Heat-sensitive element comprising (a) an optionally pre-treated substrate (b) a positive working heat-sensitive coating comprising (i) at least one novolak resin, (ii) at least one component which reduces the aqueous alkaline developer solubility of novolak, wherein said reduction in solubility is reversed upon the application of heat, and (iii) at least one acidic polyvinyl acetal comprising the structural units (A), (B), (C) (A) (B) (C) and (D) wherein (D) is at least one unit selected from (D-1), (D-2), and (D-3): (D-1) (D-2) (D-3) wherein R1 is a hydrogen atom or a C1-C4 alkyl group, R2 is a hydrogen atom or a C1-C18 alkyl group, R3 is a hydrogen atom or a C1-C4 alkyl group, R4 is a hydrogen atom or a C1-C4 alkyl group, R5 is -COOH, -(CH2)a-COOH, -O-(CH2)a-COOH, -SO3H, -PO3H2 or -PO4H2, a is an integer from 1 to 8, and X is selected from -(CR6R7)n- and -CR8=CR9- wherein n is an integer of 1 to 6, each R6 and R7 is independently selected from a hydrogen atom and C1-C6 alkyl group, and R8 and R9 are independently selected from a hydrogen atom and a C1- C6 alkyl group or R8 and R9 together with the two carbon atoms to which they are bonded, form an optionally substituted aryl or heteroaryl group, wherein components (i) and (ii) do not have to be present as separate substances but may be used in the form of an appropriately functionalized novolak.