Back to the double-glazing.

Gas filling is the last step (to date) in achieving a small but significant improvement in U-value, (according to the published U-value calculators) but is perhaps the most difficult to explain. Air is a wonderful insulator - generally accepted to be the best of all - but three of the inert gases are reported to have better insulating qualities still, despite being heavier (and therefore denser). Logic might suggest that they might conceivably be better sound insulators but worse thermal insulators. Clearly there's more to this than meets the eye, as I have seen it claimed that these heavier gases - which also convect, of course - are at their best at successively smaller glass spacings. On that basis, it appears that an argon-filled DGU with a 12mm spacer ought to perform better than one with a 16mm spacer. Not according to my BRE U-value calculator it doesn't. I rather think that the benefits are questionable. However, for my preferred 20mm DGU (12mm spacer) argon confers a demonstrable and affordable reduction of 0.2 in the U-value of an average-size domestic window installation. Whether it's cost effective is arguable. Whether the use of expensive krypton or eye-wateringly expensive xenon is ever justified is highly, highly questionable. The claim that either (or, most puzzling of all, a mixture - in just what proportions?) of these two gases miraculously tranforms a 4mm gap into an efficient insulating unit is emphatically NOT borne out by the published U-value calculators from our most reputable authorities.

On a cheering note: Yesterday, a passing trio of canvassers for a national pvc window company told my wife (working in the front garden) that they wouldn't bother to pester her, as our wood windows were clearly excellent, and she wasn't going to be interested in what they were pushing. Knowing that pvc salesmen never have an option up their sleeve for those they encounter who really do want wood, I suggested they point such leads in my direction. They jumped at the opportunity. Other canvassers, form a queue here, please.