drips as the result. Steel rails and purlins that cross the insulation layer create a linear thermal bridge in the same way.

Location	Risk	Typical measure
Screw or clip through the outer skin	Point thermal bridge, condensation spot	Thermal spacers or breaks under fixings, fewer fixings where the design allows
Steel rail or purlin crossing the insulation	Linear thermal bridge	Thermal break strip, insulation kept continuous over the support
Window or door reveal	Cold edge, surface condensation	Insulated reveal with a continuous seal
Eaves and wall-roof junction	Thermal bridge and air leakage	Continuous insulation and a sealed closure profile

Why do sandwich panels drum in the rain, and can it be reduced?

Thin metal faces act like a drum when rain hits them, especially on large roofs with light sheeting and no solid layer beneath. The sound is loudest where the building has little internal mass and no ceiling absorption. It does not mean the panel is failing, but it matters in a bedroom or living room under a metal roof.

The usual measures are a heavier sheet, a denser core, or a lining board fixed to the underside on battens.

Why does a sandwich panel look industrial on a family house, and how can it be clad?

Profiled steel with exposed ribs, visible fixings and a corrugated surface reads as hall architecture. Left bare, a metal wall on a house looks cold and utilitarian, and the module lines and joints draw the eye. The fix is to hide the panel behind a second layer, such as a ventilated facade in timber, fibre cement, brick slips or render on a carrier, or to choose flat-faced panels with concealed fixings in a restrained colour.

Cladding changes the building physics too. The air gap, the detail at openings and the moisture behaviour of the outer layer must be designed together with the panel, not added after the envelope is fixed.

What does "sandwich panel" mean in Slovak usage for a system house?

In Slovakia the term is sometimes used loosely for the whole wall of a system house, meaning the prefabricated wall package from the house manufacturer. That wall may be a timber frame with insulation, a panel with a different core, or a combination, so the word does not reveal the build-up. Two offers both labelled sandwich panel can differ in fire behaviour and joints, so ask for each layer, its core, its fixings and its test documentation.

When is a sandwich panel the right choice for a house?

Use	Fit
Workshop, garage or hall	Strong fit, the product was made for this
Family house with a visible metal envelope	Possible, if clad and detailed for the climate
Passive-house envelope	Only with thermal breaks at fixings and tested joints
Load-bearing timber house	Usually better served by a SIP or timber frame

Choose a sandwich panel when an industrial character suits the building, or when a contemporary design uses it as a deliberate, detailed element. Involve the supplier and the energy consultant early, because the panel alone does not deliver the performance of its joints and fixings.