What are the ZTA, LTA and g-value in glass

If you're looking for the right double glazing or insulating glass, you're bound to come across the terms ZTA, LTA and g-value. They have to do with to what extent sun and heat can get through the glass. Below is a clear explanation of what each term means.

ZTA: the solar accession factor

ZTA is the solar factor: the ratio of how much light passes through the glass to how much is actually blocked by the glass. For this, the angle is 45 degrees. The higher the ZTA, the more sunlight passes through the glass. The factor runs from 0 to 1. It is also used as a percentage, i.e., from 0 to 100%, where 100% equals an open window and 0% equals a wooden panel, for example.

So a high ZTA is beneficial in terms of energy consumption in the winter, but in the summer, on the other hand, it can be too hot, requiring additional cooling.

Buildings with large glass facades should not get too hot in summer, so here the ZTA value should not be too high. The factor ranges from 0 to 1.

Clear glass has a ZTA value of 0.8. Modern HR++ glass has a ZTA value of between 0.6 and 0.7. A lower ZTA value can be achieved by applying a heat-resistant coating.

LTA: light admission factor

LTA is the light transmission factor of a window. It represents the ratio of how much visible solar radiation passes through the glass to how much is blocked by the glass. It is calculated at a perpendicular angle of incidence (90 degrees). Clear single glass has an LTA value of 0.9. Modern HR++ glass blocks more visible solar radiation and has an LTA value between 0.7 and 0.8.

Thus, the LTA is the light transmission factor of visible sunlight. The ZTA value can be small while the LTA is large and vice versa.

What is the optimal ZTA and LTA?

So the trick is always to choose a type of glass that repels heat well (low ZTA) but still has a high LTA value for sufficient daylight. AA Glas will be happy to advise you on this.

G-value: the new standard for ZTA

The international standard, the g-value, is increasingly used in the Netherlands. Whereas the ZTA is calculated with an angle of incidence of 45 degrees, the g-value is calculated with an angle of incidence of 90 degrees. As a result, the g-value is always higher than the ZTA, because sunlight shining perpendicularly on the glass reflects less than sunlight falling obliquely.

The difference between the ZTA and g-value

The g value is the ratio of the total amount of solar energy entering a room through the glazing (direct and indirect) to the amount of solar energy falling perpendicular to the glass (source: https://www.joostdevree.nl/shtmls/g-waarde.shtml). The g value is between 0 and 1. The higher, the better the light transmission. Good translucent glazing has a g-value of about 0.85.