The window frame, bedded in or fastened to the warm interior wythe, is now relieved of the. substantial edge-cooling effect of the former arrangement. Advantage can be taken of the inside metal sill to collect and conduct heat to the frame, and a thermal break may be incorporated on the outside to minimize the loss of heat in winter. The relocation of insulation by itself effects very considerable improvement in relieving the frame and inner wythe of temperature variations. If, however, the insulation is merely inserted between the main wythe and the cladding, the latter being made continuous by the use of mortar at all joints as before, two potential difficulties remain. The cladding and the main wythe will still tend to move in relation to one another in accordance with their respective temperature changes.
Cracks are almost certain to form in the cladding and perhaps also in the main wythe.
Further, the cladding, being more or less continuous, is likely to provide at least a partial barrier to wind pressure so that pressure differences across it can develop and cause rain to enter wherever cracks exist. The need for some further improvement in the design is thus indicated. It may be set out to form an air space and supported by ledger angles and ties as before. The air space, being heavily vented by suitably designed open joints at both horizontal and vertical intervals, will at all times follow closely the outside air pressure so that the rain screen is substantially relieved of wind pressure differences. This not only removes the major force causing rain to penetrate the cladding, but also eliminates the wind loads on it.