One of the key performance parameters for any energy efficient window product is the center of glass U-value or thermal transmittance. This paper presents a new methodology to compute the thermal performance of fenestration systems incorporating partitioning radiant energy veils (TM) between two panes of a window. A conduction heat transfer model is used for glazing layers coupled with radiation and convection models at the layer boundary surfaces. Partitioning radiant energy veils (TM) are treated as individual layers with effective radiative properties. The effect of a wide range of optical properties, pane thicknesses, pane distances and different gas fill types on thermal transmittance of the window are considered. The novel use of partitioning radiant energy veils (TM), having application specific properties, between two panes of a window allows to customize the thermal performance of a triple or quadruple window. It is shown that using one or two internal partitioning layers, with appropriate pane/inter-layer distances and high spectrally reflective metallo-dielectric coated layers, results in highly enhanced U-Values.