Measurements of 226Ra using gamma spectrometry are associated with certain difficulties. In this work we present an improved and validated method using a radon tight geometry for the measurements of the radon progeny 214Pb. The objective was to find a radon tight material which allowed vacuum packing of an existing, and already calibrated, geometry. Among the material tested none were completely radon tight, but calculations on the diffusion resulted in a correction factor for a specific geometry. Furthermore, this study shows that two measurements were enough for determination of 226Ra. The first measurement performed as soon as possible after vacuum packing (4 h-1 d), and the second after radioactive equilibrium has been established (approximately 26 d).