Double Facades

Background

A ventilated double facade 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. There exist facade concepts where the ventilation of the cavity is controllable, by fans and/or openings, and other facade 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´ is proposed as the generic term to designate these facades.

 

Double facades 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.

Modelling Considerations

There are some important considerations when using DesignBuilder to model double facades.

 

  1. If you are using a zone to model the cavity in the double facade then the inner glazing is 'interior' and so requires the Full interior and exterior solar distribution option to get 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.
  2. 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.
  3. It is possible to set the double facade zone as a return plenum.

 

To be extended...

 

Source of background information:

 

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