Both UPVC and Aluminium windows can be energy efficient and help save heat, but they have some differences in terms of thermal performance.

UPVC windows are typically more energy efficient than Aluminium windows because they are a better insulator. UPVC windows are made from a type of plastic called unplasticized polyvinyl chloride, which has a lower thermal conductivity than aluminum. This means that UPVC windows help to keep heat in your home better than aluminum windows.

Aluminium windows, on the other hand, are made from a material that is a good conductor of heat. This means that aluminum windows can transfer heat more easily than UPVC windows. However, modern aluminum windows are designed with thermal breaks which are barriers between the inside and outside of the window frame that reduce heat transfer. This makes them more energy efficient than traditional aluminum windows.

Another thing to consider is that both types of windows can be made more energy efficient by adding Low-E coatings or Argon gas filled in the gap between the glass panes. This helps to reduce heat loss and improve the overall thermal performance of the windows.

In summary, UPVC windows are generally more energy efficient than aluminum windows due to their better insulation properties, but modern aluminum windows with thermal breaks and additional energy-saving features can also be an energy-efficient option.