Nowadays, many performing insulation materials are available on the market. However, their expected thermal performance can be affected by many sources of uncertainty due to random errors that can occur during the manufacturing and the measurement processes. In addition, the thermal performance is strongly affected by another source of uncertainty related to the insulation laying process. As a matter of fact, defects in insulation panels are introduced either for practical reasons or due to a lack of rigor of workers. These errors are still not yet properly considered for simulation, although they result in significant heat losses. This work aims at investigating the impact of four common workmanship errors on the thermal performance of insulation panels. A coupling between experimental measurements and finite element modeling allows us to evaluate the effective thermal conductivity of insulations in presence of defects. The uncertainty analysis allows us to quantify the dispersion of insulation conductivity according to different types and sizes of defects, showing that flexible materials seem to be more affected by the defects. For simulation purpose, analytical models are proposed to assess the effective thermal conductivity in terms of size and type of defects. The numerical application to insulated wall shows the impact of each defect on the energy consumption, where deep grooves and openings are found to be strongly affecting the insulation performance. (C) 2015 Elsevier Ltd. All rights reserved.