1. One room – different thermal environments

An air temperature of 21 °C does not guarantee that every location in a room feels the same. Near a large pane of glazing that is cold in winter, the surrounding surfaces, local radiant environment and air movement may differ from those at the centre of the room. An air-temperature thermometer does not fully capture these differences.

The factors that matter for people include air temperature, mean radiant temperature, air speed, humidity, clothing and activity. Under defined conditions, operative temperature combines the effects of air temperature and the radiant environment. Local discomfort, for example from draughts or radiant asymmetry, can occur even when the average operative temperature is acceptable [1, 2, 6].

2. The cold interior glass surface

In winter, the interior glass surface is usually colder than the room air. It affects heat transfer to the adjacent air and exchanges longwave thermal radiation with people, the floor and other room surfaces. This is different from solar radiation transmitted through the glass.

The temperature at the centre of the glass must be distinguished from temperatures at the glass edge, frame and installation joint. A good glazing Ug-value does not guarantee that the frame, insulating-glass edge seal or lower junction reaches the same surface temperature.

3. Calculation example: how cold is the centre of the glass?

A simplified steady-state estimate for the central glass area, without solar radiation: indoor air 21 °C, outdoor air 0 °C, internal surface thermal resistance Rsi = 0.13 m²K/W. The interior surface temperature is approximately θsi = θi − Ug · (θi − θe) · Rsi. The Ug-values used describe the glazing, not the whole window.

The results are 13.36 °C, 18.00 °C and 19.36 °C, or 13.4 °C, 18.0 °C and 19.4 °C when rounded. These are calculated values, not measurement data from the cited studies. Different surface heat-transfer conditions, insulating-glass edge seals, convection, radiation and actual installation conditions change the local temperatures.

4. Radiation: why proximity and geometry matter

A person exchanges thermal radiation with all visible room surfaces. Cold glazing has a particularly strong influence on local mean radiant temperature when it occupies a large part of the field of view. This contribution is described by geometric view factors. The same pane may therefore be more relevant to a person directly in front of it than to someone at the opposite internal wall [1].

Radiant asymmetry describes an uneven radiant environment in different directions. It must be distinguished from air temperature: a body can lose more heat by radiation towards a cold surface even when the air throughout the room is at 21 °C.

Ordinary window glass is largely opaque to the longwave thermal infrared radiation relevant at room temperatures. The floor therefore exchanges longwave thermal radiation primarily with the interior glass surface, rather than directly through ordinary glass with the cold night sky. Different optical properties apply to shortwave solar radiation.

5. Convection: cold downdraught is a separate mechanism

Room air adjacent to cold glazing cools, becomes denser and may descend along the pane. This flow is referred to as cold downdraught. Whether it is perceived as a draught depends, among other factors, on glazing height, surface temperature, air speed and the arrangement of the heating system. Floor-to-ceiling windows can extend the path of the descending air into the occupied zone [4].

Radiative heat exchange and cold downdraught may occur simultaneously, but they are physically different. Improving one mechanism does not automatically demonstrate an equivalent improvement in the other.

6. What measurements and simulations show

Zmeureanu and colleagues developed and validated a model that accounts for local mean radiant temperature and thermal sensation near windows. Validation used a climatic chamber with electrically heated surfaces representing warm windows; it was not a direct measurement of a particular window currently on the market [1].

Bessoudo and colleagues investigated a large glazed façade with interior shading systems in winter, including surface temperatures, operative temperature and radiant asymmetry. In Part II, Tzempelikos and colleagues used the validated thermal model for comfort simulations. The results depend on the weather, solar radiation and properties of the coverings in each case [2, 3].

In 2017, Joe and colleagues analysed downdraught along glazed façades with underfloor heating using computational fluid dynamics. The results show that the arrangement of the heated floor area and heat exchange with the descending air are crucial. This is a CFD study, not a measured FVS experiment [4].

Oliveira and Corvacho compared thermal conditions at the centre of classrooms and offices with those near windows. The measurements showed greater variability in comfort near the glazing. This supports the need for comfort assessments that account for location within the room [5].

7. The floor directly in front of the window

An often overlooked consequence of large glazed areas is the local temperature distribution at floor level. Radiative exchange with a cold window surface, convective airflows and the construction of the lower junction can jointly influence the surface temperature in the window zone.

Karlsson reported six experiments in a controlled test room with different window positions and heating systems. The experimental series explicitly varied the window U-value, window orientation and heating system. With vertical glazing extending down to floor level in combination with underfloor heating, the floor surface directly in front of the window was measured to be 1.8 °C colder than the floor to the side; air speed did not exceed 0.15 m/s at any measurement point. This was a specific experimental configuration, not a universally applicable temperature difference for all floor-to-ceiling windows [7].

8. Frame, glass edge and lower junction

Heat flow through a window is not distributed uniformly. Frame material, spacers, installation joints, reveals and the floor junction can lower local surface temperatures and affect the risk of draughts or condensation. Assessing the centre of the glass alone therefore does not replace an examination of the entire window recess. Even well-insulated glazing can have a cold local area where junction details are unfavourable.

9. Technical implications and dynamic thermal protection

Technical measures can address different mechanisms: better glazing and warmer interior surfaces, well-designed edge junctions, suitable heating surfaces and additional interior coverings. For interior coverings, airtightness and airflow management are just as important as thermal resistance.

Depending on its construction, dynamic thermal protection could affect thermal transmittance, the temperature of the surface facing the room, exposure to the cold pane and local airflows. Its open position can temporarily allow solar energy to enter. However, the studies discussed here do not directly test the present-day movable FVS.

The research overview ‘Interior Thermal Protection for Windows – What Scientific Studies Actually Show’ examines coverings and air cavities. The more detailed publication ‘Radiant Comfort in Front of the Window’ separately examines the possible influence of low-emissivity surfaces.

10. Conclusion

The U-value remains important, but it does not fully describe how a window affects people and the adjacent room environment. The temperature of the interior glass surface, the spatial distribution of thermal radiation, air movement and junction details jointly determine thermal comfort near windows. The same room-air temperature therefore does not automatically mean equally comfortable conditions at every location.