Two aluminium windows can look identical yet behave completely differently in winter and summer. The difference is a strip you can't see.
Aluminium conducts heat very well. In a saucepan that's an advantage; in a window it's a drawback. A solid aluminium frame is cold to the touch in winter and hot in summer, and heat escapes through it like a radiator facing the wrong way.
What a thermal break is
In thermally broken profiles, the outer and inner parts of the frame aren't joined directly but through polyamide strips, a material that conducts heat poorly. The profile is "broken" in half, so cold from outside doesn't reach the inside.
In practice: the inside of the frame stays at room temperature, condensation doesn't collect on it, and there's no damp or mould around the window.
When it's a must
For any space you heat or cool: flats, houses, holiday apartments, offices. On the coast people often forget the thermal break works in summer too, when it's 35 degrees outside and the air conditioning is running flat out, a "cold" frame lets heat in just as it lets warmth out in winter.
When it isn't necessary
For unheated spaces: storerooms, garages, utility rooms, terraces enclosed for summer use only, and interior partitions. There a simpler "cold" system makes sense and saves money.
What about the price difference?
Thermally broken systems cost more than non-insulated ones. But the difference comes back through energy bills, and the comfort is incomparable. For living spaces we always recommend a thermally broken system.
Don't forget the glass
The best frame doesn't help much with ordinary glass in it. A thermally broken profile only makes sense with good insulated glazing, ideally Low-E. Only then does the window work as a whole.
Alumil, whose systems we install, offers both versions, so for every opening we choose what it actually needs.