Guide to Modern High Performance Glazing

Don’t know the difference between soft coats and hard coats, low iron and low e, self cleaning and anti sun glass, then please read on, this guide provides an introduction into modern high performance glazing.

A window provides a view to the outside world, thermally and acoustically insulates, shields us from the elements, provides ventilation and emergency exits and adds to the architecture of a project. Some of these functions are achieved through the type of glazing employed. In recent decades much improvement has been made in this area. Glass can now perform more functions than you may expect.

Much of the current improvements in glazing are driven around improving thermal performance. When selecting the type of glass for your project consider its ‘u’ value. This is a measure of heat loss in an element or compound such as a glass unit or window. Low ‘u’ values indicate high thermal insulation and greater energy savings.

To begin with consider the following table of potential ‘u’ values.

Material U Value Comment
Total Insulation 0 No energy loss
Roof 0.25 Minor energy loss
Wall 0.3 Minor energy loss
Modern high performance double glazed unit 1.2 3 times the performance of old units
Old double glazed unit 3 10 times the energy loss of a wall

Factors influencing ‘u’ values in a glazed unit include:

  • Number of glazing panes, thickness and space between them
  • Gas within the unit
  • Size of the unit
  • The coatings on the glazing surfaces
  • The type of space bar
  • Inclination of the unit

As a standard we provide a double glazed unit using Planitherm (Total +) on the inside pane with a low iron substrate on the outside pane achieving a centre pane ‘u’ value of 1.2 W/m^2. The low iron content in the unit allows for more light transmission in comparison to units you may get from standard clear glass, and this coupled with the neutral appearance of Planitherm achieve the following visual benefits:

  • No unsightly tint
  • More Light
  • No Haze effect

The low ‘u’ value is achieved through a combination of factors. Please see diagram below.

High performance glazing 1

The Zone of Discomfort

Modern high performance glazing units don’t have a zone of discomfort compared to older double glazing. This zone of discomfort is from the surface of the inner pane into the house. It is an area of lower temperature. Traditionally this meant a build up of condensation on the inner pane. Condensation is reduced on modern double glazing, but it is not eliminated as condensation is caused when the moisture in the air comes into contact with a cold surface. Therefore you will typically find condensation in areas where there is a lot of moisture in the air, such as kitchens, bathrooms, bedrooms, and where there is poor ventilation or heating.

High performance glazing 2

Glass Coatings – Soft Coat vs Hard Coat

The thermal performance of glass is achieved through applying a coating to the surface of the glass. These coatings fall into two categories – soft coats and hard coats. A soft coat is applied after the manufacture of the float glass. A hard coat is applied during the manufacture of the float glass and is a metal oxide coating with additives applied directly to float glass while it’s still hot.

Soft coat

  • Generally soft coating offer the lowest ‘u’ values available.
  • High light and low UV transmission
  • More difficult to work with during manufacture
  • Cannot be used in single glazing applications

Hard Coat

  • Can be used if single glazing required
  • Generally lower optical clarity and higher u’ values than soft coat
  • Easier to work with during manufacture

Much of the current trending in glass manufacturing is on the varieties and enhancements in coating technologies. The type of coatings used can have a big impact on thermal performance. Below is a table of glass units with different coatings and the ‘u’ values they achieve.

Glass Type U Value 28mm Double Glazed Unit 4-20-4 U Value 28mm Triple Glazed Unit 4-8-4-8-4 U Value 44mm Triple Glazed Unit 4-16-4-16-4 Coating
Saint Gobain Planitherm Total + 1.2 1.0 0.6 Soft
Saint Gobain Planitherm 4S 1.0 0.9 0.5 Soft
Saint Gobain Planitherm One II 1.0 0.9 0.5 Soft
Planibel A 1.4 - - Hard
Pilkington K 1.5 - - Hard
Pilkington K S 1.2 - - Soft

NB - Higher U values can be obtained through different coating combinations.

Triple Glazed Units

As can be seen from the table above triple glazed units can offer slightly better ‘u’ values than their double glazed counterparts. Triple glazing may however not always be the best option for your project. The ‘u’ values achieved are probably only relevant when they fit into an overall energy saving project such as a passive house. Triple glazing may offer slightly lower light penetration and comes at a higher cost. Double glazed units are normally preferred but in certain circumstances triple glazing can be the required option.

The Saint Gobain Planitherm Range

Extremely neutral in appearance even though its low e glass

Planitherm Total +

Uses a special coating to reflect heat back into your home while allowing free solar heat in.

Planitherm 4s

Gives both thermal insulation and solar control. This type of glass will provide a comfortable environment in all seasons. Reduces unwanted heat gain in the summer and maximises solar heat in the winter. One example of where this type of glass is well suited would be a conservatory where temperature control is important (Note that Planitherm 4s is unusual in so far that it is placed on the outer pane of the double glazed unit, all other low-e glass are normally placed on the inner pane of the double glazed unit)

Planitherm One II

Represents the best thermal insulation available in the Planitherm family. A unique combination of metal oxide layers added to planilux clear float glass magnetically (soft coat). Also available in 6mm so it can be used on very large units.

The Planitherm range can be combined in a double/triple glazed unit with different substrates offering different qualities. One example of which is bioclean active glass. This has a chemically bonded active coating on the outside pane. The coating absorbs UV light from the sun which causes a chemical reaction which breaks down the dirt on the glass. When the rain comes it washes away this loosened dirt and leaves the window clean. This type of glass can fit well into projects such as conservatories or other areas in which glass is hard to reach.

Glossary of Terms

Low E Glass

Low E stands for Low emissivity glass and refers to the pane of glass that is designed to reflect heat back into the room. Low E glass is either soft coated or hard coated

Low Iron Glass

Refers to glass that allows more sunlight and energy into the room (heat gain), improving the overall energy rating of the unit.

U Value

Measures the heat loss (measured in Watts per metre squared Kelvin). The U value of the glass (often referred to as the Centre Pane U Value) is combined with the U value of the frame to give an overall U value for the window or door.

Energy Rating

Measures the difference between heat loss and heat gain. Applies to the total window, and is graded A to G, with A being the best in performance.

Self Cleaning Glass

Glass with a coating applied to the help break down dirt particles. When it rains the water washes away the loosened dirt.

Anti Sun Glass

Reflects solar radiation, helping to keep rooms cooler in the summer. Often combined with self cleaning glass and used in conservatory roof glazing.