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DOUBLE GLAZED UNITS
A double-glazed unit consists of two panes of glass with a sealed air or gas-filled space between them. This design enhances thermal efficiency, reduces heat loss, minimises condensation, and improves sound insulation compared to single-glazed windows. Typically, a double-glazed unit is 28mm wide, made up of two 4mm glass sheets with a 20mm cavity. Thanks to advancements in technology, there are now various ways to configure double or even triple-glazed units to better suit your specific needs and preferences.
U VALUES
Improving your home’s energy efficiency is at the core of our industry. A U-value, or thermal transmittance, measures how effectively a material insulates heat. It is calculated by dividing the rate of heat transfer through a material by the temperature difference across it. This applies to all building materials, including windows, which are a significant source of heat loss in a home. Measured in watts per square meter per Kelvin, the lower the U-value, the better the insulation. In other words, the higher the U-value, the more you will spend on heating your home! This makes it a crucial factor to consider when selecting glazing for your property. As shown in the examples below, different types of glazing can have a significant impact on energy performance:
4mm single glazed float glass – 5.8 W/m2K
4mm double glazed float glass with aluminium spacer, air filled – 2.9 W/m2K
Double glazed 4mm low iron, warm-edge spacer, argon filled, 4mm planitherm – 1.2 W/m2K
Triple glazed 4mm float, 16m argon, 4mm planitherm, 16mm argon, 4mm planitherm – 0.6 W/m2K
GLASS
Standard glass is referred to as float or annealed glass. Float glass has relatively low resistance to thermal stress, meaning it can easily crack or break when exposed to significant temperature differences across its surface, typically only withstanding a temperature change of around 40°c. Float glass is commonly used in windows and breaks on Impact into sharp angular pieces.
SAFETY GLASS
There are regulations that require the use of safety glass in certain situations, however at PWW&C we use toughened glass for almost all of our installations. There are two types of safety glass:
Laminated glass – made of two or more layers of glass that are fused together with a thin transparent plastic film interlayer, the glass holds together even if broken. Due to the interlayer, laminated glass is also used for UV reduction where fading due to UV is a concern. Building Regulations Part Q specify the use of laminated glass for ground floor and accessible doors.
Toughened glass – Heating annealed glass in a furnace to around 620°c followed by rapid cooling creates tension and compression in the glass making it stronger and more resistant to breaking. Around five times stronger than annealed or laminated glass, it needs to be hit a lot harder to break. If it does break, it breaks up into lots of small blunt pieces rather than sharp shards.
REDUCING SOLAR HEAT GAIN
Whilst in some circumstances solar gain is beneficial, it is not always the case and the amount of heat entering a room needs to be controlled..
There are a range of glass configurations that can substantially reduce the solar gain or G-value:
- 4mm Float 0.87
- 4mm Float/4mm Float 0.78
- 4mm Float / 4mm Planitherm 0.71
- 4mm Float/ 4mm Bronze 0.58
- 4mm Planitherm / 4mm Bronze 0.51
- 4mm Activ Neutral/ 4mm Planitherm 0.41
- 4mm Activ Blue / 4mm Planitherm 0.40
- 6mm SKN 154 / 6mm Float 0.27
Conservatories, have gained a bad reputation for being hot in the summer and cold in the winter, however with improved glass technology this can be mitigated significantly. By putting glass with a lower G-value in the roof of the conservatory, when the sun is high in the sky in summer months heat will be reflected. However, when the sun is in a lower position in the sky during winter months, using glass with a higher G-value in the vertical glass will make the most of the solar gain.
LOW IRON GLASS
Low iron glass is a specialised type of glass designed for enhanced clarity and energy performance:
Regular glass contains a higher level of iron, which gives it a greenish tint, especially at the edges. Low iron glass, made from sand with minimal iron content, appears nearly clear. The clarity of low iron glass allows more light and heat to enter the building, which is particularly useful on colder north facing rooms.
LOW-E GLASS
Low-E glass has a microscopic coating that can reduce thermal transfer. Low-E glass (Low Emissivity) glass is a type of energy-efficient glazing that has a special coating designed to reduce the amount of heat that passes through it. This coating reflects infrared light, keeping heat inside during the winter and outside during the summer, thereby improving the energy efficiency of buildings. Low-E glass allows visible light to pass through while minimizing the transfer of ultraviolet (UV) rays, which helps in reducing fading of furniture and fabrics. It can be used in both windows and skylights, providing thermal insulation without sacrificing natural light. This makes it a popular choice for residential and commercial buildings looking to improve comfort while lowering energy costs.
At Philip Whear we use Planitherm-1 low-E glass, which offers the lowest centre-pane U-value for any double glazed unit.
SOLAR CONTROL GLASS (SKN)
The COOL-LITE® SKN range is specially engineered to provide effective solar control residential projects with large windows. This advanced solar control glass is designed to prioritize both comfort and energy efficiency, making it an ideal choice for reducing overheating while improving a building’s overall energy performance.
When designing a space, factors like aesthetics, interior design, noise levels, and natural light all play a key role in creating a comfortable and welcoming environment. Thoughtfully crafted spaces can boost mood, increase productivity, and even contribute to better health.
The COOL-LITE® SKN range is designed with these factors in mind, offering high-performance double silver-coated glass that allows for maximum light transmittance while maintaining a low solar factor. This makes it perfect for large glazing applications where both comfort and energy efficiency are a priority.
Available at Philip Whear on request.
UV REDUCTION
If colour fade is of concern in a particularly bright room it may be worth considering 6.8 mm laminated glass which reduces UV light by up to 97%.
SPACER BAR
Spacer bars are used to separate the panes of glass to form a cavity that enables thermal insulation. Older or cheaper double glazed units will have an aluminium spacer bar. Warm edge spacer bars outperform the older style aluminium spacer bars due to their low thermal conductivity, thereby improving u-value. A warm-edge spacer is made of a highly insulating composite plastic material that significantly improves the window’s energy efficiency. This results in reduced heat transfer across the edge of the unit, which means greater energy efficiency. By minimising heat loss at the edge of a glass unit, these spacer bars can help reduce condensation. At Philip Whear we use Swiss-V warm edge spacer, widely known to be the best performing spacer bar manufacturer on the market. We can supply units with an aluminium spacer to match existing units if requested.
A warm edge spacer is an essential component for double-glazed units, offering both functionality and style:
Colour Options: Black, white, or grey
The spacer bar is sealed with a polyurethane hotmelt edge sealant.
CAVITY
The cavity between the two panes of glass in a double glazed unit is typically air filled. Gas fillings in double-glazed units enhance energy efficiency:
Argon Gas: Reduces the U-value of a sealed unit by 0.02 W/m²K. While the impact on global warming might be modest, it can make the difference in meeting window compliance standards.
Other Gases: Krypton and Xenon are alternatives, particularly effective but they come at a high cost. While these gases offer superior performance, their price makes them more suited for specialised applications.
At PWW&C we filled our sealed units with Argon as standard.
BROKEN GLASS
Glass doesn’t break without reason—there’s always a cause behind it. Here are some common factors that can lead to breakage:
Impact Damage: Direct force or impact to the glass surface. Most common culprits are strimmers and small stones in the garden.
Edge or Face Damage: Chips or cracks on the edges or face of the glass caused by mishandling or accidental knocks.
Nickel Sulphide Inclusions: Microscopic imperfections within the glass that can cause it to fracture over time.
Understanding these causes can help prevent glass breakage and ensure a longer-lasting, durable product.
HEAT SOAKING
In particularly tricky installations that may require scaffolding to replace a unit, we recommend heat soaking. This is an additional process whereby the toughened glass is placed in a chamber and heated. This process reduces the chances of spontaneous breakage caused nickel sulphide inclusions.
This combination of advanced materials and technologies ensures optimal energy efficiency and performance for your installation.
HOW CAN WE HELP?
Get in touch with us today – we would be delighted to hear from you, and look forward to potentially working with you.