Double Skin Facades

Background

A ventilated Double Skin Facade (DSF) can be defined as a traditional single facade doubled inside or outside by a second, essentially glazed facade. Each of these two facades is commonly called a skin . A ventilated cavity - having a width which can range from several centimetres at the narrowest to several metres for the widest accessible cavities - is located between these two skins. In some DSF designs the ventilation of the cavity is controllable by fans and/or openings, and other concepts where this ventilation is not controllable . The indoor and outdoor skins are not necessarily airtight (see, for example, the "louvre" type facades). Automated equipment, such as shading devices, motorised openings or fans, are most often integrated into the facade. The main difference between a ventilated double facade and an airtight multiple glazing, whether or not integrating a shading device in the cavity separating the panes, lies in the intentional and possibly controlled ventilation of the cavity of the double facade.

The standard prEN 13119, specifies the terminology associated with curtain walls and of a double-skin facade. These are defined as:

Curtain walling

External building facade produced with framing made mainly of metal, timber or PVC-U, usually consisting of vertical and horizontal structural members, connected together and anchored to the supporting structure of the building, which provides, by itself or in conjunction with the building construction, all the normal functions of an external wall, but does not contribute to the load bearing characteristics of the building structure.

Double-Skin Facade

A curtain wall construction comprising an outer skin of glass and an inner wall constructed as a curtain wall that together with the outer skin provide the full function of a wall.

This definition includes only references to the constructive and structural aspects of the facade. No reference is made to the ventilation of the cavity. Thus formally, the term "double-skin facade" does not designate a ventilated double facade. It is for this reason that the term ´Ventilated Double-Skin Facade´ has been proposed as the generic term to designate these facades.

DSFs can be categorised according to the type of ventilation.

Type of ventilation	Name of the facade concept
Natural	Passive facade
Mechanical	Active facade
Hybrid	Interactive facade

This table contains the commonly used names referring (only) to the type of ventilation and thus giving no indication about the partitioning of the facade or the ventilation modes. In the literature, the double facades adopting the air supply ventilation mode are called airflow window or also ventilated window. The facades adopting the air exhaust ventilation mode are also called extract air or exhaust airflow.

Double Skin Facade Example

A simple example DSF model is provided with DesignBuilder (Double Skin Facade Example.dsb). A screenshot of the model at block level is shown below.

The model was created using a single block with a partition to divide it into 2 zones, one zone to represent the occupied space and the other for the ventilated cavity. The glazing for the partition was set up at surface level.

Modelling Considerations

There are some important considerations when using DesignBuilder to model double facades. Firstly, the Zone type on the Activity tab should be set to 3-Cavity at zone level in the cavity zone and this causes the following further changes to be made to the model:

The zone is set as unoccupied by loading <None> Activity, HVAC and Lighting template data.
The 5-CavityInternal convection algorithm is set for Cooling design, Heating design and simulation calculations to correctly model the air space.
The 3-Full interior and exterior Solar distribution algorithm is set for Cooling design and Simulation because it allows solar radiation to be accurately transmitted through the interior glazing in the partition.

In the example model a simple ventilation control system has been defined in the cavity zone to provide 10 ac/h of outside air into the cavity when the air temperature in the cavity is greater than 24°C. This helps to ensure that the cavity does not overheat, reducing cooling loads in the occupied zone. The schedule and setpoint could be adjusted to fine-tune the ventilation. The model assumes no transfer of cavity air into the occupied space. This could be modified using an approach similar to that in the Trombe wall example model where vents in the partition between occupied and cavity zones are used to transfer air between the 2 zones.

Finally, an overhang has been added to control the amount of summer sun exposure.

Because the model uses a zone to model the cavity in the double facade, the inner glazing is 'interior' and so requires the Full interior and exterior solar distribution option to allow solar radiation to pass through exterior and then through interior windows. You should read the information about the restrictions associated with this option before using it.
Take care when comparing the exterior solar radiation transmitted into the double facade zone with the interior solar radiation transmitted into the occupied zone. These do not include the re-reflection of solar back out of the external windows. Read the definitions for the output data.

Source of background information on DSF:

http://www.bbri.be/activefacades/new/content/1_home/en.html