Method of kerfing insulation boards to form duct liners

A method of kerfing insulation boards for forming ducts of preselected transverse dimensions having round or flat oval transverse cross sections, comprising:

• providing a rigid fibrous insulation board for forming a duct of preselected transverse dimensions;
• said insulation board having a coated first major surface forming an interior surface of a duct formed from said insulation board and a second major surface forming an outer surface of a duct formed from said insulation board;
• said insulation board having a length extending parallel to a longitudinal centerline of a duct formed from said insulation board, a width and a thickness;
• forming a plurality of kerfs in said first major surface of said insulation board which make said insulation board flexible in a direction of said width of said insulation board and foldable into a tubular configuration of preselected transverse dimensions having a round or flat oval transverse cross section;
• each of said kerfs having a depth;
• each of said kerfs extending parallel with respect to and being spaced from other of said kerfs;
• each of said kerfs extending said length of said insulation board;
• each of said kerfs having sidewalls which extend from a kerf base portion adjacent but spaced inwardly from said second major surface of said insulation board to said first major surface of said insulation board;
• each of said kerfs having shoulder portions adjoining said first major surface;
• and each of said kerfs having a width at said first major surface of said insulation board said width of said kerfs at said first major surface of said insulation board pressing said shoulder portions of said kerfs into abutment with said insulation material in said shoulder portions in compression, when said insulation board is formed into a duct of said preselected transverse dimensions with said round or flat oval transverse cross section, to retain said insulation board in said round or flat oval transverse cross section with no exposure of said kerf sidewalls at the interior surface of the duct and any loose fibers or dust from said kerf sidewalls confined within said kerfs;
• and
• controlling said flexibility of said insulation board in said direction of said width of said insulation board by at least one of the following:
• said depth of said kerfs formed, said widths of said kerfs formed at said first major surface, and said spacing formed between said kerfs.