Building infiltration measurement is a challenge. To address current challenge, a scalable and low cost Building Infiltration Estimator with Ultrasonic Thermometry (BLAST) is proposed. The proposed method contains the physical measurements and a model-based estimation. This paper is focusing on the model-based estimation part, which uses an Extended Kalman filter (EKF) to inversely estimate the building infiltration using measurements of surface temperature and total heat flux, and a low-order state-space model. An EnergyPlus-based emulator is used to generate a virtual building and measurements to test the proposed estimation method. Nearly 80% estimated infiltration resistances are within the 20% error band compared to the calculated infiltration resistance from EnergyPlus. This preliminary study shows the EKF based estimator with the proposed measurements is promising for building infiltration estimation.