A sheet for use in a heat exchange apparatus. The sheet includes a first vertical rib that extends in a first direction and protrudes in a second direction out of the plane. The sheet also includes a second vertical rib that extends in the first direction along the sheet, between the first and second edges of the sheet. The second vertical rib also protrudes in the second direction out of the plane. The sheet further includes a first horizontal rib that extends in a third direction along the sheet between the third and fourth edges of the sheet, and protrudes in a fourth direction opposite said second direction. The sheet additionally includes a second horizontal rib that extends in the third direction along the sheet between the third and fourth edges of the sheet. The second horizontal rib protrudes in a fourth direction and intersects said second vertical rib.