performance of several topics for buildings up to five floors. The acoustic performance is one of them. The standards are in approval process and measurements in real buildings will be necessary. Different professionals using different equipment will emit certificates establishing which levels of insulation a certain flat provides and its uncertainties. The expanded measurement uncertainty is necessary to make different measurement results comparable. The international Guide to the Expression of Uncertainty in Measurement, the ISO/IEC Guide 98 (GUM), is a widely accepted document to assess and evaluate the uncertainty of a measurement result, and was used in this work. The standards concerning sound insulation measurements are ISO 140 series and ISO 18233. Uncertainty estimates are available only for the classical technique described in ISO 140, based on repeatability and reproducibility tests performed in laboratories. Field measurements present some characteristics that can contaminate the results. Independent measurements were carried out in a single floor building using ISO 18233 specifications and the ISO/IEC Guide 98 was applied to obtain the uncertainty for measurement results of airborne sound insulation between rooms in situ.