Roof lights can be very useful in buildings because of the direct daylight that enters the building. But the other side of roof lights is the overshoot on solar gain that can cause overheating in buildings. In climates with cold nights without clouds, the roof light temperature will go down and a lot of internal heat will disappear by radiation. This can be useful in summer but probably not in winter. Spaces with roof lights may require more ventilation to keep the indoor temperature acceptable without air-conditioning.
Basic designs of roof lights.
Several options for roof lights are:
- flat glass roof almost at the same level as the roof,
- tilted glass roof that comes above the surrounding area or
- lifted glass roof (flat or tilted) that have vertical glass or openings for ventilation.
Three possible roof light constructions.
We have to think about the following aspects: daylight, solar gain, radiation and ventilation.
Daylight is in general welcome for most office or work spaces. For the design there are two aspects that have to be considered. These are brightness and contrast (glare). In a working environment with computers the light from the ceiling can be too much to work comfortable behind a screen. Especially reflection of light in the computer screen is an issue. The contrast between the background and the ceiling can be too high and will give complaints and let people’s eyes get tired. So some semi-transparent screening can improve the situation and solve brightness and contrast.
Shading lamellas for roof light.
Solar gain can be useful or not depending on the location and the season. The summer sun is more powerful and more direct then the winter sun. To gain passive solar energy in the winter and to keep excessive solar gain out in the summer, the roof light design should be modelled for this situation. Outside shading lamellas that have the right angle for the winter situation can let passive solar energy enter the building while reflecting it in the summer.
Thermal radiation. This will happen in land climates that have a large temperature difference between day and night. Without any reflection (from clouds) the radiation will be endless and can make the roof light very cold. A problem that can occur is condensation. In case of high day temperatures and humidity, the temperature of the roof light will drop down and will be a perfect place for condensation. This problem ask for good detail design and suitable materials that withstand the moisture.
Ventilation. Ventilation of a construction can solve or prevent many problems. Not only will the excessive heat disappear but also the humidity can be regulated. In general a roof light should always have a ventilation possibility. Because it is easier to make ventilation openings in a vertical are then in a horizontal area, it is preferred to lift the roof light enough to have space for ventilation openings.
Relation with BIPV.
To integrate BIPV modules in roof lights is a logical solution. The solar modules can reduce the amount of daylight, the amount of solar gain and the thermal radiation. In fact it does not make a big difference if the solar modules are integrated in the roof lights or placed above the roof lights, although placed above the roof lights will create a ventilated cavity that makes shading more efficient.
Solar shading lamellas placed above the glass at Zürich airport.
Integrated BIPV module in lifted roof light (De Kleine Aarde Boxtel NL)
Ventilation windows under PV roof (De Kleine Aarde Boxtel). Integrated BIPV module without ventilation.
Sky lights with PV shading and Skylights with ventilation grills or lamellas.