Double-skin facade

What is a double-skin facade?

A double-skin facade is an outer wall of two glazed layers that enclose an air cavity. Designers use the cavity for ventilation, shading and heat control. The outer skin takes weather and wind, and the inner skin holds the windows and doors of the rooms. In Slovak the term is dvojplášťová fasáda.

Engineers use the system to improve the building envelope. It adds a second glazed layer and an airflow cavity between the two layers. The first built examples date from the 1980s. Northern Europe developed the technology in the 1990s and 2000s, first for sound insulation and thermal comfort, and later for natural ventilation.

It is not the same as a ventilated facade. A ventilated facade has one insulated wall behind a cladding. A double-skin facade is usually a glazed system, related to the curtain wall, with a second glass layer in front of or behind it.

What types of double-skin facade exist?

Technical literature classifies the system by how the cavity is divided. Four types are common: box window, shaft-box, corridor and multi-storey.

Type	How the cavity is divided	Known drawback
Box window	Each room has its own cavity, closed horizontally and vertically, with separate air inlet and outlet	Grilles close together can pass warm, used air between rooms, and noise is harder to control
Shaft-box	Box-window cavities connect to a vertical shaft that draws air up through the building	Airflow is fast and hard to control in a tall building
Corridor	The cavity is divided at each floor level and ventilated floor by floor	More vents in the outer skin mix clean and used air and add noise
Multi-storey	One cavity with no divisions, which can cover a whole elevation, with intake at the bottom and exhaust at the top	Overheating if ventilation is inadequate, sound and odour transfer, and fire safety questions

How deep is the cavity and how does it ventilate?

Cavity depth in built examples ranges from about 10 cm to 2 m. Multi-storey types tend to be deep. Box-window types sit at the shallow end, typically 10 to 20 cm, which suits a prefabricated unitised facade for tall buildings.

Three drivers of airflow exist: natural, mechanical and hybrid. In natural ventilation the air in the cavity warms and rises, leaves through the top vent and draws fresh air in at the bottom. This is the chimney effect, or stack effect. Mechanical fans can assist the flow where the cavity would otherwise overheat.

The path of the air also varies. In the multi-storey type the air can pass from the cavity into the rooms through openings in the inner skin. This allows natural ventilation even on upper floors. In the other types each floor or room has its own inlet and outlet.

Does a double-skin facade overheat in summer?

It can. The cavity air in a multi-storey facade overheats if the ventilation is inadequate. The cavity also suits shading devices. Blinds between the skins stop solar radiation before it reaches the inner glass, and the outer skin shields them from weather. A cavity that vents at the top carries the heat away instead of passing it into the rooms.

The same shading blocks daylight, as any shaded glass wall does. Each extra glass layer also lowers the light that reaches the room. See external venetian blind for the simpler single-skin version of this idea.

How does fire spread in a double-skin facade?

A vertical cavity can work as a chimney for smoke and flames. A 2024 simulation study of a three-storey office with a cavity 1 m deep compared the four types. It found that multi-storey and shaft types, which have a continuous vertical cavity, involve the chimney effect. In its scenarios the fire started on the ground floor. The continuous cavities carried flames to the offices on the upper floors. The office temperature stayed below 60 degrees Celsius in all four types.

The study also reports that glass cracks at high temperature. Cracking of the inner glass raises the risk of fire developing in the room, while cracking of the outer glass lets smoke out. It points to smoke extraction through the cavity and fire dampers where necessary as design options. A contractor article lists the lack of fire safing between floors in a multi-storey cavity as a code issue. Fire rules for a real building decide the detail, and the designer must show how the cavity is divided or protected.

How are the cavity and the glass cleaned?

Both skins need access. Deep cavities can hold maintenance platforms. An article by a facade contractor notes that cavities narrower than about 75 cm limit the movement of workers. Shading devices and operable vents inside the cavity must be reachable, which complicates a unitised facade. If the units are not hermetically sealed, dust and humid air can enter the cavity. They can leave dirt and condensation on the inner glass and raise the maintenance effort.

Does a double-skin facade save energy?

The evidence is mixed. A 2016 systematic review analysed more than 50 studies of double-skin facades in temperate climates. It compared their results with a base case such as a single-skin facade. Results are grouped by configuration, cavity width, ventilation, orientation and climate, and the operational energy values are widely spread. The review also notes that almost no life-cycle energy data exist. For that reason this page gives no saving in percent. Designers and owners also remain sceptical of the payback of the system, and cost is seen as the main barrier to its use.

Where does a double-skin facade make sense?

Situation	Fit for a double-skin facade
Large glazed office or public building with strong sun exposure	Typical use, because the cavity can house shading and give ventilation
Glazed facade where sound insulation matters	Sound reduction was one of the first reasons for the system
Single-family house with a compact plan	Rarely a fit, for the reasons below
Small extension or single room with a large window	A better window or a shading device does the job

Why do double-skin facades rarely suit a family house?

The system needs a cavity with room for access, maintenance and shading, and it doubles the glass to buy, clean and maintain. A family house has little spare depth at its envelope and a small glazed area to share the cost. Heating demand is reached more simply with a continuous airtight insulated envelope and good windows, which is the logic of the passive house. The U.S. Department of Energy notes that two or three panes, a gas fill of argon or krypton and a low-emissivity coating reduce heat transfer. It adds that shades help against overheating but block daylight.

What do Slovak rules say about glazing and daylight?

The Ministry of Health decree 259/2008 Z. z. on the indoor environment of buildings sets the daylight rules. Its § 4 ods. 3 requires rooms where people stay for long periods to have adequate daylight. Its § 4 ods. 6 requires glazing with a light transmittance factor of at least 0.6, measured perpendicular to the surface, for long stays. Its § 4 ods. 12 requires daylight to be verified by measurement when calculated values cannot be used. The numeric daylight values sit in an annex that this page does not quote. A second skin adds glass, so the designer must show that the combined wall still meets these rules.

What should a family house use instead?

Use a continuous insulated envelope and windows with low-emissivity glass and gas fill. Add external shading on the sunny elevations. If the wall needs a finished outer layer, a ventilated facade protects the insulation without a second glass wall. Good double glazing with shading meets most goals that a double skin is asked to meet.