A fill pack having plurality of sheets (12, 14, 100) each having first, second, third and fourth edges along, for use with a heat exchange apparatus having a first axis and a second axis, wherein each sheet (12, 14) generally lies in a plane that is generally parallel to the first axis and wherein the plurality of sheets (12, 14) combine to form a plurality of conduits, each sheet (12, 14) further comprising: a first vertical rib that extends in a first direction generally parallel to the first axis along the sheet (12, 14), substantially all the way between the first and second edges, wherein said first vertical rib protrudes in a second direction out of the plane; a second vertical rib that extends in the first direction along the sheet (12, 14, 100), substantially all the way between the first and second edges generally parallel to said first vertical rib, wherein said second vertical rib protrudes in the second direction out of the plane; a first horizontal rib that extends in a third direction generally parallel to the second axis along the sheet (12, 14, 100) substantially all the way between the third and fourth edges, wherein said first horizontal rib protrudes in a fourth direction opposite said second direction, into the plane, wherein said first horizontal rib intersects said first vertical rib; a second horizontal rib that extends in the third direction along the sheet (12, 14, 100) substantially all the way between the third and fourth edges generally parallel to said first horizontal rib, wherein said second horizontal rib protrudes in the fourth direction opposite said second direction, into the plane, wherein said second horizontal rib intersects said second vertical rib, wherein said first and second vertical ribs and said first and second horizontal ribs form a boundary that surrounds at least one generally planar region, characterised in that said at least one generally planar region further comprises performance ribs disposed thereon at an angle to the first axis, at least one performance rib being continuous from one horizontal rib to one vertical rib. a first vertical rib that extends in a first direction generally parallel to the first axis along the sheet (12, 14), substantially all the way between the first and second edges, wherein said first vertical rib protrudes in a second direction out of the plane; a second vertical rib that extends in the first direction along the sheet (12, 14, 100), substantially all the way between the first and second edges generally parallel to said first vertical rib, wherein said second vertical rib protrudes in the second direction out of the plane; a first horizontal rib that extends in a third direction generally parallel to the second axis along the sheet (12, 14, 100) substantially all the way between the third and fourth edges, wherein said first horizontal rib protrudes in a fourth direction opposite said second direction, into the plane, wherein said first horizontal rib intersects said first vertical rib; a second horizontal rib that extends in the third direction along the sheet (12, 14, 100) substantially all the way between the third and fourth edges generally parallel to said first horizontal rib, wherein said second horizontal rib protrudes in the fourth direction opposite said second direction, into the plane, wherein said second horizontal rib intersects said second vertical rib, wherein said first and second vertical ribs and said first and second horizontal ribs form a boundary that surrounds at least one generally planar region, characterised in that said at least one generally planar region further comprises performance ribs disposed thereon at an angle to the first axis, at least one performance rib being continuous from one horizontal rib to one vertical rib.