Concrete Experimentation

A Special Door

With one exception, all doors are 35 mm thick (1-3/8”) solid concrete. They offer the major advantages of dimensional stability and resistance to the elements; they do, however, have two disadvantages, present because of lack of insulation: heat loss/gain and condensation. (The latter was mostly a winter problem before we installed a centralized heating system.)

Although these are not serious problems in the mild climate of Kythnos, we decided that attempting an insulated door was in order. We settled upon the door for the guest bedroom of GBB III (the third structure), which was being developed into a master bedroom suite with the addition of a bathroom. As this was seen as the most “luxurious” space of the compound, we thought that a more elaborate door was also appropriate.

The design resulted in a door 80 mm thick at the perimeter, comprising four layers: 35 mm of concrete reinforced with a 50 x 50 mm galvanized wire mesh; 30 mm of rigid (extruded polystyrene) insulation; a 5 mm plexiglass sheet, and a 1 mm copper sheet. The concrete extends to the full dimensions of the door vertically and horizontally and forms a tray with a 40 mm thick frame, into which the insulation, plexiglass and copper sheet are placed. The purpose of the plexiglass is to provide a rigid surface as a backing for the copper sheet.

The door was cast in one operation, with the copper, or outside, door surface against the bottom of the form, and the inside concrete door surface screeded smooth. White cement was used for both the door and its frame, and was left unpainted but sealed with a primer, which is clear and soaks into the concrete.

The copper sheet is held flat along its perimeter by a continuous z angle, custom bent from a piece of the same sheet; the z angle is cast into the concrete, forming a reveal that covers the edge of the copper sheet and allows space for expansion.

The copper is further held in place with a grid of brass studs, 10 cm on center, which go through the door so that the same grid is formed inside, on the concrete surface. The studs comprise a 10 cm long piece of 8 mm-diameter threaded rod held in place with a brass washer and acorn nut on each side. The rods pass through the door in plastic tubing that was installed during the casting process.

While it seems contradictory to introduce thermal bridges when thermal insulation was one of the original objectives in designing the door, the overall visual impact is worth the compromise. (Besides, the vines and the bougainvillea on the arbor outside are maturing fast enough to soon provide full shade during the most critical noon hours of the summer.)