interdome

An interdome is the air cavity sandwiched between the inner and outer domes of a double-shell cupola or rotunda. This void typically ranges from 30 centimeters to over a meter in thickness, depending on the structure's span and the architect's intentions. The space exists because many large domes are built as two separate curved surfaces rather than one solid shell, a technique that dates back to the Pantheon but was systematized during the Renaissance.

The primary function of an interdome is structural. The outer shell bears wind and weather loads while remaining thin and light; the inner dome provides the finished ceiling visible from below and carries its own weight plus any suspended fixtures. The gap between them distributes stresses differently than a monolithic shell would, and allows the two surfaces to move independently slightly under thermal expansion. This separation is essential for domes wider than about 15 meters, where a single shell would be either prohibitively thick or prone to buckling.

Practical concerns in the interdome

The interdome space must be ventilated or it will trap moisture, leading to rust on any metal reinforcement and accelerated deterioration of both shells. Historic domes like that of the Florence Cathedral or St. Peter's have elaborate systems of radial ribs or buttresses running through the interdome to brace the outer shell and channel water toward drainage points. Modern domes sometimes use web-like reinforcement to serve the same purpose while keeping the space partially open to air circulation.

Access to the interdome is rarely straightforward. Maintenance workers must navigate narrow galleries or squeeze through hatch openings, often in conditions of poor light and high temperature if the space is in direct sunlight. Inspection of the inner surface of the outer dome and the outer surface of the inner dome is difficult and dangerous, which is why preventive maintenance and sound original drainage design are critical.

The term interdome appears primarily in architectural and structural engineering contexts, less often in general construction vocabulary. It is sometimes called the interstitial space or, in older texts, the cavity or void, though those terms lack precision. Understanding whether a dome has an interdome and how it is ventilated and buttressed is essential before undertaking any repair or restoration work on the dome itself.