Ventilated (rainscreen) facade
An insulated wall in which the cladding hangs on a substructure, separated from the insulation by a clear ventilated cavity of roughly 25 to 40 mm.
What are the layers of a ventilated facade, from the inside out?
A ventilated facade is a layered construction in which each layer does one job and does not stand in for the others. That separation of function is the whole idea, and it is what distinguishes the system from a bonded one where a single composite does everything at once.
- The load-bearing wall carries the structure and provides the fixing substrate.
- The substructure of metal brackets and rails, or a timber frame, holds the insulation and the cladding and forms the cavity.
- The thermal insulation does all of the thermal work, laid tight and without gaps.
- The clear ventilated cavity, roughly 25 to 40 mm of free gap, runs continuously from a low-level inlet to a high-level outlet.
- The cladding keeps the weather out and sets the character of the facade.
The cavity is not insulation and contributes practically nothing to thermal resistance. Its job is to let air move: that air removes moisture from the insulation and from the masonry, and carries away water that gets behind the cladding. The condition is continuity. If the cavity is interrupted at a window head, at a sill, or where a rail was packed tight against the wall, the flow stops and the system stops working in exactly the band where the moisture is.
What is the rainscreen principle?
The cladding is a first line of defence, not a seal. The design assumes that some water will always pass the joints, laps and fixings, and answers it with a drainage plane behind the cladding: the water runs down that plane and is ventilated away. This is precisely why open joints between boards or battens are possible at all, when in a bonded system the same joint would be a defect.
The same airflow dries the insulation and the substrate after driving rain or after a wetting during construction. A ventilated facade therefore tolerates an uneven, damp or hard-to-assess substrate far better than a bonded system does: an old stone wall, cinder concrete, or masonry of uncertain history. In renovation that tolerance is often the deciding argument, because it removes the need to guarantee something about the existing wall that nobody can actually guarantee.
What does the substructure cost in thermal performance?
The system's real penalty is the thermal bridging of the substructure. Brackets and rails cross the insulation from the wall to the cladding and conduct heat through its full thickness, and their effect cannot be ignored: it has to be included in the declared U-value. It is addressed with thermally broken brackets and by calculating the point thermal bridges explicitly rather than applying a rule of thumb.
STN 73 0540 Part 2 recommends a U-value of 0.22 W/(m²·K) for the external wall of a new dwelling and sets a target of 0.15 W/(m²·K) for new buildings from 1 January 2021. A ventilated build-up reaches either figure comfortably, but only if the substructure penalty was counted at design stage. A wall designed to the target value on paper and then hung on unbroken steel brackets does not reach it in reality, and the shortfall is invisible in every document except the calculation that included it.
Why does fire safety shape the specification?
A continuous cavity is also a chimney. Cavity barriers are therefore designed into it at floor levels and at fire compartment lines, fire safety of buildings is governed by STN 92 0201, and the design is signed off by the fire authority for the specific project. This is not a formality on a ventilated facade; it is the reason several otherwise attractive material combinations are unavailable on larger buildings.
For the same reason mineral wool is the usual insulant here. Both stone and glass wool are reaction to fire class A1, non-combustible, whereas polystyrene boards are class E and timber cladding is combustible. Insect and rodent mesh is fitted at the cavity inlet and outlet, but it must be chosen so that it does not choke the airflow, which is a detail routinely got wrong with fine mesh chosen for the wrong reason.
How does a ventilated facade compare with a bonded system?
| Criterion | Ventilated facade | Bonded system (ETICS) |
|---|---|---|
| Cost | Higher, substructure and cladding | Lower, the Slovak default |
| Build-up thickness | Greater, cavity plus cladding | Smaller |
| Tolerance of an uneven substrate | High, the substructure adjusts | Low, the wall must be reasonably flat |
| Drying performance | Excellent, the cavity ventilates | Limited, the build-up is closed outwards |
| Cladding options | Heavy, timber, metal, open-jointed | Render finishes only |
| Repair and replacement | Cladding only, insulation undisturbed | Insulation, mesh and render together |
| Thermal bridging | Substructure must be calculated | Continuous, almost none |
The comparison should be made fairly. A bonded system is cheaper, thinner and simpler, and on an ordinary Slovak house with a flat substrate it is entirely adequate, which is exactly why it is the default. A ventilated facade earns its cost where its specific strengths are needed, and it is paid for with a substructure that must be designed rather than improvised on site.
What does the cavity mean for the build-up behind it?
| Design point | Requirement |
|---|---|
| Inlet and outlet | Free openings at the base and at the top of every ventilated zone |
| Continuity | Cavity maintained past window heads, sills and floor junctions |
| Insulation face | Boards with a stabilised surface, so airflow does not erode the fibres |
| Cavity barriers | At compartment lines, without closing the ventilation path elsewhere |
| Mesh | Insect and rodent protection sized not to choke the flow |
| Fixings | Thermally broken brackets, point bridges calculated |
Because the cavity ventilates outwards, the wall behind it can be a vapour-open build-up without difficulty, which is a genuine advantage on old masonry that needs to dry. The cladding itself is almost irrelevant thermally: timber cladding conducts at roughly 0.13 W/(m·K) and adds practically nothing to the thermal resistance of the wall, so it is chosen for appearance, durability and fire behaviour rather than for performance.
Frequently asked questions
- What is the cavity in a ventilated facade for?
- It lets air move. That air removes moisture from the insulation and from the masonry behind it, and it carries away water that gets past the cladding. The cavity is not insulation and contributes practically nothing to thermal resistance, so making it wider does not make the wall warmer.
- How wide should the ventilated cavity be?
- A clear gap of roughly 25 to 40 mm is the usual range, and what matters more than the exact figure is that it is continuous from a low-level inlet to a high-level outlet. A cavity interrupted at a window head or squeezed shut where a rail was packed tight against the wall stops working precisely where the moisture is.
- Why is mineral wool almost always used in a ventilated facade?
- Because the cavity behind the cladding is a continuous vertical duct that behaves like a chimney in a fire. Stone and glass wool are reaction to fire class A1, non-combustible, whereas polystyrene and wood fibre are combustible. Cavity barriers at floor levels and compartment lines are designed in for the same reason.
- Does the substructure spoil the insulation performance?
- It costs some of it, and the loss has to be counted rather than ignored. Brackets and rails cross the insulation from the wall to the cladding and conduct heat through the full thickness. The remedy is thermally broken brackets and an explicit calculation of the point thermal bridges, which then enters the declared U-value.
- When is a ventilated facade worth its extra cost?
- When the substrate is uneven, damp or of uncertain history, when the cladding is heavy or timber, when the facade may need to be repaired or replaced without disturbing the insulation, or when drying performance genuinely matters. On an ordinary flat new-build wall, a bonded system is cheaper, thinner and entirely adequate.