A parametric study of window-to-floor ratio of three window types using dynamic simulation

The windows can be responsible for unnecessary energy consumption in a building, if incorrectly designed, shadowed or oriented. Considering an annual thermal comfort assessment of a space, if windows are over-dimensioned, they can contribute to the increase of the heating needs due to heat losses, and also to the increase of cooling needs due to over-exposure to solar radiation. When under-dimensioned, the same space may benefit from reduced heat losses through the glazing surface but does not benefit from solar radiation gains. Therefore, it is important to find the optimum design that minimizes both the heating and cooling needs. This paper presents a parametric study of window type (single, double and triple glazing), orientation and opening size, located in the city of Coimbra, Portugal. An annual and a seasonal assessment were done, in order to obtain the set of optimum values around 360 degree orientation.

💡 Research Summary

The paper presents a comprehensive parametric investigation of how window‑to‑floor ratio (WFR), glazing type, orientation, and opening size influence the heating and cooling energy demands of a typical office space located in Coimbra, Portugal. Using a dynamic simulation platform (EnergyPlus 9.5), the authors generated a matrix of 1,344 design scenarios by combining three glazing options (single, double, triple), eight cardinal orientations (N, N‑NE, E, SE, S, SW, W, NW), seven WFR values ranging from 0 % to 70 %, and three opening‑area fractions (10 %, 30 %, 50 %). The building model follows standard office geometry (150 m² floor area, 3 m ceiling height) and incorporates typical internal loads for lighting, equipment, and occupants. Climate data for Coimbra (1991‑2020 averages) provide hourly outdoor temperature and solar radiation for a full year of simulation.

Key findings can be summarized as follows:

1. Orientation‑dependent optimal WFR – In the south‑facing façade, solar gains dominate; a WFR between 45 % and 55 % minimizes heating demand while keeping cooling demand manageable, provided high‑performance glazing is used. Exceeding 60 % WFR leads to excessive solar heat gain and a sharp rise in cooling load. For north, east and west façades, where solar exposure is limited, increasing window area primarily raises heat loss. Here the optimal WFR stays below 15 % to keep heating loads low; beyond 30 % the heating demand grows almost linearly.

2. Impact of glazing type – Triple glazing (U‑value ≈ 1.2 W/m²·K) offers the greatest resistance to heat transfer, effectively mitigating both winter losses and summer gains when large windows are required. Double glazing (U ≈ 2.8 W/m²·K) provides a balanced cost‑performance solution, especially for low‑WFR façades. Single glazing (U ≈ 5.8 W/m²·K) is only viable for very small window areas because its high heat‑transfer coefficient dramatically increases both heating and cooling loads.

3. Opening‑area fraction effects – Larger operable openings improve natural ventilation and can reduce summer cooling loads, but they also increase infiltration of cold outdoor air during winter, raising heating demand. The trade‑off is most pronounced with high‑performance glazing, where the benefit of ventilation can be partially offset by the glazing’s low U‑value. A 10 % opening fraction yields the lowest overall energy use for north‑oriented façades, while a 30 %–50 % fraction may be justified on the south side if summer cooling is a priority.

4. Seasonal load distribution – Annual energy consumption is split roughly 55 % heating, 35 % cooling, and 10 % internal loads and ventilation. Consequently, design optimization must prioritize heating reduction while not neglecting the cooling penalty on sun‑exposed façades. Seasonal analysis shows that the winter optimum WFR for north/east/west façades lies between 15 % and 20 %, whereas the summer optimum for the south façade is 45 %–55 %.

Based on these results, the authors propose practical design guidelines:

• For south‑facing windows, adopt triple glazing, target a WFR of 45 %–55 %, and limit operable opening area to 10 %–30 % to balance solar gain with heat‑loss control.
• For north, east, and west façades, use double or triple glazing with a WFR no greater than 15 %–20 % and keep operable openings at the minimum (≈ 10 %).
• Apply orientation‑specific WFR values in whole‑building energy models rather than a uniform global ratio, as this yields the greatest annual energy savings.

The study acknowledges limitations, including the focus on a single building type, a single climate zone, and the exclusion of shading devices, detailed cost analysis, and internal load variability. Future work is suggested to expand the parametric framework to multiple building typologies, incorporate shading and daylighting controls, and perform multi‑objective optimization that includes economic and environmental criteria.