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Wickes show 24mm thick glazing 70mm profile. Giving 1.4Watts/sqm
28mm has been "the standard" thickness for a fair while and 1.4W/sqm is rather poor for a new install.
90mm thick window frames aren't uncommon now and warm edge spacers between the glass panes with argon fill doesn't cost any more than aluminium spacer.
Even with 70mm frames, the number of internal boxes varies between cheap and decent profiles so you do get what you pay for (to a point). The problem being they mostly look the same from the outside.Theres some good info here.
https://aspirebifolds.co.uk/2018/03/what-are-typical-u-values-on-windows-and-doors/Still £5k can >>>>>
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I didn't think I'd be recommending them, but once you get past a truly awful sales team Anglian have been brilliant... We have some fairly large tilt and turn and non-opening windows (2m x 1.6m largest single frame) fitted this week, price point, fitting and finish and warmth all on the good side. Even their scaffolders were less manic than you'd usually expect...
Standard sized uPVC window from Wickes - £100-400
Big custom size window but still uPVC - £5000!
Think I might get another quote.