One of the most common measures of improving energy efficiency in buildings, both new and within the energy renovation of existing ones, is setting up thick layers of thermal insulation. For this purpose, various insulation materials, mostly expanded polystyrene, have been used. Because of some environmental issues, more versatile ecological materials are becoming more attractive in building industry. In the renovation of the insulation in buildings, it should be decided whether to erect new insulation over the old one or to replace it after completely removing the old insulation. This decision mainly depends on the old insulation and the situation between the insulation and the outside wall. The moisture in the space between insulation panels and outside wall makes an appropriate environment for growth of microorganisms. To detect and measure the moisture content in the space between the insulation panel and the outer wall, it has been necessary to apply destructive methods. In this work, a nondestructive method was developed for measurement in the insulated walls. The measured values are the air temperatures and humidities inside and outside the building and in the space between the insulation panels and outer wall. A new instrument was developed and used to examine the hygrothermal properties of the building envelope and the results were mathematically analyzed. (c) 2012 Elsevier B.V. All rights reserved.