In this paper, two ways to improve the PVC window frame thermal transmittance without frame geometrical dimension and material variations are presented. The first variation considered relies on inserting polyurethane foam into the air gaps. The second variation counts on low-emissivity coating on PVC surfaces in the frame air gaps. To investigate these modifications two-dimensional CFD simulations of PVC window frames were used which then were validated by measurements performed in a calorimetric chamber. A hot box methodology was implemented for the measurements. The experimental work was focused on verification of simulation results of modeled frame thermal transmittances. A calorimetric chamber was used, consisting of a metering box, simulating indoor conditions (warm side), and a climate box, simulating outdoor conditions (cold side). It was concluded that the air gap filling with polyurethane foam in window frames can reduce window frame thermal transmittance by about 27% while covering PVC surfaces with low-emissivity coating can reduce window frame thermal transmittance by about 28%. These variations do not change the frame geometry or its total thickness. To answer the question what would be the expected impact of proposed frame variations on annual energy demand of different types of buildings additional simulations of two buildings with a Design Builder with EnergyPlus engine software were performed for a set of different climatic conditions. (C) 2017 Elsevier B.V. All rights reserved.