Thermal performance discrepancies between theory and practice in buildings is a well known issue. Reducing this gap is crucial for enhancing energy efficient building construction and renovation. To measure the actual transmission heat loss coefficient of a building from in situ testing, an evaluation of time-varying infiltration losses needs to be quantified to improve the result’s precision, especially for short dynamic methods. This study first presents a theoretical analysis to explain why this evaluation is necessary and how to perform it using different technical approaches (simplified aeraulic models and tracer gas). An experimental comparison of four of these solutions on a small shed reveals that blowerdoor and wind speed measurements can be sufficient to evaluate infiltration losses with acceptable accuracy. Results of the identified transmission losses coefficient Htr using the In Situ Assessment of Building EnvoLope pErformances (ISABELE) method shows how much the bias is reduced by including permeability specifications and local wind speed measurements. ISABELE is a measurement method of the overall transmission heat loss coefficient of a building.