An insulating enclosure for a ceiling opening of the kind typically provided for a pull-down folding stair provides an inexpensive and simple structure for enclosing and insulating the opening. The structure comprises wall portions and a top portion overlying the wall portion when the apparatus is operatively disposed, the wall portions and top portions comprising modular insulating block-like members. The block-like members may be made of foam, encased in corrugated board or plastic sheet, interconnected by tabs or tape.