The thermal conductivity of building envelope materials varies according to the conditions of temperature and moisture content. Precisely determining the thermal conductivity is a primary challenge in developing more appropriate, realistic thermal designs for buildings. However, a detailed implementation of environmental conditions requires either the use of sophisticated software programmes and the collection of a large set of climate data at each location or a laborious analytical calculation. To simplify the thermal design of buildings, regulations use normative conductivity values based on constant, standardised conditions of temperature and moisture content, which are independent of ambient conditions. This article proposes a correction factor that considers these conditions to functionally approximate the design thermal conductivity of materials based on their normative values. For this calculation, a procedure that simplifies the standard ISO 10456:2007 using the mean annual temperature and relative humidity data for each location is proposed. The procedure is applied in 52 Spanish cities to correct the conductivity values established by the Spanish regulation for facade materials. A correction map of Spain is presented. Finally, by using the ISO standard, the corrections are validated by comparing the values with the design thermal conductivities calculated for different case studies. (C) 2014 Elsevier B.V. All rights reserved.