An odd-one-out in our range! Our generalised advice is to avoid aluminium windows. Windows should keep heat inside. Metal (especially aluminium) conducts heat. It’s clearly not a good combination… However, there are a lot of UK homes that still have very narrow steel windows, and it’s physically impossible to squeeze a high performance timber window into that type of building construction. The solution is definitely an Alitherm Heritage unit: very slim thermally-broken aluminium frames provide far better insulation compared to the steel units they’re designed to replace, have a modern multi-point security setup and there genuinely is no better option available for windows set within stone mullions.