Sealed units explained

The ‘sealed unit’ is the cornerstone of modern, well-insulated window construction, but what exactly is it and how does it achieve its insulating properties?

We explain what a sealed unit is, its benefits and how to spot the signs when windows are ready to be replaced or upgraded.

What is a sealed unit and how is it constructed?

A sealed unit is the glazed element of the window that sits within the frame. It usually consists of two or three panes of glass (double or triple glazing), which are separated by a vacuum or gas-filled pocket of air.

It is this area between the glass panes that gives the sealed unit is ability to reduce the transfer of heat and achieve insulation. Argon is the gas most commonly used to provide this insulation.

‘Spacer bars’, usually made of aluminium, are used to separate the panes of glass to form a cavity that enables thermal insulation.

The spacer bars hold the ‘desiccant’, an absorbent substance such as silica pellets. Through small holes in the spacer bars, the desiccant adsorbs any moisture that enters the sealed unit during its lifetime.

Finally, sealant is applied around the edge of the glazing, to achieve an airtight seal and further enhance the insulating properties.

Year-round thermal efficiency

Sealed units are designed to keep your home cooler in the summer and warmer in the winter. They work hard year-round so that fans and heating can work less hard, which is better for your pocket and for energy consumption.

The glazing in sealed units is often treated with a solar-control coating that manages the UV light coming into the home, which in turn assists insulation.

Warning signs that a sealed unit may need replacement

There are certain signs that a sealed unit may need to be replaced:


If you begin to notice fogging between the panes of glass and your windows are still in good condition and do not need to be completely replaced, it may be possible and cost-effective to replace the sealed unit.

Damaged glazing or seal

Windows with broken glass or seal should be replaced. When the airtight seal is compromised, the windows are no longer fully insulated. Replacing the sealed unit will make your home safer and more energy efficient.

Sometimes replacing the sealed unit is not enough and a full window replacement needs to be considered:

Single glazing

If you only have single pane glazing, you may wish to realise the benefits of energy-efficient double or triple glazed windows.

Damaged or rotten frames

When window frames become damaged, warped or show signs of rot, they will no longer be airtight or provide efficient insulation for your home.


If you notice draughts or cold air around your windows, your home is not sufficiently insulated or energy-efficient.

View examples at our showroom

Window Wise fit sealed unit windows in and around Sussex. If you would like to see examples of the available options, please visit our Haywards Heath showroom.