A freezer structure (12) includes a floor structure (20), side walls (22) and a ceiling (24), each of which includes a substantially hollow inner or upper layer or zone (30) behind or beneath which is disposed an intermediate layer or zone (32) substantially filled with insulating filler material and a bottom or outer layer or zone (34) that is substantially hollow. A monitoring system (16) monitors the temperature, moisture level and pressure of a substantially dry gas circulating through the first layer or zone to maintain the first layer or zone in substantially dry condition.