Insulating Glass Units

Heat will migrate from a warmer surface to a cooler surface when there is a difference in temperature between two surfaces.

A transparent glazed surface is subject to solar radiation, G Factor, resulting in heat gain.

Double or triple glazing will give better thermal insulation than single glazing.  The dry, still air between the two sheets of glass improves the thermal insulation.  Heat exchange by convection is reduced and the low thermal conductivity of the still air limits heat loss by conduction.

The U-value of an insulating glass unit can be improved by reducing the heat transfer by conduction, convection and radiation.  This is achieved by reducing the heat exchange between the glasses incorporated in the insulating glass unit.

A low-emissivity (low-e) coating will reduce the radiated heat transfer.

Argon gas filling of the cavity will reduce heat loss by conduction.

A solar control glass towards the exterior, with high solar radiant heat absorption or reflectance, will reduce solar energy heat gain.


Frequently Asked Questions about Insulating Glass Units

What is U-value?

The rate of heat loss per square metre for a temperature difference of 1 degree Kelvin, or Celsius, between the interior and exterior.

What is the "greenhouse" effect?

Clear glass will allow heat from the sun to enter the building, contributing significantly to internal temperatures.

What standard are the units made to?

EN 1279 Parts 1 to 6

Is there a date from which all units must be made to EN 1279?

April 2007

How can I improve the U-value?

  By using either a hard or softcoat low-e glass with Argon Gas in the cavity if necessary.

Can heat be kept out?

Solar heat gain can be reduced by using a solar control glass.

Does the thickness of glass improve the insulation value?

Very little.

Is there a self cleaning glass?

There are clear and tinted self cleaning glasses produced by the glass manufacturers with a hard hydrophilic coating to help the removal of dirt when it rains or hosed down.  There are other applications available that are applied to the surface of the glass prior to installation, the life of which should be checked with the manufacturers.

How should my frames be constructed?

A frame needs the U-value or WER (the Architect, Specifer or Frame Maker will give you the U-value or WER for the frame); from this information we can give you the required makeup of the insulating glass unit.  Make clear if your U-value is a centre pane U-value for the glass alone or a combined frame & glass U-value.  The size of the rebate in timber frames especially, is critical, the height of the rebate must be at least 15mm & the width, thickness of the unit, plus 6mm for glazing materials, plus the width of the timber bead (normally a minimum of 12mm).

What cavity widths (1) are available and what colours (2)?

  1. 4mm, 6mm, 8mm, 10mm, 12mm, 14mm, 16mm, 18mm & 20mm.
  2. Warm Edge Spacer GREY or BLACK. Aluminium spacer SILVER, WHITE, BRONZE, BLACK & GOLD.  Some cavities and colours are only available on request.

If I do not specify a spacer colour, what will I get?

Default spacer colours are silver for aluminium & black for warm edge.>

Are there inserts available to go into the cavity?

Georgian Bar - Straight or curved in White or Brown, 25mm/18mm × 8mm.
Gold Victorian Grill - Please submit your enquiry.
Duplex Bars - The pitch, the dimension of the flat of the bar to be applied is required.  Duplex bars are only incorporated in units that are constructed toughened glass to minimise the risk of thermal fracture.

Can units be leaded?

A self adhesive lead strip is applied to the inner and outer surfaces of the outer leaf of the insulating glass unit.  In squares, diamonds and colour film designs, details with any necessary drawings and colour references are required.  Standard designs are available.

How should units be glazed?

Glazing should be in accordance with BS8000.  See our Arbo Heel Bead Glazing System page for the basic method; contact us for other types of glazing and glazing of self cleaning glass.

Are there special requirements for glazing glass in a roof?

  1. The outer leaf is normally toughened to resist thermal fracture.
  2. The inner leaf can be toughened or laminated depending on the application.  If laminated it must be stepped back to be within the building.
  3. If the bottom edge of the insulating glass unit is exposed, the edge should be encapsulated by a U-channel or the top leaf should be longer than the bottom leaf to run water clear of the seal.
  4. Both top and bottom leaves must be "stopped" in the rebate to prevent the glass sliding down.

Can I have a cat flap in an insulating glass unit?

Yes, buy a cat flap with a round hole for fitting into glass.

After glazing what maintenance is required?

Follow the maintenance instructions below for all types of installation:

- Flush Edge Units

Inspections should be carried out after about one year and periodically,thereafter, as deterioration could take place as a result of incorrect application or vandalism.

Damaged sealant should be cut out and replaced in the affected areas.  The Sealant Manufacturers' recommendations should be followed.

Where gaskets have been displaced or damaged, or where there are gaps, they should be re-fitted or cut back and capped with silicone sealant.

With vented glazing methods, checks should be made to ensure the drainage or ventilation holes or slots, have not become blocked with dirt.  While checking these, it is also advisable to examine the effectiveness of the seal of the exposed frame joints, which may determine the weather tightness of the rebates.

- Stepped Edge Units

Routine maintenance is similar to that required for single glazing with the putty fronting. i.e. ensure that painting is carried out regularly to provide protection for the putty.  The paint line to extend onto the glass inside and out.

- Micro Porous Frames and Stained Frames

Special care must be given to the inspection of treated and stained softwood frames.  Lack of routine maintenance causes the finish to flake off and joints in the frame to open up.  This may allow water to permeate the seal and cause failure of the unit.

Visual quality standard for installed insulating glass units constructed from flat transparent glass.

It is necessary to apply a viewing standard when it is deemed an insulating glass unit may have a fault.  The glass manufactures apply a standard to their glass for imperfections in the main body of the glass and any coatings they may apply to it during their manufacturing processes.  The quality of glass used in the manufacture of insulating glass units is the same as that for single glazing.  Working with the materials supplied to us to the relevant EN standards in turn dictates the quality of the insulating glass units supplied to you and has to be taken into account for the finished product.

The viewing standard will be as follows:  Annealed glass will be viewed at 2 metres.  Toughened, laminated or low e glasses will be viewed at 3 metres.  Other coated glasses will be viewed from 3 metres.  The viewing will be at 90 degrees, in natural daylight, imperfections may not be marked to highlight their presence and will only be visible to the naked eye.  A band 50mm wide around the perimeter of the insulating glass unit is excluded as an area to be viewed.  The insulating glass unit will be looked through, not at the surfaces.

Flat transparent glass, including laminated and toughened shall be deemed acceptable if the following phenomena are neither obtrusive nor bunched: totally enclosed seeds, bubbles or blisters; hairlines or blobs: fine scratches not more than 25mm long: minute embedded particles.

Due to the manufacturing process seeds and bubbles are deemed acceptable in patterned glass.

Duplex and Back to Back Bars

The method of construction compounds tolerances and allowance should be made for this when determining the width of the bar to be applied to the surface of the glass.  The width of the bar should be the pitch of the Duplex or Back to back bar plus 3mm.  Deviation from parallel along the bar lines is acceptable.