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Insulating Glass

What is an insulating glass unit (IGU):


Insulating Glass Units (IGUs) are hermetically sealed combinations of two or more panes of glass separated by a dry airspace. IGUs improve the thermal performance of windows, thus significantly reducing heating and air-conditioning costs. IGUs also reduce interior condensation in cold climates, and increase comfort near windows, thus maximizing usable interior space.

The area between the glass panes, called a cavity, must be filled with dry air or gas. To ensure this dry environment, the spacer bars are filled with desiccant. A thin strip of butyl is applied on both sides of the spacer bar and it is then placed between the two glass pieces. The butyl serves as primary/interiror sealant and prevents gas exiting and moisture entering the cavity. A secondary/exterior sealant is then applied for mechanical strength. Silicone, polysulphide or polyurethane is usually used as a secondary sealant.

For a quality IGU, all materials must be mechanically strong, UV resistant, have a good moisture penetration protection and be stable in cold and hot conditions. Placing the primary sealant evenly without any interruptions, especially on the edges is vital for a quality production. If any of the above conditions is not met, there’s a risk of moisture penetration, which will result in the inside of the IGU becoming wet.


Gas penetration comparison table between primary and secondary sealants:
Lower value is better.

MateriaApplicationGas penetration value (l/mm2d)
ButylPrimary sealant0.01
PolysulphideSecondary sealant0.03
PolyurethaneSecondary sealant0.3
Structural siliconeSecondary sealant10


From the table above it becomes clear how important the primary sealant is for the gas penetration/leakage protection of the IGU. It is important that enough butyl (at least 4mm) is laid on each side of the spacer and that there are no interruptions, especially on the edges.



Having the right reliable equipment for IGU production ensures right planning and high quality product.

"Bul-It Glass” Ltd. owns two IGU production lines manufactured by one of the world’s leaders in the field – Lisec, Austria.


Technical specifications:

Maximum dimensions2700 х 5000 mm
Minimal dimensions200 х 300 mm
Maximum IGU thickness60 mm
Maximum thickness of a single glass15 mm
Argon gas fillingYes, automatic up to 3500 mm and manual between 3500 mm and 5000 mm




How to choose the right IGU:

Nowadays there is a wide variety of glass types with different coatings and properties. This allows us to combine them differently into a double (single cavity) or tripple (double cavity) IGUs in order to achieve our goals when glazing. In order to choose the right structure it is important to prepare the anwers to the following questions:




Main IGU properties:

Light properties - by chooising tinted glass or glass with a special coating, different light properties can be achieved:

Energy properties - by chooising tinted glass or glass with a special coating, different energy properties can be achieved:

Noise reduction properties:


Main functions of the insulating glass units:

Energy savings – keeping the heat inside the room during cold weather.

The energy saving effect of the IGUs are achieved by reducing the heat transfer through the unit. This can be done if multiple ways:


Solar protection – reflecting the heat from the sun out of the room in the hot sunny days.

Clear glass and energy saving Low-E glass do not provide any solar protection. It is achieved by either retaining the sun energy inside the mass of the glass by using dark glass with a high EA value*, or by reflecting the sun energy/heat by using glass with a reflective/solar protecting coating. There is a direct correlation between solar protection (solar factor SF) and light transmission (LT) and achieving maximum comfort is a matter of finding the right balance between these two properties.

*Glass with property ЕА ≥ 60% must be tempered due to high risk of thermal stress. For glass with ЕА ≥ 40% it is highly recommended to have the edges arised.


VIDEO: What is Low-E glass. Source: glassed.vitroglazings.com

VIDEO: How does Low-E glass work. Source: glassed.vitroglazings.com


Noise protection/noise cancellation.

The coating and the color of the glass do not play any role in the noise cancellation properties of the glass. Noise protection is achieved by:


Appearance and light transmission.

The appearance of the IGUs is determined by the color of the outer glass when looked from outside. Different colors/appearances are achieved by:

Darker colors and coatings with stronger solar protection are inversely proportional to light transmission (LT), i.e. stronger solar protection darkens the room. The proportion between light transmission (LT) and solar factor (SF) determine the glass selective properties, or selective coefficient. The regular clear 4mm glass has the worst selective coeffiecient: 83% (LT) / 80% (SF) = 1.0375. A value of 2 and above is considered to be a very good selective coefficient. This is achieved by using 2 or 3 silver coatings, usually on low iron glass.


Safety and anti-burglary.

Safety in IGU can be achieved by using temepred and laminated glass. When temepred glass breaks, it falls apart into many small pieces that are not sharp and can not cut human skin. The worst injury that can happen to a person is a bruise or a scratch. The laminated glass remains in one place when broken. This makes it safer than temepred glass, albeit not stronger. Strength is actually the drawback of the lamniated glass in regards to temepred glass. Roghly, we can measure the strenght of the laminated glass to be equal to the strenght of monolithic glass with thickness about 60% of  the total thickness of the laminated glass. In other words, we can say that the strength of a 10mm laminated glass (5+5) is about the same as the strenght of a single 6mm glass. In order to combine the best of both worlds it is best to use a combination of temepred and laminated glass in an IGU. A tempered glass can be placed outside the unit in order to withstand rought outside conditions, like strong winds, thermal load and deviations. A laminated glass can be placed inside the IGU in order to achieve the safety of the inhabitants.

The strength of the temepered glass alone is not enough to prevent burglary. It is susceptible to breakage by being hit with a sharp object.  In order to achieve better burglarly protection it is advisable to use a combination of two or three laminated glasses in an IGU, where at least 4 layers of laminating foils are  used between at least 3 glasses. Due to the tensile properties of the laminating foils it would take a really long time to bust through such a glass, even if one manages to break the glass panes. Oddly enough, in such cases it is important to install a better and stronger framing system, as regular frames might be easier to "unclip” if the IGU is hit hard enough.


"Warm edge” spacer - improved heat insulation around the perimeter of the IGU:

The spacer bar is the thermal bridge between the perimeter of the IGU and the environment. The standard aluminum spacer is a really good heat conductor, which makes it not the best choice if one is looking for maximum performance. By using "warm edge” spacer bars, usually made out of plastic of composit materials, we get a much better insulation around the perimeter of the IGU. This improves the combined performance of the glass and framing system by up to 10% and also prevents condensation appearing around the edges of the IGU. From an aesthetic point of view, the warm edge spacers are produced in many colors ( black, grey, brown, white and more). This makes this solution a nice improvement in interior design. "Bul-It Glass” partners with the swiss based manufacturer of warm edge spacers Swisspacer.


Comparison table with parameters, suggested applications and sample IGU structures:

StructureLight propertiesHeat propertiesU-coeff.Noise propertiesAdditional information
LTLRUVТEAЕRSF     Air    Argon 90%dBreduction [%]
4 clear – 16 – 4 clear8315-914802.72.631-Base IGU, no advantages.
6 clear – 16 – 4 clear8215-1213782.72.635--
6 clear – 16 – 6 clear8115-1413782.72.633--
4 green – 16 – 4 clear7112-448532.72.631--
6 green – 16 – 4 clear6611-547452.72.635--
6 green – 16 – 6 clear6511-557452.72.633--
4 clear – 16 – 4 Low-E8211-1527651.41.131--
6 clear – 16 – 4 Low-E8111-1826641.41.135--
6 clear – 16 – 6 Low-E8011-1926631.41.133--
4 green – 16 – 4 Low-E6910-5111441.41.131--
6 green – 16 – 4 Low-E649-608381.41.135--
6 green – 16 – 6 Low-E639-608381.41.133--
4 four season– 16 – 4 clear6122-2438401.31.031--
6 four season– 16 – 4 clear6022-2736391.31.035--
6 four season– 16 – 6 clear6022-2836391.31.033--
4 clear– 16 – 4 clear– 16 – 4 clear7621-1319731.81.732--
4 clear– 16 – 4 clear– 16 – 4 Low-E7518-2029601.10.932--
4 Low-E – 16 – 4 clear– 16 – 4 Low-E7314-2530530.80.632--
4 four season– 16 – 4 clear– 16 – 4 clear5625-2739371.00.832--
4 four season– 16 – 4 clear– 16 – 4 Low-E5524-3039360.70.532--
6 clear – 16 – 4 clear– 16 – 4 clear7520-1518711.81.736--
6 clear – 16 – 4 clear– 16 – 4 Low-E7417-2227581.10.936--
6 Low-E – 16 – 4 clear– 16 – 4 Low-E7314-2828520.70.636--
6 four season– 16 – 4 clear– 16 – 4 clear5625-2937371.00.836--
6 four season– 16 – 4 clear– 16 – 4 Low-E5523-3337350.70.536--
6 clear– 16 – 4 clear– 16 – 6 clear7520-1718711.81.737--
6 clear– 16 – 4 clear– 16 – 6 Low-E7417-2327581.10.937--
6 Low-E – 16 – 4 clear– 16 – 6 Low-E7314-2828520.70.637--
6 four season– 16 – 4 clear– 16 – 6 clear5625-3037371.00.837--
6 four season– 16 – 4 clear– 16 – 6 Low-E5423-3337350.70.537--
6 Cool Lite SKN176 – 16 – 6 clear7013-2837371.31.033--
6 Cool Lite SKN176 – 16 – 4 clear– 4 Low-E6315-3138350.70.536--
4 clear– 16 – 4.4.1 clear laminated8115-1713772.72.635--
4 clear– 16 – 4.4.2 clear laminated8115-1913772.72.637--
4 four season– 16 – 4.4.1 clear laminated6022-2738391.31.035--
4 four season– 16 – 4.4.2 clear laminated6022-2738391.31.037--
4.4.1 clear laminated– 16 – 4 Low-E8011-2522591.31.135--
4.4.2 clear laminated– 16 – 4 Low-E8011-2721591.31.137--
4 clear– 16 – 4.4.1 Low-E laminated8011-2027641.31.135--
4 clear– 16 – 4.4.2 Low-E laminated8011-2127641.31.137--
6 clear– 16 – 4.4.1 Low-E laminated8011-2326631.31.138--
6 clear– 16 – 4.4.2 Low-E laminated8011-2326631.31.139--




Risk of breakage after installation and prevention methods:




Type of breakageReasonPrevention
Glass Breakage 1

Точков удар с остър предмет или твърд предмет, попаднал между стъклата.

Добра сепарация и по-внимателно складиране и манипулиране на стъклата.

Glass Breakage 2

Точков удар с остър предмет или твърд предмет, попаднал между стъклата при температурно заздравено стъкло.

Добра сепарация и по-внимателно складиране и манипулиране на стъклата.

Glass Breakage 3

Прекомерно механично натоварване на стъклото (силен вятър, подпиране с тяло или друг тежък предмет) при стъклопакет, който вече е термично натоварен (загрят от слънцето).

Обработка на ръба на стъклата.
Закаляване на външното стъкло.

Glass Breakage 4

Счупване поради термично натоварване.

Избор на стъкла с по-ниска абсорбция на енергия.
Обработка на ръба на стъклата.
Закаляване на външното стъкло.

Glass Breakage 5

Продължително неравномерно механично натоварване.

Обработка на ръба на стъклата.
Закаляване на стъклата.

Glass Breakage 6

Твърде голямо напрежение от температурно разширение на газа в стъклопакета поради разлика в температурата на производство и инсталация или надморска височина.

Обработка на ръба на стъклата.
Закаляване на стъклата.

Glass Breakage 7

Счупване поради осукване на стъклото или допир в твърд предмет поради разместване на конструкцията или температурно разширение.

Премахване на конфликтни точки с твърди предмети.
Увеличаване на дебелината на стъклата.
Обработка на ръба на стъклата.
Закаляване на стъклата.

Glass Breakage 8

Счупване поради точково механично натоварване, причинено от твърде голяма тежест на стъклата и неправилно манипулиране.

Правилен подбор на дебелина на стъклата и правилно манипулиране със стъклата.

Glass Breakage 9

Счупване поради точков удар в ръба, най-често от удар или грешката при монтаж.

Правилно боравене със стъклата.

Glass Breakage 10

Счупване поради натоварване на ръба на стъклата или престягане при укрепване.

Правилно боравене и укрепване на стъклата.

Glass Breakage 11

Счупване поради разлика в температурата на центъра и периметъра на стъклото.

Обработка на ръба на стъклата.
Закаляване на стъклата.

Glass Breakage 12

Засенчване отвън или наличието на обемисти предмети в непосредствена близост до стъклената повърхност отвътре. Външни условия за такива счупвания създават: козирки, дървета, сгради или други обекти причиняващи частично засенчване на стъклените повърхности. Вътрешни фактори могат да бъдат: мебели, части от строителната конструкция, щори и др. разположени в непосредствена близост до стъклената повърхност възпрепятстващи добрата вентилация и охлаждане.

При възможност да се махне причината, която създава засенчването (скеле, мебел, рекламни материали).
Избор на стъкла с по-ниска абсорбция на енергия.
Обработка на ръба на стъклата.
Закаляване на външното стъкло.


VIDEO: Risk from breakage due to thermal stress. Source: glassed.vitroglazings.com


Size of the IG units and recommended thickness of the glass and spacers:
(all dimensions are in millimeters)

Glass thickness
Max width
not tempered/tempered*
Max height
not tempered/tempered*
Min. spacer widthMax size when square
not tempered/tempered*
Max sides ratio
4 / 3.3.1 / 3.3.22200 / 2800     1300 / 1800     8 up to 2 m2
10 up to 3 m2
12 up to 4 m2
14 up to 5 m2
16 over 5 m2
1300 / 1800     1:4
5 / 4.4.1 / 4.4.22800 / 3400     1700 / 2400     1300 / 2000     1:6
6 / 5.5.1 / 5.5.23500 / 4000     2000 / 2400 **2000 / 2400     1:8
8 / 6.6.1 / 6.6.24000 / 4800     2500 / 2400 **2500 / 2400 **1:10
10 / 8.8.1 / 8.8.24500 / 4800 **2700 / 2400 **2700 / 2400 **1:10
125000 / 4800 **2700 / 2400 **2700 / 2400 **1:10

* - tempered ot heat strengthened monolithic glass

** - limited by the maximum temperening dimensions

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