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Insulated Glass Units
aisglass.com   9/23/2010



IGUs’ are basically multiple layered glass panes that have very dry air or inert gas between the glass panes. The glass panes are hermetically sealed.

Any use of two thick nesses of glass, separated by an air space, within an opening, to improve insulation against heat transfer and/or sound transmission. In factory-made double-glazing units the air between the glass sheets is thoroughly dried and the space is sealed airtight, eliminating possible condensation and providing superior insulating properties.

Very dry air is a poor conductor of thermal heat, and an IGU provides excellent thermal insulation. A typical 6mm clear glass + 12mm air-gap + 6mm clear glass IGU provides a U-value of 2.8 Watts/m2, often significantly lower than many low-emissivity glasses. Contrary to popular opinion IGUs do not provide good acoustic insulation.

 Related Links
Gas Filled IGUs for High Performance Glazing

Spacer: Spacer performs the role of keeping the glass panes apart as well acts as a housing for the desiccant.

Desiccant: Or, the drying agent performs the most important role in an IGU; that of absorbing the moisture from the hermetically sealed space.

Secondary Sealant: This sealant is applied on the outside of the unit. It performs several roles, chief among them of providing structural strength.

Primary sealant: First line of defense for the hermetic seal.



There are many benefits of utilizing insulated window and door products in your home. They not only lower one's energy costs, but also make homes cozier by improving comfort, decreasing condensation, as well as, diminish the effects of fading. Insulated units are also highly effective in reducing unwanted sound transmission.

In the summer, heat is transmitted through the glass and is absorbed by the walls, furniture, floors, etc. By using window and door products with a low solar heat gain coefficient, the solar radiation coming into one's home is decreased. This reduces the amount of heat entering the home, thereby improving one's homes' comfort level and lessening the amount of energy needed for cooling during warm weather months.
Moreover, during the cooler seasons, insulated products create warmer interior glass surfaces, reducing frost and condensation from forming.

Another benefit is that during daylight hours, sunlight is transmitted through the glass and is absorbed by the walls, furniture and flooring. Many materials, such as carpet, curtains, upholstery, artwork and wood may fade upon exposure to sunlight. Window glass selection influences the type and intensity of ultraviolet (UV) rays, which causes fading and degradation.


Key Performance Requirements

1. Thermal Insulation: The primary reason for using IGUs is to cut down on thermal heat coming from outside. Thermal insulation primarily depends upon the dryness of the air inside the IGU, as well as the quality of the hermetic seal.

Working Principle: IGUs work on the principle of reducing conduction and convection heat transfer across the unit.

Dry air is a poor conductor of heat. Due to the air-gap, the conducted heat through glass is reduced significantly as it reaches the second pane.

2. Structural Strength: IGUs are typically extremely heavy. For example a 2m x 1m IGU using 6mm +4 mm glass weighs around 60 kg. Additionally in certain applications like structural glazing, IGUs often act as structural members of the building. It is paramount that the strength (direct and shear strength) of IGUs is maintained. This strength is primarily a function of the type of spacers, number of spacer joints, type of spacer joints (corner joints Vs side joints), type & quality of the secondary sealant, and adequate quantity of the secondary sealant .
3. Fogging: IGU's provide better protection against fogging than a single pane of glass. However, IGU's made without proper quality control, poor desiccant filling practices, inadequate quantities or incorrect type of desiccants, or a poor hermetic seal are more prone to fogging.
Key quality areas

There are a few key areas that are of immense importance to ensure the quality of IGU’s to ensure that clients get the performance they pay for.
i) Appropriate Type & Quantity of desiccant : Desiccants perform the most important role of absorbing moisture and keeping the air inside the IGU’s dry. The glass processor must use the correct type and quantity of drying agent inside the IGU. Not using the correct quantity or type of desiccant is one of the ways for a glass processor to significantly cut costs by cutting corners.

ii) Automatic Vs. Manual Desiccant filling: The main issue is that the drying agent should never make contact with ambient air during the process of filling the desiccant inside the spacers. As soon as the desiccant makes contact with air, it starts absorbing moisture reducing life cycle performance.

Auto Desiccant Station

Desiccant is filled & sealed automatically without contact with ambient air

iii) Number of joints in the Spacers: Ideally the spacer should have no joints. Joint-less spacers are structurally stronger. However, cost & performance tradeoffs require that an automatically bent spacer system in a single piece assembly with one side joint be used. Many processors use 4 corner joints for the spacers. This reduces the structural shear strength of IGU’s. The practice is not allowed by many European standards.

Spacer Bending Station

Automatic Spacer Bending creates spacers in a single piece assembly with one side-joint

iv) Enclosed Environment: In India, IGU's should be made in an enclosed & conditioned environment; ideally at a relative humidity of less than 62% and an ambient temperature of less than 24 C. This prevents excess moisture being absorbed by the IGU's during production, as well as prevents dust from entering the units.

v) Secondary Sealant quantity: The secondary sealant along with the spacer provides the unit with rigidity. The gap between the spacer and the edge of the glass pane that controls the amount of secondary sealant to be filled should be appropriate for the size of the insulated glass unit.

vi) Equipment: IGUs should be made using the proper IGU manufacturing line. A manually made IGU by sticking 2-panes together will not perform over the lifecycle of glazing.

A Modern IGU Line

Enclosed IGU Line for
Indian Environment at AIS Plant

vii) Standards: Due to the absence of any standards in India, it is highly recommended that IGU’s should be made to an international standard keeping several quality areas, including the ones above, in mind. Some applicable standards for IGUs are:

a. EN 1279-2
b. BS 5713
c. ASTM E 546-88


Production process

First and foremost the right glass combination is selected according the clients requirement. The choices are limitless, for e.g. the glasses can be monolithic, annealed, tempered, Laminated, Tinted and/or reflective.

Step 1.
All spacer bars are cut to size, and are then filled with a desiccant for moisture absorption. The spacer bar is an aluminum bar that comes in various widths to customize the amount of airspace, which is required within the unit. The spacer bar sizes are as followed:

5.5mm / 7.5mm / 9.5mm / 11.5mm / 13.5mm / 14.5mm / 15.5mm / 17.5mm
19.5mm / 23.5mm.

Alternatively a spacer bending option can also be done.

Step 2.
The spacer bars are filled with a high quality desiccant, and then assembled and prepared for the primary seal. This system ensures that the desiccant does not come into the contact of air and hence its moisture absorbing capability is intact till it goes inside the insulating glass unit.
Step 3.
Then, P.I.B(poly isobutyl) is applied to the spacer bar as the primary seal. The primary seal acts as the first seal that keeps the spacer bar at place and sticks both the panes of glass together separated by a uniform distance.
Step 4.
Glass is then put through the washer and arrives on to the quality control check point.
Step 5.
Quality control must then inspect to ensure the glass is clean and flawless. After the inspection has been done, the pre-prepared spacer bar is then applied. The glass with the spacer bar attached is then sent to the holding point.
Step 6.
The second piece of glass is then also put through the washer and inspected all the same, and then let through to the holding point, in which case the machine automatically aligns the two sheets of glass.
Step 7.
The two sheets are then pressed in the large glass press, and the automatically released out the other end. Then the Insulated Glass Unit is transported using the glass lift and is then ready for the application of the secondary seal.
Step 8.
The secondary seal is applied all around the unit, in either butyl or silicon depending on the size and what the purpose of the unit is.
Step 9.
Once the secondary seal process is complete, the unit is left to dry, which brings us to the end of the manufacturing process.
Properties of Insulated Glass 


•  Double or Triple pane Insulated glass units separated by aluminum spacers

Spacer Thickness

•  6, 8, 10, 12, 15, 18, 24mm available in stock
•  Thicker spacers made on special order

Spacer Bending

•  Full automatic
•  Single piece assembly with one side joint
•  Corner key assembly available (not recommended)

Desiccant Filling

•  Full automatic
•  No contact of desiccant with ambient air

Desiccants type

•  Molecular sieve IV


•  Enclosed air-conditioned
•  £ 24 ° C temperature
•  £ 62% Relative humidity

Secondary sealants

(performance parameters)

•  Polyurethane, Poly sulphide, and/or Silicone based from reputed    manufacturers.
•  In service temperature range -30 ° C to 80 ° C
•  Short exposure (up to 1 hour) 100 ° C to160 ° C

Space between glass edge & spacer

•  please see table below.



Joint depth

Beneath depth

2x 4mm glass (short side)

•  > 500mm

•  <500mm






2x 5mm glass (short side)
•  > 500mm
•  <500mm



2x 6mm glass (short side)
•  > 500mm
•  <500mm



2x 8mm glass (short side)
•  > 500mm
•  <500mm



Arched units 2x 4mm



Arched Units > 2x 4mm thick





•  Stepped glazing
•  Any shape for which a drawing can be made, E.g. Oval, arches, triangular, hexagonal, etc (min size limitations for bending apply)

Edge processing

•  Rough Grinding; CNC ground arise for tempered IGU
•  Belt grinding for annealed

Rectangular Dimensions (Maximum)

•  3050mm x 1830mm
•  Tolerance ± 2%

Rectangular Dimensions

•  280 x 280mm (rectangular)
•  450mm x 480mm (shaped glass, depending on shape)


•  74mm max.
•  Tolerance ± 1mm


•  Maximum diagonal difference of £ 4 mm

Evenness/war page

•  < 1% for annealed glass
•  < 2% for tempered or heat strengthened glass

International Standards
(Meet or exceed)

•  EN 1279-2:2002

Publisher link: http://www.aisglass.com/insulated_glass_units.asp


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