When a DG unit fails and mists up,how's it U value affected?

[cjard]

Towards the bottom of this thread: http://forums.moneysavingexpert.com/showthread.php?t=923211
is a discussion about how failure of the seal around the edge of a DG pane affects its ability to perform thermally. I've currently got tenants who are complaining that windows are leaking water in and it's because the DG seal has failed, yet I know this isn't technically sensible as a DG unit misting up says nothing about the watertightness of the frame. I'm more convinced that the water they find on the windows at the moment, is condensation because they don't run the heating often. They say they've made all the necessary lifestyle adjustments, and the water is always there whether they open the windows or not, so it got me wondering by how much a DG unit underperforms if it's no longer gas tight. My personal feeling is "not much" because the insulation comes from trapping a layer of air between two panes so that it cannot convect or conduct. Sure, wetter air in there might have a slight performance impact but I think the main reason we go to efforts to keep the air in a DG unit dry is so we can still see out the window, rather than for thermal performance

I need to go and have a look at the frames to check that they aren't actually leaking/drains are clear, and if the panes are misted up really badly then I'll consider replacing them on aesthetics. I just get the feeling that the tenants are expecting it to be a magic bullet that will solve the problem when I'm more inclined to think that it's a lifestyle+current climate combination

 

[AronSearle]

My understanding is that there is little difference in thermal conductivity between dry and humid air.

Wet materials do suffer from a lower thermal resistance, but I wouldnt expect any hydroscopic materials to come into play here.

Air movement would still be fractional, the build up of moisture might effect the glazing bars.

So I would expect it to have little effect, I am not aware of any evidence however.

If the DG pane has failed it does raise questions about the overall quality of the frame, fitting and thermal brakes used. It would be usefull to go along with an IR temperature probe to read internal surface temperatures, if the internal surface is hitting single figure temperature it becomes very hard to avoid condensation without the use of mechanical ventillation, or open windows (trickle vents may not cut it with low temperatures).

EDIT: to clarify I mean more than opening windows in the morning to dry the air, but having to leave them on a crack for significant periods, or permanent ventillation such as positive pressure loft systems.

 

[cjard]

I've got an IR thermometer. Something like this:
http://www.ebay.co.uk/itm/Nice-Non-..._Measurement_Equipment_ET&hash=item19fa00f3b6

The nuisance is that I'm the landlord, so I'm not in the flat at relevant times.. Will see if I can get some measurements and report back..

 

[AronSearle]

Ideally you turn up when it is near as possible to 0-5c and they have the heating on, but it is not necessary.

As long as you know the external temperature, and the internal surface temperature, you will know the approximate difference and can calculate. If the internal surface temperature is likely to hit single figure C when the outside is 0-4C, you will likely need more ventilation than trickle vents. Lots of variables but loads of stuff on the forum about ventillation.

If the glazing is crap are you really prepared to upgrade it rather than simply replace it?

The colder the internal surface temperature, the more they need to ventilate, depending upon the tenant there is a point where they may consider it unreasonable to provide X ventilation, unscrupulous landlords or ones on a genuine budget can always legally get away with 'ventilate more'. Also tenants may vary, some may be happy with positive pressure systems.

 

[ronniecabers]

The thermal properties could well be effected if it had Argon gas in it , as it would have seeped out by now