I wrote this in a previous post about an online advice centre called
Window Advice CentreJust copied it for here since when choosing double glazing there's quiet a lot of difference between a good window and a bad one. A bad one has no strength, discolours and no great benefit for heat, cold or condensation.
Nothing wrong with sites like these really, they get you to put forward the job you require then put it out to tender to a few companies (from 3 to 10... tho 10 is a lot) and the company then contacts you to arrange to send a rep round or if they've taken your measurements they then give you a quote and hopefully a detailed spec of the window they are selling.
Here's a few pointers on what to look out for
internally beaded window frames for security
all joints welded together not screwed and glued
20mm air gap between glass and proper sealed units NOT sealed with rubber which erodes and causes misting of unit. You can tell quite easily on a medium to large window if it's a good sealed unit by knocking it. If it soounds like you're knocking glass it is not good. If it sounds like a thud it is !
pilkington glass, has to be k glass (low emmission)in house windows
Grade A uPVC is best, first call on uPVC so stays white for years aposed to going grey and dirty looking
Multi point locking - are all the corners covered from being jemmied as well as the bottom
Hmmm... i think that's about it
Tho for prices, that varies hugely. The small local company always seems to be favoured on here tho for my opinion medium sized local companies are best as they have a proper factor for making the windows aposed to buying them in or making them up out the back.
Have fun !