Thermal Insulation
The use of glass in buildings offers a wide variety of advantages as follows:
1- Decorating & distinguishing the building.
2- Providing good comfortable working conditions for the employees.
3- Energy reduction.
1-Decorating & distinguishing the building:
Glass for buildings (especially administrative buildings) is considered a vital and very effective factor of aesthetic appeal of the building.
(more information is available inthe glass external fascades section).
2-Providing good comfortable working conditions for employees:
This is maintained by using glass of different transparency levels to provide colour-natural visibility for undistorted, natural views from the interior together with excellent light transmittance that enhances energy reduction during daytime, meanwhile offering quiet and undisturbed working environment by reducing visual & accoustic distractions.
Moreover, It maintains the sensation of thermal comfort since the feeling of excessive heat or cold causes loss of mental concentration span.
3-Energy Reduction:
In view of the fact that maintaining thermal balance of human body needs electrical heating in winter and air conditioning in summer (dominant season in Egypt), glass offers a wide contribution in saving electrical energy used for the above purposes by controlling thermal transmittance levels.
*The higher light transmittance the less electrical lighting is needed=Energy reduction.
*The more efficient thermal insulation performance is, the less air conditioning is needed=Energy Reduction.
Glass designers can maintain both advantages (thermal insulation + high light transmittance) if needed (mainly in cold sunless countries).
If the architect needs to maintain both thermal insulation and low light transmittance of different degrees, it is best to use one or more layers of colored or reflective glass (very suitable to overcome hot weather conditions like in Egypt). This is because of the fact that more light insulating glass is (as in tinted or reflective glass), the more heat absorption and thermal insulation it provides.
Glass and Its Energy Performance
In Brief, the following points are going to be tackled:
1- International thermal standards.
2- Light transmittance & thermal controllers.
| a) thickness. b) color. c) air spacer (insulating glass). d) coating (reflective layers). |
4-Insulating glass as environment friendly.
1-The International Thermal Standards:
Glass characteristics like light transmittance or even its colours can be easily selected by consultants or consumers when objecting glass samples to light. But as for glass thermal control characteristic (heat insulation) it has more than one standard. Or definition that highlights this functional resistance.
Thet most of the commonly used standards are as follows:
1- Direct solar energy reflectance (D.E.R.)
2- Direct solar energy Transmittance (D.E.T.)
3- Energy Absorption (E.A.)
4- Solar Factor (S.F)=Total Solar Energy Transmittance=g-value
5- Shading
Coefficient (S.C.)=b-Factor 6- K (U)-value.
7- U-Value (U-factor)=ASHRAE U-Value American
8- R-Value
9- Dew Point
10- Solar Heat Gain Coefficient.
11- Relative Heat Gain (R.H.G.)
12- Efficacy factor (Ke)
13- Dr.Greiche unit (*DG*unit)
Note: The numbers shown above are considered merely illustrative figures which vary according to glass type & characteristics.
2-Thermal Controllers:
a) Thickness:
The thicker the glass, the more it absorbs heat and accordingly the less solar heat it gains.
Meaning, glass thickness is:
* Inversely proportional to solar heat gain.
* Directly proportional to heat insulation.
b) Colour:
Colours come from adding oxides to the glass mixture.
These colors are not considered superficial.
The most common colours of glass are:
(Bronze - Green - Blue - Grey - Rose - Amber)
For Example:
- Adding iron oxide gives green colour
- Adding selenium & nickel oxide gives bronze colour
- Adding cobalt oxide gives blue colour
* Basically, colouring glass enhances its ability of heat absorption and accordingly its thermal insulation.
For example:
* Green colour enhances thermal control while maintaining natural visibility (without colour shifts) and undistorted vision. This is why it is used in auto glass.
* Grey colour enhances thermal control and reduces light transmission.
c) Insulating Glass I.G:
Glass is generally considered as a good thermal conductor.
However, in order to enhance its thermal characteristics, two layers of glass with an intermediate air space are enough to fulfill that purpose (since air has a less thermal conducting factor compared to glass).
- If the width of air space is up to 15 mm maximum, the thermal insulation capacity increases.
- However, air spacer of 12 mm ensures maximum thermal insulation and is suitable & practical under most installation conditions.
- Adding a third glass layer and 2 more air spacers further effecively enhances the level of thermal control.
d) Coated Reflective Glass:
* Reflective glass is coloured glass or clear glass coated with a special coat or a number of coats of one or many metals like: (silver Ag - chrome Cr - silicon Si - stainless steel SS (Ni Cr)
* This type is called coated glass or reflective glass as it reflects the light falling on its surface with different levels, and this does not mean that this kind of glass is heatproof.
* Metal coating of glass aims at enhancing the thermal characteristics and in this condition, the glass is called:
| - Solar energy reflecting glass. - High performance reflecting glass. - K-glass or solar control glass. |
* Deposition methods of the metal coating vary. The most common methods are:
| 1- CVD Chemical Vapor Deposition 2- Sputtering (vacuum coating method) |
| 1- Soft Coating. 2- Hard Coating. |