Light through clouds

The Window as a “Solar Panel”: Light Transmittance & Solar Factor G

In this article, we cover two very important aspects of glazings: light transmission and the solar heat gain coefficient.

These parameters are extremely important for performing buildings and passive houses, however, they are often overlooked by both designers and window manufacturers.

We already covered a variety of topics on windows and glazings as part of the thermal envelope, such as the thermal transmittance of glass, Ug, the glass edge thermal bridge PSIg, as well as finite elements analysis to estimate design surface temperatures (fRsi factor), in order to avoid the risk of mold and condensation.

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IGU faces
The design of an insulated glass unit (IGU).

We’re going to describe how silly it is to evaluate the quality of a window or a door by its average Uw value alone, even if it is calculated according to ISO 10077-2. About ISO 15099, we reserve judgment until we gain more confidence with the norm.

Thinking of window quality through its Uw value is as silly as saying that a zebra is a “light grey” or “dark grey” horse: it does not make sense.

Plain’s Zebra at Kruger National Park
If you think that a zebra is a grey horse, then the Uw value is all you need.

A mediocre window may look to be performing well, due to a very low transmittance value of glass, Ug, regardless of the fact that you may have condensation (or ice!) forming along the edge of the glass.

Close up of window
Ice forming along the edge of the glass, on the room side, due to very low window quality.
Square window with water view
Window in a passive house near Riga, Latvia.


What’s the purpose of an opening? In a building, windows and doors:

  • connect the inside to the outside;
  • contribute to the overall aesthetic of the building;
  • create views;
  • allow for natural ventilation;
  • provide daylight to the interior environment.
Palazzo Rucellai in Firenze, designed by Leon Battista Alberti. Doors and windows have always played a fundamental role in the aesthetic composition of architecture.

The aesthetic value of openings in a building is undeniable.

As far as natural ventilation, we dedicated a previous article, Meet the Mechanical Ventilation System, to describe how this is not enough to guarantee comfort conditions in a building. Natural ventilation through building openings has therefore to be integrated, to prevent the risk of mold and/or condensation.


Daylight requirement and energy performance of glass units may be in contrast with one another. To lower the thermal transmittance of glass, Ug, insulated glass units (IGUs) are created with double or triple glass panes, with low-e coatings (made of metal oxides).

With the same type of glass, the more the number of panes, the less light gets through. Choosing the type of glass (e.g. extra clear glass), can have a very important effect on the quality of the internal environment, in terms of daylight.

As far as low-e coating, if their quality is not excellent (standard coatings), a low thermal transmittance Ug is associated with a low light transmittance.

In order to conciliate low thermal transmittance and high light transmittance, the low-e coatings need to be “selective”. They need to be able to reflect the infrared component of the spectrum and transmit the visible light. This is why a proper specification of a glass unit includes the light transmission value (to be 70% or higher), besides the thermal transmittance Ug.

Daylight visualizer
Example of a daylight simulation using Velux Daylight Visualizer.

We’re going to publish an article on daylight, and how this is affected by the type of IGU (double pane vs triple pane; standard low-e vs selective).


Solar heat gains represent the portion of total solar radiation (visible light + infrared), that is transmitted through a structure, and penetrates into the building. For insulated opaque assemblies (walls, roof, etc.), the amount of transmitted solar radiation is negligible; for glazed elements, it is extremely important.

The term “passive” itself derives from the fact that a passive house is such an efficient thermal envelope, that the solar heat gains become a determining factor (as well as the interior heat gains).

The solar factor g, also called SHG (solar heat gains) represents the amount of total solar radiation transmitted by the IGU – it is expressed as a percentage, or as a number (from 1,00 to 0,00). As in the case of the thermal transmittance of glass, Ug, the solar factor g also has to be expressed with two decimal figures.

In a well-designed building, the openings can become the most cost-effective “solar panels”, and provide “free gasoline” (solar gains), to keep the building comfortable in winter and reduce the demand of energy for heating.

The solar factor g represents the amount of “free gasoline” provided by the windows to the building.

Let’s get used to seeing the solar factor g as “free gasoline”, provided by the glazed components of the thermal envelope (Image source: TurboSquid)

The solar factor g is a very important element in the energy design of the entire building, next to the thermal transmittance of the glass, Ug, and the light transmission.

The question a designer needs to ask is: does the building need solar gains?

  • In residential building, even in moderately warm climates (e.g. Northern Italy): probably yes;
  • In non-residential buildings, with high interior heat gains (high number of people, appliances, etc.): maybe no.

The solar factor has to be designed as an integral part of the thermal envelope so that its effect can be evaluated for both winter and summer.


Let’s remember that any unshaded glass (even single pane), can cause overheating: the cause of that does not lie in the thermal performance of the glass per se, but in poor design or use of the building.

car in the sun
A car parked in the sun. This is equal to an uninsulated building, provided with single pane glazings. Overheating is caused by human error (poor design, poor management, or both), not by the energy performance of the building or of the glass.

To avoid overheating and guarantee daylight, the best strategy is to provide the building with external adjustable shading devices. This allows building users to manage daylight and solar gains based on season and daily activity schedule.

A building without shading is like a car without brakes: if you lose control (if it overheats), it is not the fault of the accelerator.

If solar gains need to be constantly kept low (e.g. non-residential buildings, with high interior heat gains), the glass units can be provided with a low solar factor on purpose.


Let’s have a look at a data sheet we received from a window manufacturer for a residential project near Reggio Emilia, Italy.

In the eyes of the manufacturer, the glass unit was extremely good, with a Ug value of 0,47 W/m2K (0,08 BTU/h*ft2*°F, about R12). In his mind, this would have allowed his 92 mm solid timber frame window to meet passive house requirements in the local climate (which is not true, because he was considering the bare Uw value, without installation thermal bridges).

Bad window glass
The data sheet of the glass. Top left: the widow manufacturer highlighted in green the Ug value, which made him think that the IGU could be very performing.

If you keep in mind what described above, it is evident how this glass unit is really not very good, because is provided with low-quality low-e coatings, not selective ones.

A light transmission of 50% would probably cause the failure of the windows to provide the minimum daylight factor (2%), with electrical lighting being required during daytime.

On the other hand, a solar factor of 0,30 (30%) dramatically reduces the overall energy performance of the building, and increased the demand for heating, even if the thermal transmittance Ug is better than other glazings.

Is this a “bad” glass?

Glazings are very dynamic components of the thermal envelope. In order to understand if a glass unit is appropriate for a specific building, in the local climate, you need to carry out an analysis on the entire thermal envelope. This cannot be done by the window manufacturer: it needs to be done by the designer of the building.


Glazed openings provide a determining contribution to the energy balance of high-performing buildings and passive houses.

The selection of insulated glass units has to be made based on climate, urban context, and use of the building. Thermal transmittance of glass, Ug, has to be paired with light transmission and solar factor g.

The quality of glass has consequences on the overall quality of the building, in terms of comfort, daylight and energy efficiency. For this reason, the design of the glass units has to be carried out by the designer of the building, not by the window manufacturer.

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