Sash Windows and Climate Change

Roger Whitfield sent me the following email…

Re sash windows – the first answer is to (retro-) draughtseal them – otherwise air movement through the window will lose more heat than the single glazing, and would render any more sophisticated glazing irrelevant.

A second answer alters the architectural appearance to some extent, and is to secondary glaze. This glazing can be single or double but will lead to a more cluttered look in the window reveal, especially since it must open both for ventilation and so as not to compromise egress reqs. If the inner glazing system is draughtsealed and the original outer glazing is vented to outside, then condensation issues shouldn’t arise.

A third answer is to re-sash the existing boxes to incorporate double units. You ’steal’ the original depth of the flat between rebate and mould, gaining 12.7mm, and extend the sashes slightly (say 3mm) into the parting bead gap, and if you can achieve a rebate of 25mm thereby that could comprise a 9mm seating for a bead aginst a 14mm unit, the rest being seating/sealing materials, presumably in this case silicone from a gun (low modulus, like most glaziers use), since it’s thinner than applied glazing strips. The unit itself would be 4-8-4K with krypton fill. The problem is the weights, and how you solve that will depend on the proportions of the window. If square section lead to the max length insertable into the pockets won’t do it then you can add make-weights to the cords, but only up to the max length that will allow full travel of the weight stack inside the boxes.

Or junk the weight idea and substitute spring balances recessed into the new sashes rather than the box frame (which isn’t thick enough).

Or make a complete new window, but as in all double glazing options small panes = thick bars and the only alternative that mimics visual proportions is fake bars applied with adhesive to the glass surfaces. Not so good close-up. Partially effective from a distance.

There may come a point at which energy conservation must outweigh heritage in most cases, & I’m tempted to suggest that that point is now, Philistine though it may seem.

Then a 2nd email suggesting…

Even 6mm krypton wld be good – poss U of about 1.5? More energy intensive (embodied) than the less effective argon, though, & what’s the life of a glass unit seal?

Most site glaziers seem to use silicone as a default – for ease of use and adaptability? – but apart from them there seems to be a consensus that dry glazing with a vented (drained) rebate is healthiest for the unit seals.