A heating apparatus includes an elongated flexible envelope (4) comprising two sheets (6), (8) that are attached to each other along permanent side seams (10), (12) and along a permanent bottom seam (14) and a moisture-activated chemical heater (16) disposed inside envelope (4), wherein envelope (4) is folded over on itself and secured by temporary side seams (18), (20) and a permanent top seam (22) to form a continuous moisture barrier surrounding the moisture activated chemical heater (16) and wherein heater (2) may be opened for use by removing top seam (22) and unfolding envelope (4) by unpeeling temporary seals (18), (20) to provide unfolded envelope (4), wherein sheets (6) and (8) remain sealed along permanent seams (10), (12) and (14), but wherein envelope (4) now has an open top end (30).