For measurements, the prewarmed (40°C, 1 h) and rehydrated (saturated water vapor atmosphere) samples were placed in home-built sample cells with Teflon sealing and either PE foil or kapton windows. A setup was applied at which the RH could be adjusted by PID control, remaining stable within 0.1% (Aeffner et al. 2009 ). The temperature (T) of the sample chamber, the water reservoir, and the pipings was controlled as well. The RH/T sensors and mass flow controllers were interfaced with the diffractometer controls (in-house and beamline), enabling the usage of long and fully automated scan macros including RH and T as parameters. Unless otherwise stated, GID experiments were carried out at RH = 94%, and the reflectivity experiments were performed at RH = 90%, both at 20°C. The chambers were mounted to the respective goniometers with the sample oriented either horizontally (beamline) or vertically (in-house) depending on the diffractometer setup. The X-ray beam enters and exits the chamber through kapton (in-house) or PE foil (beamline) windows.