Information about insulating double glazing

What is insulating double glazing?

Insulating double glazing consists of two glass panes connected together with an airtight space between them. One side faces the outside of the home, the other side faces the inside. The space between the panes is created by a profile placed between the glass panes around the edges. We call this space the “cavity”. At the factory, the glass unit is sealed airtight around the profile. You can choose the exact composition of the glass unit yourself by combining different elements. These elements determine the total glass thickness and insulation value — more about that later. These are the components you can choose from:

• Which type and thickness of glass pane is used on the outside of the home
• Which type and thickness of glass pane is used on the inside of the home
• Which type and/or colour spacer bar you want between the glass panes
• How wide this spacer/cavity should be
• Whether the glass unit should be Argon gas-filled or air-filled
• Whether the standard seal around the glass unit is sufficient or whether a silicone seal is required

 

When is glass classified as HR, HR+ or HR++?

The insulation value classification is expressed as a U-value (W/m²K). This unit indicates how much heat, in watts, is transferred per square metre of material surface, per degree of temperature difference, in kelvin, between the inside and outside of that material. The lower the value, the better the material insulates. This is used for windows, walls and other building materials to describe their thermal performance.

HR: with a U-value greater than 1.7 and less than or equal to 2.0 W/m²K
HR+: with a U-value greater than 1.2 and less than or equal to 1.6 W/m²K
HR++: with a U-value less than or equal to 1.2 W/m²K

When we refer to “HR glass”, one of the glass panes has a high-performance coating and the cavity is filled with Argon gas.

 

Which glass do I need?

It is important to know what the purpose of the glass should be, which safety requirements apply and what glass thickness will fit into your frame. If you want thermal insulation, you need a different composition than when you want sound insulation. And if you want to achieve a high insulation value but your frame cannot accommodate the required glass thickness, you will need to find a compromise.

In the past, “standard double glazing” was often made for cost-saving reasons: glass without gas filling and without a high-performance coating on the glass. This has a U-value of approximately 2.8. Because the production costs of HR glass are now almost the same, we only offer HR coated glass. For almost the same price, you achieve much better insulating performance.

 

Which spacer profiles are available between the glass panes?

As standard, an aluminium spacer profile is used around and between the glass panes to create the space between the two panes. This is available in a standard aluminium colour or in black. The disadvantage of this profile is that there is a metal connection between the inner and outer pane. As a result, the cold from outside can be transferred through the profile to the inner pane. This may result in condensation forming along the edges of the inner pane, on the home side.

To prevent this as much as possible, so-called “thermally broken spacer bars” are available. This is a profile made up of several parts, which interrupts the thermal bridge between the inner and outer pane. As a result, there is much less cold transfer and therefore less condensation on the inner pane. This spacer does not make the glass unit retain heat inside the home more effectively, but it does help keep the cold from outside out of the home. This contributes to comfort in the home and, in the long term, to the maintenance of interior paintwork due to reduced moisture and condensation.

 

Which double glazing should I use for thermal insulation?

For this, you use HR coated glass with Argon gas filling. Preferably in combination with an aluminium spacer bar of 13 or 15 mm. With this combination, you achieve the HR++ classification. It is even better to use a thermally broken spacer bar with a width of 14 mm. With this profile, the thermal bridge between the inner and outer pane is also interrupted.

If you were to use a thinner spacer bar between the glass panes, the unit would fall back to HR+ or even HR classification. Sometimes this cannot be avoided because your frame simply does not offer enough space for a thicker cavity/spacer bar.

 

Which glass type and thickness do you need?

With double glazing, you need to choose the outer pane, on the outside of the home, and the inner pane, on the inside of the home. The glass type and thickness are determined by the size of the glass, the installation location and the safety requirements that apply. For example, with a larger pane, you will need to use two thicker glass panels. The following points are important to know:

• A slightly thicker pane is often used on the outside of the home due to weather influences such as wind load
• Preferably choose laminated safety glass on each side of the glass unit where a person can normally come into contact with it. In many cases, this will result in double-sided laminated HR+(+) glass
• Consult this page about the use of safety glass: “do I need to use safety glass?” 
• The HR coating is always on the pane on the home side
• Check carefully whether the thickness of your insulating glass unit will fit into your frame

 

How does the product name of insulating glass work?

With insulating glass, the composition is always viewed from the outside of the home, looking inwards. The elements of the glass are named in that order. For example, if you have a glass unit called: 5 - 13A - #33.1, this can be broken down as follows:

• outer pane: 5 mm clear glass
• cavity: 13 mm space, Argon gas-filled (“A” stands for “Argon gas”)
• inner pane: laminated 33.1 glass with an HR coating on the cavity side of the pane. The “#” symbol indicates where the HR coating is located

To calculate the total glass thickness, add the different elements together: 5 mm + 13 mm + 6.38 mm, laminated 33.1 glass, = 24.38 mm thick.

 

How much additional space do I need in my frame?

To install the glass according to the guidelines, you need to place 4 mm thick glazing tape on both sides of the glass. This amounts to a total of 8 mm. This is explicitly described in NPR 3577, which is regarded as the guideline for warranty purposes. If glass is not installed according to these guidelines, the warranty becomes void. You can find NPR 3577 here: NPR 3577

In addition to the thickness of the glazing tape, you also need to take the glazing beads into account. These also require a certain amount of space in the frame. If you unexpectedly run out of space, you can replace them with cover beads, allowing you to fit a slightly thicker glass unit into your frame. Often, the glazing beads are already present, or you may want to replace the existing beads with the same type to match the other windows. In that case, you need to measure the space between the frame and the glazing bead to determine which total glass thickness plus glazing tape will fit into your frame.

 

How do I determine the glass size, width and height?

The NPR 3577 states that, when installing glass in wooden frames, a rebate depth of at least 17 mm is required. This ensures that the insulating glass unit sits deep enough in the frame so that the spacer bar is not visible and the seal is protected against weather influences. If the rebate depth is lower, the frame will need to be adjusted to meet this requirement. If this is not possible and the glass is installed in a frame that does not comply with the guidelines, the glass warranty becomes void because the installation has not been carried out in accordance with the guidelines.

To determine the correct glass size, you first need to calculate the perimeter clearance. The perimeter clearance is the space that must be kept free around the glass between the frame and the glass. Use the following calculation: 1/3 x rebate depth = perimeter clearance, with the result always being at least 5 mm. With a rebate depth of 17 mm, this gives: 17 : 3 = 5.6.
Because we are dealing with tenths after the decimal point here, you can safely use 5 mm as the perimeter clearance. This still meets the minimum requirement of 5 mm. Ultimately, the important thing is that the glass sits sufficiently deep in the frame, that there is enough space underneath the glass for drainage and ventilation, and that the glass has some room to move if the timber expands or shrinks due to weather influences.

With a perimeter clearance of 5 mm, you therefore subtract a total of 10 mm from the exact height and 10 mm from the exact width. You then use glazing blocks with the same height as the perimeter clearance. So with a 5 mm perimeter clearance, you use glazing blocks that are 5 mm high.