# Gas filled unit

A gas-filled unit is a type of double or triple glazing where the space between panes is filled with an inert gas-typically argon, and sometimes krypton-to reduce heat transfer and improve thermal insulation.

These gases are denser than air, slowing conduction and lowering the window’s U-value by up to 30%. In heritage windows, slimline gas-filled units (14–16mm thick) offer a practical balance of energy efficiency and authenticity, though they require high-quality edge seals to prevent long-term gas leakage.