Membranes with tunable gating functionality can be produced by modification with thermo-responsive polymers and colloids. We propose a straightforward method to gain this responsiveness by adsorption of microgels to the membrane surface. To be able to predict the filtration properties of the resulting composite membrane, a comprehensive understanding of the behavior of adsorbed microgels on porous substrates under convective flow is required. Previous studies focus on constant pressure experiments at various temperatures neglecting changes in viscosity and flux. In this study, we investigate the permeability and selectivity of poly-n-isopropyl-acrylamide ( PNIPAM) monolayer microgel membranes under convective flow in cross-flow mode applying a constant flux. We use in-situ spectroscopic ellipsometry on a porous substrate to prove a reproducible and confluent monolayer microgel deposition. Supporting scanning electron micrographs and confocal laser scanning micrographs confirm the high reliability of the ellipsometry data set. The monolayer microgel membranes show distinct switching between 25 and 50 degrees C. When increasing the temperature above the microgel's volume phase transition temperature, the molecular weight cut-off (MWCO) of the functionalized membrane shifts to lower molar masses controlling the dextran retention between 30% and 70% only when the pore size of the support is in an appropriate range. The confluent monolayers are also mechanically stable when backwashed. Our data give insight into the responsive behavior of the PNIPAM network under convective flow - a key issue for custom-made filtration tasks.