Good quality PVC windows

Can anyone recommend a good quality, good U-value, PVC window supplier? We are building in Galway and we have already got quotes from Wright Window Systems and Grady Joinery.
What should we be asking for in a PVC window to make sure that they are well made?

Sorry to all the timber window fans but this is the route we are going to take.

try senator windows.

what you need in a good window is.

steel reinforcing , gas filled , welded joints , 10 year guarantee , rated window and so on