Vacuum insulating glass thermal property
The emerging of vacuum insulating glass increased glass heat insulation property significantly ,its U value is just 1/5 of normal insulated glass,less energy through the glass in summer and keep warm inside in winter.This posts will analyse the typical window glass U value and how VIG bring comfort to your life.
Heat transfer types
There are three ways of heat transfer: conduction, convection and radiation.
- Conduction heat is transferred from the higher temperature part of the object to the lower temperature part.
- Convection relies on the flow of liquid or gas to transfer heat,gas convection is more obvious than that of liquid.
- Radiant heat is emitted from the object in a straight line,no medium is needed, so radiation can be carried out in a vacuum The heat from the sun on the earth is transmitted by the sun through radiation.
The heat transfer through glass is combination of convection,radiation and conduction.
- Conduction: The heat conducted from the higher temperature exterior or interior side to relatively lower temperature side,the medium is glass, spacer,and other materials that bonging the 2 glass lite.
- Convection: It happened in insulating glass with gas between glass.
- Radiation: Solar radiation,it include the solar direct radiation,and other objects that emit radiation after absorb solar energy or A/C heating.
Parameters to measure glass thermal property
To measure how much energy through the glass,there’re 2 parameters we should pay attention to:
- Thermal conductivity (U value) U value is used to measure the thermal conductivity, indicating the ability of the material to allow heat to pass through per unit area, the unit is W/m2·K
- The solar heat gain coefficient (SHGC) ,refers to the ratio of the amount of indoor heat gained through the solar radiation of the transparent enclosure (doors and windows or transparent curtain walls) to the amount of solar radiation projected on the outer surface of the light-transmitting enclosure.
In one word,U value means the ability of energy transmit through the glass,SHGC means the glass ability to block the solar energy transmitted the glass,both them are the lower,the better.
Typical glass thermal property
The glass thermal property is decided by its structure and emissivity.
Glass has standard emissivity,0.84. Low-E coatings was developed which decrease the glass surface emissivity to 0.15 or even 0.03 to decrease glass ability to emit radiation after absorbing the energy,meanwhile it reflect certain solar energy and decrease glass U value and SHGC at the same time,also it has many light transmittance options to suit different climate requirement.
So below chart shows the typical glass thermal property:
|Glass Combination||LT %||U value-EN673||SHGC|
|6mm clear glass||88||5.74||0.83|
|6mm grey glass||45||0.74||0.67|
|6.38 clear lami||88||5.59||0.81|
|6.38 lami with hard coated low-E||81||3.8||0.70|
|3+12A+3 with S1.16 coating||81||1.71||0.61|
|3+12A+3 with D80 double silver low-E||69||1.65||0.40|
|3+12A+3 with T70 triple silver low-E||67||1.62||0.30|
|3+0.3V+3 VIG with D80 double silver low-E||69||0.47||0.40|
Based on above chart,we can get below conclusions:
- Complicated glass combination has better thermal property.
- With the help of low-E coating and argon gas,glass U value decreased to 0.6W/M2.K,but glass itself is becoming extremely thick and heavy,which require thicker and stronger frames.Glass light transmit has an limit of 55%,cannot increase further because of 2 layers low-E coating.
- Vacuum insulating glass has simple structure,thin and light while U value is even 25% better than triple glass units,light transmittance can reach 69% and even 80%.
- Vacuum insulating glass can help save frame costs and overall windows weight,especially for historic buildings,it can replace current glass without changing the windows and original style.
Vacuum insulating glass thermal transmission principle
The thermal transmission inside insulated glass include radiation (the exterior piece absorb thermal and release radiation again),conduction ( the edge,spacer thermal conduction),and convection (gas-air/argon inside the insulated glass), in which the convection account most.
In previous post vacuum glass guide we got the conclusion that the principle of thermos flask has 2 principles:
- Eliminate most of the heat convection and transfer through high vacuum conduction;
- Reduce radiant heat transfer through highly reflective coatings.
The VIG low thermal transmission also benefit from these points.
Below our video shows clearly how excellent the VIG heat blocking performance is:
1.VIG thermal convection
Vacuum insulating glass, as there’s 0.3mm vacuum space between glass panels, it eliminate the convection caused by the air, the vacuum degree is 10-4Pa,so the remaining convection is almost 0.
Meanwhile, for insulated glass,the U value is affected by glass installation angles because of air flow,Max 20% U value increased when glass were installed horizontially,for vacuum insulated glass,because no air flow inside glass,it eliminate the factor that cause glass U value changes,so VIG has same low thermal insulation performance whether it’sinstalled vertically or hoizontially.
2.VIG radiation transfer.
The emissivity of low-E coatings used in vacuum insulating glass is 0.08,compared with 0.84 original emissivity,the coating prevent glass from transmit glass after absorbing the solar radiation or far infrared.Which means that the heat cannot transmit heat from the higher temperature one to interior one.
3.VIG conduction transfer.
The connection between vacuum insulating glass is , the micro supporting pillars, and edge sealing materials.
The micro supporting pillars inside VIG is steel balls or other inorganic materials diameter 0.3-0.5mm,distance is 30-50mm,the overall pillar area contacting glass is really small.
Edge transfer: the vacuum insulating glass is sealed with glass powders or metals,this is the main component of VIG thermal transmission.
Typical U value of vacuum insulated glass is 0.4-0.7W/M2.K,people can also calculate VIG thermal performance through LBNL series simulation software.
Vacuum insulating glass is now the best energy efficient glass till now,as for weight and coating technology has reached its best and hard to develop further more,the VIG not only has low thermal conductivity,but also has thin thickness,light weight,low condensation point,excellent acoustic performance.
HaanGlas is your vacuum insulating glass supplier and manufacturer in China,kindly contact us for more info.
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