(Latin: “island”), in architecture, block of grouped but separate buildings or a single structure in ancient Rome and Ostia. The insulae were largely tenements providing economically practical housing where land values were high and population dense. Distinct from the domus, the upper-class private residence, they were inhabited primarily by the labouring class.

Insulae were constructed of brick covered with concrete and were often five or more stories high despite laws limiting them to 68 feet (21 metres), under Augustus, and then 58 ft, under Trajan. The street level characteristically housed artisans' workshops and commercial establishments. The residences above were reached by an interior common staircase, receiving light and air from the street and an inner court. Many insulae were encircled with open or enclosed balconies of wood or concrete. Pumping devices could raise water only to lower apartments; tenants of higher apartments had to use public water and sanitary facilities. Cheap construction and a limited water supply caused frequent collapses and serious fires.