USEFUL TERMS :

Visible Light Transmittance :

This is percentage of visible light (380-780nm’s) that is transmitted through a glass.

Visible Light Reflection :

The percentage of visible light that is reflected from the glass surface.

Solar Energy Transmittance :

The percentage of solar energy that is directly transmitted through glass type.

Solar Energy Reflectance :

The percentage of solar energy that is reflected from the glass.

Relative Heat Gain :

This is commonly used by ASHRAE codes and is used for classifying window Glass. It is the total heat gain of a window relative to a window using 3mm glass. RHG is the sum of the product of U-value and temperature difference and product of solar factor (SF) and amount of incident solar energy.

Relative Heat Gain = (U x Temperature Difference) + (SF x Amount of Incident Solar Energy).

Solar Factor or SF also known as Solar Heat Gain Co-Efficient (SHGC) :

The direct solar heat gain in a building is the sum of energy directly transmitted (i.e. short wave) and the direct solar energy aborted and re-emitted inside (i.e. long wave) by it’s envelop. Solar Factor is the ratio
between the total solar heat gain and incidental solar energy. The solar factor range is between 0 and 1 of the total solar heat transfer. Lower the solar factor value of a glass, the more efficient is the glazing at blocking the entry of solar energy. A solar control glass with SF of 0.4 mean that 60% of the solar radiation is blocked inside and only 40% passes inside. Similarly north facing facades non solar heat is more important. Hence orientation of building determines the glazing requirements of the building.

Shading Coefficient :
Shading Coefficient is a term generally used in Air conditioning industry.

Solar Factor of 3 mm clear glass is 0.87

Lower the shading coefficient, lower is the amount of solar heat transmitted.

U Value : Heat transmitted through a surface by conduction, convection and Radiation is expressed by
its U Value. U-Value is the amount of heat transferred that is lost or gained due to temperature differential of 1 degree centigrade through 1 square meter. Lower the U-Value, better is the insulation property of glass.

Factors Affecting Solar Heat Gain :
1. Orientation of the Building
2. Choice of Glass
3. Colour of Glass
4. Internal or External Glazing

Orientation of the Building :

Sun facing facades take about 80% of the total solar heat transfer. Similarly north facing facades non solar heat is more important. Hence orientation of building determines the glazing requirements of the building.

Choice of Glass :

Solar Control is the percentage of solar energy coming through a pane of glass. This can be improved by:

• Thicker Glass
• Body Tinted Glass
• Coated Glass (online or offline)
• Insulated Glass
• Combination of these Glasses

Colour of Glass :

Solar Factor of Clear Glass is 88%, whereas it is 56% in green glass. This solar factor is available from manufacture of glass as per performance chart.

Internal and External Glazing :

Using proper glazing can reduce the overall cost of energy upto around 30-35 %.

Selection Criteria :

The two main criteria for selecting solar control glasses are aesthetics and performance.

Aesthetics :

In addition to performance, glass should give a pleasing and aesthetic appeal when looking at and when looking through. Transparency, colour of the basic glass substrate, colour of the solar performance coating on glass and the light reflecting properties of the coating will combine to give an aesthetically pleasing ‘look through’ and ‘look at’.

Performance :

The glass should control the heat transmitted, control glare & heat losses.

Control of U-Value :

To achieve a lower U-Value, we have to reduce conduction, convection and IR radiation. U-value of some glasses and combinations as examples are given as under:

Factor not affecting U-value :
1. Colour of the glass
2. Tempering or Lamination
3. Typical reflecting coating

Total Heat Gain (solar & Non Solar) :

1. Total heat gain depends upon solar & non-solar heat

2. Each of these depends upon many factors, for example:
(a) Geographic location, and solar intensity
(b) Orientation of façade
(c) Area of glazing
(d) Type of glass (SF & U-value)
(e) Environmental factors
(f) Temperature differences between inside and outside

3. Detailed calculations can be made assuming some generalization and some averages..

Selection of Glass :

Key parameters that describe the performance of solar control glass include

1. Visible light transmission
2. Solar factor (SF)/ Solar Heat Gain Coefficient
3. U-value i.e. W/m2k
4. Selectively:- Visible light/ total energy

TL/SF for Indian condition the ideal selectively is around.

The contribution to heat gain inside building is :

U Value X Temperature difference 8%
Solar Factor X Amount of Solar Energy incidents 92%

Additional factors to be considered are
1. There is heat being generated from inside the building too.
2.External heat is outside heat coming into building by conduction, convection & direct radiation.
Internal load is all light fixtures, equipment, activities and people generating heat.
3. Glass Coatings are generally intended to reflect heat but most often, they also reflect light. The reflection of light could be Internal reflection and External relfection.

A high External reflection will cause a lot of glare and give the building a metallic look.

A high Internal reflection is not preferred in a night use building as Call Centre. Since the occupants will see their own reflected image as in the mirror, during the night use.