Our Slim Double glazing options

Slim double glazing units often called slimline sealed units are designed to fit into traditional or narrow window frames while still providing the performance benefits of modern glazing. They use a combination of advanced components to improve thermal efficiency without altering the look of heritage or period windows.

Option 1   

Standard 12mm slimline sealed unit

Standard 12 mm double-glazing units are commonly used in traditional sash and casement windows where the glazing rebate is shallow and cannot accommodate full-depth modern units. These slimmer sealed units are designed to offer enhanced thermal performance while maintaining the original appearance of period or heritage windows. At 12 mm overall thickness—including two panes of glass and a narrow cavity and typically with an aluminium spacer they are a practical compromise between energy efficiency, weight, and compatibility with older frames.

Even with a 12 mm profile, this combination allows slim units to perform significantly better than traditional single glazing while remaining visually discreet.

For sash windows, the lightweight nature of a 12 mm unit is especially important because it preserves smooth operation and avoids overloading the existing sash weights. In casement windows, slim units help maintain the slender sightlines and period detailing that are often lost when deeper, heavier glazing is used. Overall, 12 mm double-glazing units provide an effective balance of heritage appearance and modern performance, offering improved comfort, reduced draughts, and lower heating costs without compromising the character of the window.

Option 2 is our standard 12 slimline units with the up-grade option of Low E glass and a warm edge-spacer.

This option is the most popular when option 3 is not available to you because your windows will not accommodate the thicker units.

(when adequate space is available )

Our up-graded slim sealed units start at just 14mm. When your windows have enough space to accommodate this size of unit this option would be available to you, we can advise if this option is available if you book free survey. The units have a warm edge spacer, the glass has a low-E coating & the gap between the inner & outer panes is filled with argon gas.

Three of the most important features in these units are warm-edge spacers, low-emissivity (Low-E) coatings, and argon gas filling, each of which plays a key role in reducing heat loss and improving comfort.

Key Features

Warm-edge spacers sit between the panes of glass and keep them the correct distance apart. Unlike older aluminium spacers, warm-edge versions are made from materials with very low thermal conductivity, such as insulated polymers or stainless steel hybrids. This greatly reduces heat transfer between the inside and outside of the window. The benefits include a warmer internal glass edge, which reduces condensation, improves overall energy efficiency, and increases the longevity of the sealed unit by reducing stress caused by temperature differences.

Low-E coatings are ultra-thin metallic layers applied to one of the glass surfaces inside the unit. They allow sunlight and daylight to pass through while reflecting heat back into the room. This helps retain warmth during colder months and can even limit excessive heat gain in summer, depending on the coating type. The advantages of Low-E glass include lower heating costs, more consistent indoor temperatures, and less strain on heating systems. Crucially, because the coating is virtually invisible, it doesn’t affect the appearance of slim traditional windows.

Argon-filled units replace the normal air between the panes with argon gas, which is denser and conducts heat far less effectively than air. This slows down heat transfer through the glazing, boosting insulation without changing the thickness of the unit. Argon also reduces convection currents inside the cavity, helping the window achieve a better overall thermal rating. The main benefits are improved energy efficiency, reduced cold spots and draughts near the window, and quieter indoor environments thanks to slightly enhanced sound insulation.

Together, warm-edge spacers, Low-E coatings, and argon filling significantly enhance the performance of slim double-glazing units while preserving the aesthetic qualities required for heritage or finely detailed frames. This combination delivers a discreet, high-performance glazing solution that improves comfort, efficiency, and long-term durability.