Rate of LT (Light Transmission) – Visible light transmittance
The glass’s rate of light transmission shows what percentage of light can pass through the glass into indoors. Implicitly, this parameter is between 0-100%, but the light transmission of a simple, one-layered water-clear glazing is only 91%.
In case of the standard ’4-16arg-4low-e’ insulating glass this rate decreases to 82%, and it is only 74% if we examine a triple-layered standard insulating glass unit with low-e coating.
The more natural light we can let in the less artificial light, in other words energy, we have to use. There are also other needs that must be considered in choosing, but if we choose a unit with greater light transmission from several very similar units, we make the right decision.
Rate of LR (Light Reflection) – Visible light reflectivity towards exterior
The glass’s light reflection shows what percentage of light is reflected back by the glass surface. The higher this rate, the more the surface can reflect the sunlight (the more reflective it is).For example, a strong reflecting glass (LR=36%) can be three times as reflective as a standard 4-16arg-4low-e insulating glass (LR=12%).
Theoratically, this parameter can be between 0-100%, and there are solutions for reaching nearly 0% (thanks to the anti-reflective coating), but 100% is not available for those structures which need to be somewhat see-through as well. For example, the so-called one-way glasses’ outer light reflextion is only 60 % (meanwhile, their light transmissivity is only 3%).
Glasses with reflective coating cannot prevent seeing through at night (when the strenght of lighting is stronger indoors than outdoors), so we have to make a decision knowing this fact, when we use them.
Rate of SF (Solar Factor) – Total solar energy transmittance
The total solar energy transmittance rate shows what percentage of solar energy is let through by the glass structure, which hits the facade.
It is constituted by the solar energy which gets indoors directly andby the heat energy which is first absorbed by the glass, then emanated back inside again.
The less the SF rate is, the less air-conditioning we need in summer, thus our cost of cooling, that is, our electricity bill will decrease.
The SF rate is often indicated as ’G’, but it is the same parameter.
Ideally, this parameter is between 0-100%, but a simple singlelayer water-clear glazing’s total solar energy transmittance is only 88%!
This decreases to 64% in case of the standard ’4-16arg-4low-e’ insulating glass unit, and it is only 53% fortriple-layer insulating glass units with low-e coating.
In case of passive houses, we try to utilize as effectively as possible the energy of winter sunlight in the spirit of maximazing energy consciousness. Due to this reason, a new development was achieved which allows triple layer insulating glasses with low Ug to have a 62% SF value.
Irrespectively of passive houses, the general developmental tendency is to produce glass units with which we can build facades with higher light transmittance and lower energy transmittance.
See also: selectivity.
S-rate (Selectivity) – The relation of visible light transmittance to total solar energy transmittance
S = LT / SF
The higher its rate, the more modern the glass-structure is considered to be.
Its physical maximum was defined at 2.0 for a long time by glass-manufacturing firms, but other solutions have appeared since the 2010’s, which reached a value over 2.0.
Nowadays, every significant glass-producing company has a type of glass with a rate of selectivity of about 2.2. The latest goals aims at over 2.4.
Glasses with high selectivity can filter out the energy waves from light waves and they let through less.
A simple single layered water-clear glazing’s selectivity is 1,03.
It increases to 1.28 in case of a standard ’4-16arg-4low-e’ insulating glass and to 1.41 in case of a triple layer standard low-e coated insulating glass.
The modern sunscreen glasses have a selectivity over 1.7 in any case.
Rate of EA (Energy Absorption) – Energy absorption
Part of the sun’s energy hitting the the glass is always absorbed by the glass.
The energy absorption of a simple single-layered water-clear glazing is 5.5%.
It increases to 13.9% for a standard ’4-16arg-4low-e’ insulating glass and to 20.5% for a triple-layer standard low-e coated insulating glass.
If we use solar reflecting or body tinted glass for light protection or aesthetic purposes, we have to expect that the rate of energy absorption will increase.
In case of double-layer or glasses or units with more layers, it is also worth to define this parameter of each layer separately.
So long as we have the opportunity, we have to take into consideration that if one of the plate’s EA rate reaches or exceeds 50%, the danger of heat breakage occurs.
If we do not have the chance to determine the data from layer to layer, then the EA rate of the whole insulating glass unit will be available certainly. If this rate reaches or exceeds 60%, the risk of heat breakage occurs.
You can and should defend against heat breakage by heat treating (tempering or toughening) the affected glass lite (which is usually the outer one).
Not every basic glass can be tempered or toughened, and additionally, the tempering process incurs additional cost, therefore analysis, and possible avoidance, of energy absorption has significance. In this topic we can meet more complex situations, and the glass-manufacturers can help us with the investigation and evaluation – taking into consideration those varied environmental factors which influence the glass-structures we intend to build in.
Please, ask our salesperson’s advice for evaluatiing specificcases!