Vacuum double glazing is one of the most efficient ways of reducing heat loss in your home. When you have more efficient windows, you are able to retain more heat in your home. This is especially important at the moment as energy prices continue to rise and the cost of heating our homes has more than doubled recently.
In this article, we will tell what vacuum glazing is, and why it needs a flexible edge to seal the units. If you are thinking about replacing your old windows for more efficient units, you will find this article useful as we will explain how vacuum glass is made, and why it might be the best option for insulating your home.
What is Vacuum Double Glazing?
Vacuum glazing is a technology that can improve the efficiency of your home. It’s a type of double glazing that was developed by the Japanese to make buildings in in earthquake zones lighter and, therefore, safer, whilst delivering the same levels of energy efficiency as typical double glazed units. Since these early units, the technology has developed to deliver industry leading energy efficiency, longevity and acoustic performance.
Vacuum glazing is similar to standard double glazing units, being made of two panes of glass. Standard units have a noble gas such as Argon in the cavity between the two sheets of glass. This gas is denser than air – around 1.8 times heavier than the atmosphere. This means that heat and sound find it much harder to cross the cavity, helping to maintain constant temperatures in buildings.
Vacuum double glazing uses a vacuum in the cavity. This has two major advantages over standard double glazing:
- It is much more efficient at insulating rooms
- It’s more aesthetically pleasing as the cavity can be very small without reducing the performance of the unit
Why is vacuum glazing more efficient than double glazing?
Vacuum glazing is more efficient than double glazing because the vacuum cavity has better insulation properties. As all Sci-Fi fans will know, heat and sound cannot cross a vacuum (‘In space, no-one can hear you scream’ etc).
Even though these windows only have a narrow gap (more on that later), the vacuum is still highly efficient – you can’t have more of ‘nothing’ so whether the gap is a 0.2mm cavity between two sheets of glass, or the vastness of space, it’s insulating, and sound proofing qualities are just the same.
Why is vacuum glazing better looking than standard glass?
Vacuum glazing is better looking than standard double glazing because of that narrow gap. Standard double glazing has a cavity of anything between 16 and 25mm. This wide gap means that all the ugly inner areas of the units are exposed. It also means that the units are quite bulky.
Vacuum glazing has a gap of less than 2mm, and the units are so slim that they almost give the appearance of single glazed windows. This is a vast improvement in style for any home, but it’s especially important for heritage projects where single glazed windows would have been fitted originally. Listed buildings and those in conservation areas would be legally required to make any renovations as close as possible to the original appearance of the buildings. Therefore, where planning officers would normally require single glazing to be installed, ultra slim vacuum glazing can be an acceptable option. These types of windows are called Heritage Windows – you can read more about them here.
How is Vacuum Double Glazing Made?
Vacuum glazing is much harder to make than standard double glazing and requires some special construction techniques.
Firstly, the vacuum between the two panes of glass effectively pulls the two panes of glass together. To stop the glass bending and filling the void, tiny spacers are placed in a grid between the two panes of glass. These small discs are barely visible, and are only 0.15mm thick, but they are enough to maintain the gap.
Secondly, the vacuum between the two panes has to be sealed. This is difficult to do on the edge, as this seal is difficult to make and maintain (more later), so one of the panes of glass typically has a hole in it, which is sealed when the vacuum is made. This is done in a special vacuum oven that removes all gas and moisture from the cavity. The hole is then plugged so the vacuum can be maintained. On modern vacuum glazing units, the plug is usually situated in one of the corners to allow it to be concealed in a glazing rebate.
Note: while the vacuum is good – usually less than 0.1 Pa, there is still some air and moisture left in there, so a ‘getter’ is used in the cavity to maintain the vacuum – you can find out more about that here.
One of the biggest challenges in making Vacuum Glazing is sealing the unit. There have been a few methods attempted to seal the vacuum so far:
- Solder Glass
- Indium Alloy
- Metal Foil/Laminate
It is possible to create a vacuum seal between the two panes using powdered glass, sometimes mixed with borax flux. The finely ground glass powder is mixed with the flux to make a paste which is then spread around edges of the two panes of glass and the panes are pressed together.
The unit is then heated to 200ºC for an hour to dry it out before the main firing. It’s then heated to 620ºC. The glass powder melts and joins to the two panes. Pressure is applied, and the unit is allowed to cool.
This process is very time and energy consuming. The glass needs be heated and cooled slowly or cracks can appear. Also, the units bend and stretch due to the heat, and if pressure is not maintained then more irregularities can occur.
Also, once the units are placed into application, there can be differential movement of the two panes caused by the vastly different temperatures of the two panes. If the edge seal is not flexible, then this could lead to cracking of the glass panes and unit failure.
Idnium alloy as a vacuum seal was developed at Ulster University. A strip of the alloy is heat sealed around the edge of the unit. This is a better method as the glass is under much less stress from the changing temperature and extreme heat. However, the seal is not perfect and micro-pin holes have been found. Another con of using Idnium Alloy is the high cost of the raw material.
Metal Foil / Laminate
Some manufacturers had the idea of making a vacuum seal from strips of stainless steel foil. This flexible seal is attached using ultrasonic bonding. The process is simple, the materials are relatively cheap and because there is no heat treatment, the glass is not put under any stress. The quality of the seal has been found to be better than when using solder glass or indium Alloy.
Because the unit is put under much less stress, they last longer, and vacuum windows made with flexible edge sealing techniques are expected to maintain a healthy <0.1 Pa pressure vacuum for the duration of the units service life.
So now to answer the question:
What is the Flexible Edge Sealing in vacuum double glazing?
It’s a strip of stainless steel foil that is ultrasound bonded to the two sheet of glazing to form a vacuum seal.