bi-fold door glazing options

Low E or Low Emissivity Glass

Low-e glass stands for low emissivity glass. This glass varies from normal clear glass in that one side of the glass has a special metal coating, technically known as a low emissivity, or Low E. coating. Low-E glass is a type of insulating glass, which increases the energy efficiency of windows by reducing the transfer of heat or cold through glass.

That means in the winter your house stays warmer, and in the summer it stays cooler.

There are two types of Low-E (low emissivity) glass available - Pyrolytic (hard coat) is considered to be a medium performer, and sputtered (vacuum deposition or soft coat) is considered to be the highest performer.

As energy saving will become more and more important in our daily lives, implementing low-e glass in our houses is becoming a standard we all have to comply to.

Typically double glazing using Low-E glass has energy conservation properties as good as normal triple glazing but without the 50% increase in weight.

The advantages of using Low-E glass are:

• Better heat insulation
• You save money with reduced heating bills
• Reduced carbon dioxide emissions into the atmosphere
• Reduces condensation on your window frames internally

Installing argon gas within the sealed units instead of air can have even greater energy savings. Argon is an inert gas, which has better thermal properties than dry air. In the UK we express the rate of heat loss in 'U values'. The lower the U value, the greater the thermal insulation and savings on your fuel bill.

Typical U values:

• The U value of single clear glass is 5.4
• With ordinary double glazing this is improved to 2.6
• With Low-E glass the U value is reduced to 1.8
• If argon gas is used to fill the air gap, the U value will reduce to 1.6

Low E Glass and Document L of Building Regulations

Most people are aware that effective 1st April 2002 - all replacement window installations in England & Wales are subject to BUILDING REGULATIONS.

This new regulation in particular affects the minimum levels of insulation that replacement windows must have when fitted in your home. Levels of insulation are measured as U values. The lower the U value, the better the insulation level. To get the required level of insulation some sort of LOW E glass (typically Pilkington K in the UK - although there are other brands) will have to be used. It may also be necessary for the sealed double glazed units to be Gas Filled (probably Argon).

There are two ways that you can ensure compliance with relevant regulations, firstly you may employ a contractor or installer who is registered under the FENSA self-certification scheme. This contractor will be approved to carry out the work in accordance with relevant regulations without inspection by the council and will inform FENSA when installation has been completed. Random inspections of completed work are carried out.

If you are not using a FENSA registered contractor or if you're doing the work yourself (DIY) then you will need to arrange building regulation approval yourself. Contact the building control department at your local council regarding this.

Low-e glass, how does it work?

The radiation coming from your heating system and your furniture and furnishings is long wave radiation. This type of radiation should be contained in your room as best as possible, while the radiation from the sun should be shielded and reflected back outside.

Winter Time
The sun's energy is "SHORT WAVE RADIATION" which passes through the window and is absorbed by carpet, furniture, etc. The energy is then transformed into long wave radiation. The long wave radiation wants to flow from warm to cool. Naturally, it will try to escape from the glass. At night the heat produced from radiators, wood stoves, etc, will also want to escape out through the glazing. The Low-E coating prevents this when the radiant room-side heat is reflected back into the building. This results in a lower winter U-value. For winter comfort, the higher the indoor glass temperature, the better the product is for comfort.

Summer Time
The Low-E coating manages the sun's heat in the summer by reducing the amount of heat transferred through the window in the summer. The Low-E coating filters the sun's short-wave radiation, which cuts down on the amount of solar heat gain into your building. For summer comfort, the lower the indoor glass temperature, the better the product is for comfort.

More on the two types of Low-e glass

Hard Coat Low E
Hard coat Low E, or pyrolytic coating, is a coating applied at high temperatures and is sprayed onto the glass surface during the float glass process.

Advantages

• The advantage is that the coating is relatively durable, which allows for ease of handling and tempering.
• Can be tempered after coating application.
• Can be used in single glazing applications.
• Utilizes passive solar heat gain.

Disadvantages

• Higher U-values compared to soft coat Low-E products
• Slightly higher haze levels
• Higher solar heat gain coefficient compared to soft coat Low-E products.
• Hard coat glass also has the possibility of a slight haze, which can be visible under certain angles.

Soft Coat Low E
Soft coat Low E, or sputter coating, is applied in multiple layers of optically transparent silver sandwiched between layers of metal oxide in a vacuum chamber. This process provides the highest level of performance and a nearly invisible coating.

Advantages

• High visible light transmission
• Ultra-low emissivity's giving optimum winter U-values
• Up to 70% less UV transmission compared with standard clear glazing
• Optical clarity - minimal colour haze

Disadvantages

• Soft coat Low E must be used in a double glazed unit; the soft coating is sensitive to handling.
• Most soft coat Low-E products require tempering the glass prior to the coating application.
• Edge deletion of the coating is required to insure a proper seal in an insulated unit
• There can be slight colour variations of coating.
• Generally speaking, a more expensive alternative than Hard Coat Low e glass.

What is the U Value of a typical Double Glazed sealed unit?

The following table shows some typical U values for vertical insulating glazing Pilkington Insulating Units and the effect of using K Glass, Argon and varying cavity widths. The lower the U Value , the better the insulator. U-value is the measurement of heat transfer through a given building material, glass, etc.

In the UK all U Values are quoted as W/m²K.

• Optifloat/air/Optifloat
  • 12 mm Cavity : 2.9
  • 16 mm Cavity : 2.7
  • 20 mm Cavity : 2.8

• Optifloat/argon/Optifloat
  • 12 mm Cavity : 2.7
  • 16 mm Cavity : 2.6
  • 20 mm Cavity : 2.6

• Optifloat/air/Pilkington K Glass
  • 12 mm Cavity : 1.9
  • 16 mm Cavity : 1.7
  • 20 mm Cavity : 1.8

• Optifloat/argon/Pilkington K Glass
  • 12 mm Cavity : 1.6
  • 16 mm Cavity : 1.5
  • 20 mm Cavity : 1.5

• Overall width of Insulight Unit (mm)
  • 12 mm Cavity : 20
  • 16 mm Cavity : 24
  • 20 mm Cavity : 28

U values based upon 4mm Pilkington Optifloat thickness. (NOTE: Optifloat is the name given by Pilkington to "normal" float glass)

Figures determined in accordance with the requirements of BS EN673: 1998 for 'normal' exposure conditions. U values are rounded to the nearest 0.1W/m²K as described in clause 9 of the standard. The total overall U value of a window can be calculated taking into account the insulation of the glazing, the unit spacer and framing.

We are often asked what cavity width is the best to use in a double glazed sealed unit, and as you can be seen above there is very little difference between a 16 mm and 20 mm cavity.

You will also note that adding Argon Gas alone has very little effect in improving the U Value. Only when the Argon is combined with Pilkington K Glass is any significant improvement made.

What glazing must be safe?

The updated British Standard "BS 6262: Part 4: 1994 Code of Practice for Glazing for Buildings", introduced new requirements that glazing fitted in 'critical locations' in domestic buildings must be safe.

This may be achieved for example by fitting glass which breaks safely; small panes of ordinary glass; thicker ordinary glass; by protecting the glass with a permanent robust screen; or using plastics glazing sheet. Further details are given below.

The GPSR and the standard apply to all domestic glazing installations whether new build, replacement or refurbishment.

What is a critical location?

Certain internal and external areas are considered 'critical locations' in terms of the safety of vertical glazing, as they are at risk from accidental human impact. The critical locations defined by the standard are similar to the Approved Document N of the Building Regulations 1991.

The 'critical locations' in any internal or external domestic area are:

Doors
Any glazing or part of that glazing in a door, which is between the finished floor level and a height of 1500mm above the floor level, is in a 'critical location'.

Side Panels to Doors
Any glazing or part of that glazing, which is within 300mm of either side of a door edge and which is between the finished floor level and a height of 1500mm above the floor level, is in a 'critical location'.

Windows, partitions, and walls
Any glazing or part of that glazing, which is between the finished floor level and a height of 800mm above the floor level, is in a 'critical location'.

Examples of locations

The diagram below, gives examples of glazing in windows, partitions, walls, doors and side panels. 'Critical locations' are shaded grey. Any glazing within a shaded area must comply with BS 6206.

In the diagram, glazing unit No. 10 falls wholly within a 'critical location' and so the glazing must comply with BS 6206.

Where only part of a glazing unit falls within a 'critical location' the whole of that unit must comply with BS 6206. In thr diagram this applies to units Nos. 2, 3, 5, 6, 7, 9, 11 and 12.

In the diagram only glazing units Nos. 1, 4 and 8 fall wholly outside the 'critical location' and need not comply with BS 6206.

Glazing in Windows, Partitions Glazing in Doors and Side Panels and Walls

What glazing is 'safe' in 'critical locations'?

Safety Glass and Safety Plastics
Safety glass, which complies with 'BS 6206: 1981 (1994)' may be fitted in 'critical locations'. This standard requires the glass to pass stringent tests involving impacts from a "punch bag" containing lead shot. Providing the glass does not break or breaks safely it is categorised as Class A, B, or C with A being the highest grade of safety glass.

Different types of glass can be classified as safety glass:

• Toughened Glass (also called tempered) categorised as Class A
  This looks like ordinary glass but receives a special heat treatment process to toughen it. It is much stronger than ordinary glass and on impact disintegrates into small granular pieces, which are not sharp, reducing the risk of injury.
  
  
• Laminated Glass available in Class A, B or C
  Consists of two or more sheets of ordinary glass which are attached together by a plastic interlayer. The plastic layer provides a barrier and on impact any broken shards of glass will remain attached to the plastic reducing the risk of injury.
  
  
• Wired Glass (also called Pyroshield safety clear/textured) categorised as Class C
  This glass has a network/mesh of wires embedded in it. Certain types of wired glass can satisfy the impact requirements for safety glass while giving a level of fire resistance.
  
  
• Plastics Glazing Sheet
  Certain types of transparent plastic sheet can satisfy the impact requirements for safety glass. Please Note: Glass in doors and side panels may only be glazed in Class C materials where the smaller dimension is a maximum of 900mm. Where this dimension is greater than 900mm glazing categorised as Class A or B is required.
  
  

What should I look for on safety glazing?

'British Standard 6206: 1981 (1994)' requires that each piece of safety glazing used within 'critical locations' should be marked with the all of the following:

• The British Standard number 'BS 6206'.
  
  
• Identification of the type of glass used i.e. 'L' for laminated, 'P' for plastics, 'T' for tempered (toughened), 'W' for wired or 'SFB' for safety film backed.
  
  
• The category of safety glass used i.e. 'Class A', 'Class B', or 'Class C' (based on its performance under impact tests). A suffix 'o' denotes front impact test only e.g. mirrored wardrobe doors.
  
  
• An identifiable name, trademark or other identification mark of the manufacturer. If any of the above markings are lost by cutting, then the company which carries out the cutting must replace markings a) to c) and add their own identifiable name or trade mark.