Condensation between double glazing

bootman

I seem unlucky with my windows as well, another seal went last week. I have a local chap who is a window man, her charges me £35 each one. Mine are leaded as well.

I am amongst 5 other houses but i am the only one who has this problem.

Mutton_Geoff

When a sealed unit breaks down it in effect becomes single glazing. This can create as much as a 70% drop in thermal efficiency (still better then single though). Many of the bigger companies make hermetically sealed units in a moisture free environemnet such as argon or helium when making the units.

Still don't agree (in the nicest possible way!). Most of the heat lost through a window (excepting drafts) would be through conduction of the surface of the glass. With a single glazed unit and cold outside, warm air inside the house would be cooled by conduction with the cold glass layer, sink as it became denser thus setting up a convection current feeding more warm air against the inside of the glass, cooling it as it passed. The warm air, when cooled would have it's temperature lowered towards it's dew point (wet bulb temperature) and condensation *may* occur on the inside of the glass. Evaporation of the condensate would create a further cooling effect.

Double glazed units provide an air barrier between the inside and outside of the house. As the unit is sealed, there is very little convection inside the sealed unit and as air is a good insulator, there is much less transmission/conduction between the outside surface and the inside, providing the insulation level that double glazing is famous for.

If the sealed unit breaks down (at points along the edge), any heating/cooling causes outside air (with high water vapour levels) to be drawn inside the unit. As there is not a free flow of air (the leak is usually through a fairly small gap), then the moisture cannot escape and condenses on the inside of the sealed unit.

The damaged unit may now contain more moist air, or even lost it's helium/argon or whatever (which has slightly better insulating properties than air), but there still exists a mechanical barrier between outside and inside panes, filled with a gas that is 1'000s of times less conductive than solid glass, so the insulation effect will not be greatly affected by a breakdown of the unit. Certainly not by 70%!

If there was a vacuum between the panes in the original unit (impossible or the glass would bend inwards and be under stress) and the unit failed, there would be a greater loss of thermal efficiency, but again the majority of the insulating factor comes from the dual layers with little mechanical contact between them or convection of any conducting gas (air, helium, argon etc) inside the panel.

It might be 70% more unsightly with condensation, but I'd bet the thermal efficiency drops by only a tiny amount with a failure of the edge seal.

ukwoody

Ok, Basically I am wrong about the 70% - it's nearer 30%.
I have in front of me a leaflet done by Pilkingtons approx 4 years ago. It is that which I took the 70% from, However, when doing my calculations from this sheet, which is all done in U values, I didn't see they had changed one of the figures to show the benefits of K Glass - and sneakly buried the fact they were using a different size air gap as well!.

So, very basically, an old 12mm air gap filled with normal air, compared with a 20mm air gap filled with Krypton and using K Glass on the inside will give about a 70% greater thermal loss. ie, The newer one is 70% more efficient.
Thats if you believe their somewhat dubious methods of calculating this.

However the interesting points are that, over 16mm the size of the air gap for thermal saving is minimal. Krypton is only fracationally better than Argon, and Argon is only fractionally better then hermetically dry air - but if the units are made with normal air in a glaziers workshop you could suffer about an 18% drop in efficiency over the dryer air.
But when the units break down, wet air can be pulled by thermal conraction into the unit and there can be enough water trapped inside the units to create direct thermal conduction causing a u value loss of approx 29%.
Phew does that all make sense ? I've just pent 30 mins trying to get my head round this!
Woody

sashman

Thats all very interesting (promise) but the unit has lost its seal, has mist on the inside of the unit which is unsightly and there's no cure for it other than replacement of the sealed unit.

My point is to prevent a similar unit failing, do look at the drainage and ventilation (PVCu,timber of aluminium) to ensure the new unit does not sit in moisture.......that WILL reduce the life of the sealed unit.

sashman