Hygrothermal simulation is an important tool in the practice and policy of deep energy retrofit and other aspects of design. A key constraint on its use is availability of material data, in particular absorption coefficient. Practitioners must usually select data from databases with little or no useful information for historic materials. Improved automatic methods, such as those used here, have existed for over two decades; a further refinement is proposed. Data for two types of limestone from Lincolnshire, UK are presented, and the automatic method is compared with the manual method.