The Princeton Envelope Group (PEG) is a design and research unit at the Princeton University School of Architecture. Headed by Alejandro Zaera-Polo and coordinated by Urtzi Grau. The unit is engaged in a three year research program on the Politics of the Building Envelope the conclusions of which will be published in a forthcoming book by Actar Press, Barcelona+New York.


Flat Vertical

The category of the flat-vertical envelope, better known as a slab, includes those envelopes whose predominant dimensions are parallel to gravity and distributed along a line. Flat-vertical envelopes are generated by the horizontal displacement of a section of space, which, in order to support a specific function, optimizes density, daylight, ventilation, structural constraints, and the building’s relationship with public and infrastructure. Land uses and orientation are crucial drivers for this envelope type. Most mid-rise residential and many office buildings fall into this category, as they respond to the need to host a large volume of homogeneous program. The flat-vertical envelope is primarily determined by the facade-to-facade or facade-to-core depth, hence its laminar organization. Modern urban fabrics are predominately matrices of flat-vertical envelopes combined in various configurations suited to a particular climate, use, and culture. The flat-vertical envelope usually has a high level of environmental performance and a relatively low level of expressive performance.

Flat Horizontal

The category of flat-horizontal envelopes includes those in which the horizontal dimensions are considerably larger than the vertical. Buildings like airports, train stations, factories, trade fairs, convention centers, markets, and retail and leisure complexes generally belong to this category. The political performance of flat-horizontal envelopes lies in the delimitation of edges, frontiers, and boundaries, and in the sheltering of large-scale atmospheres operating primarily through the articulation between natural and artificial. Since a comprehensive perception can only be obtained from an aerial perspective, flat-horizontal envelopes are experienced in a fragmented manner are, therefore, less concerned with representational and figural performance, than with the organization of material flows: traffic, ventilation, daylight, security, etc. The flat-horizontal envelope usually presents relatively low affective and environmental performances.


The spherical envelope’s dimensions are approximately equivalent in all directions; cubic, spheroidal, and polygonal geometries are also particular cases in this category. In principle, the spherical envelope has the lowest ratio between its surface and volume it contains. The specificity of this type is the relative independence that the skin acquires in relation to its programmatic determinations, as functions are not strongly determined by adjacency to the outside and, therefore, by the form of the envelope. This often implies a wider variety of programs inside and a hetrogeneous environmental content. Spherical envelopes generally enclose a wide range of spatial types with specific functions, rather than a single spatial condition. Unlike other envelope types in which the border between public and private occurs on the surface of the container, the spherical type often contains gradients of publicness. Spherical envelopes often correspond to public buildings that gather a multiplicity of spaces, such as city halls, courthouses, libraries, museums, and arenas. In the spherical envelope the gap between expressive and environmental performances is at a maximum, with low environmental and high expressive performances.


The vertical envelope has a predominantly vertical dimension and, unlike the flat-vertical type, a multidimensional orientation in plan. The specificity of this envelope category is an intense relationship between physical determination and performances. Because of its scale and technical complexity, functional and environmental performances such as daylight penetration and natural ventilation need to be maximized, while the formal qualities of the envelope play a crucial role in the building’s structural stability. The vertical envelope’s geometric determination crucially impacts both the spaces that it encloses and its surroundings. In addition, the visibility of the vertical envelope makes it particularly conducive to iconographic performance. If in the spherical envelope the gap between representative and environmental performances reach a maximum, in the vertical envelope both sets of performances are at their highest level. The collusion between extreme technical performance and high visual impact produces maximum tension between efficiency and expression, a condition that runs deep in the history of this building type.