Related links:
Double-skin façades and natural ventilation performance | building roster

Double-skin façades and natural ventilation

Heat extraction double skin façades | Night-time ventilation | Mixed-mode and natural ventilation

Heat extraction double-skin façades 

Heat extraction double-skin façades rely on sun shading located in the intermediate or interstitial space between the exterior glass façade and interior façade to control solar loads. The concept is similar to exterior shading systems in that solar radiation loads are blocked before entering the building, except that heat absorbed by the between-pane shading system is released within the intermediate space, then drawn off through the exterior skin by natural or mechanical ventilative means. Cooling load demands on the mechanical plant are diminished with this strategy. 

This concept is manifested with a single exterior layer of heat-strengthened safety glass or laminated safety glass, with exterior air inlet and outlet openings controlled with manual or automatic throttling flaps. The second interior façade layer consists of fixed or operable, double or single-pane, casement or hopper windows. Within the intermediate space are retractable or fixed Venetian blinds or roller shades, whose operation can be manual or automated.

During cooling conditions, the Venetian blinds (or roller shades) cover the full height of the façade and are tilted to block direct sun. Absorbed solar radiation is either convected within the intermediate space or re-radiated to the interior and exterior. Low-emittance coatings on the interior glass façade reduce radiative heat gains to the interior. If operable, the interior windows are closed. Convection within the intermediate cavity occurs either through thermal buoyancy or is wind driven. In some cases, mechanical ventilation is used to extract heat.

The effectiveness of ventilation driven by thermal buoyancy, or stack effect, is determined by the inlet air temperature, height between the inlet and outlet openings, size of these openings, degree of flow resistance created by the louver slant angle, temperature of the louvers and interfacial mixing that may occur at the inlet or outlet openings if there is no wind. Box windows are single-story double-skin façades that are divided by structural bay widths or on a room-by-room basis. Shaft-box façades couple single-story box windows to multi-story vertical glass chimneys via a bypass opening at the top of the box window. The vertical height of the glass chimney creates stronger uplift forces due to increased stack effect. However, the upper stories of the shaft can become appreciably hot, lending to increased heat gains and thermal discomfort. Corridor façades are single-story façades that have no vertical divisions except those required at the corners of the building or elsewhere for structural, acoustic, or fire protection reasons. Here, air flow is expected to take a diagonal path across the face of the façades and inlet and outlet openings are staggered to prevent air exchange between the two openings. 

The position of the Venetian blind within the air cavity affects the rate of the heat transfer to the interior and amount of thermal stress on the glazing layers. Placed too close to the interior façade, inadequate air flow around the blind may occur and conductive and radiative heat transfer to the interior are increased. The blind should be placed toward the exterior pane with adequate room for air circulation on both sides. With wind-induced ventilation or high velocity thermal-driven ventilation, the bottom edge of the blind should be secured to prevent fluttering and noise. 

Heat recovery strategies can be implemented using the same construction to reduce heating load requirements during the winter. This strategy is normally not useful for the California climate and for commercial buildings, which tend to be cooling-load dominated year-round. Heat recovery strategies can be used for east- to south-facing façades to offset early morning start-up loads that occur typically on Mondays or periods following a holiday but careful engineering is required to avoid overheating during late morning hours.

1. Exterior upper air outlet 
2. Controllable solar control device 
3. Interior upper operable window (air inlet) 
4. Interior operable or fixed view window 
5. Exterior glazing layer 
6. Air cavity 
7. Interior lower operable window (air inlet) 
8. Exterior lower air inlet

The position of the Venetian blind within the air cavity affects the rate of the heat transfer to the interior and amount of thermal stress on the glazing layers. Placed too close to the interior façade, inadequate air flow around the blind may occur and conductive and radiative heat transfer to the interior are increased. The blind should be placed toward the exterior pane with adequate room for air circulation on both sides. With wind-induced ventilation or high velocity thermal-driven ventilation, the bottom edge of the blind should be secured to prevent fluttering and noise. 

Heat extraction 

Heat recovery

Heat recovery strategies can be implemented using the same construction to reduce heating load requirements during the winter. This strategy is irrelevant for the California climate and for commercial buildings, which tend to be cooling-load dominated year-round. Heat recovery strategies can be used for east- to south-facing façades to offset early morning start-up loads that occur typically on Mondays or periods following a holiday but careful engineering is required to avoid overheating during late morning hours.

References 

Oesterle, Lieb, Lutz, Heusler. 2001. Double-Skin Façades: Integrated planning. Munich: Prestal Verlag.

Question/Information: eslee@lbl.gov