Misting DG window and drilling glass: a result!

[Dave Plowman (News)]

Strange the makers seal them, then.

[Tim+]

Not if you understand how DG works.

Tim

[Dave Plowman (News)]

It would seem many here don't.

[Tim+]

Is that your arm I see waving?

Tim

[Bob Minchin]

Thanks Chris

I recall the start of the drill question but not the conclusion.

Cheers

Bob

[polygonum]

I didn't suggest for one moment that fully sealed panels wouldn't be better. And even that argon filling might not be a good idea. Just that the difference that a small hole will make is probably not that great.

[Tim+]

Save your breath. It's just Dave trolling again.

Tim

[PeterC]

Mix 'em with water - easier to get in and self-levelling.

[Dave Plowman (News)]

Right. You have some figures for this - or just another of your wild guesses?

[Chris Hogg]

Thanks for all the comments, here and in the earlier thread. For those wanting more information on the factors affecting the insulating ability of DG units, there's this:

[Ian Jackson]

In message , "Dave Plowman (News)" writes

My house used to have metal Crittall windows, set in wood, with rather Heath Robinson sliding secondary glazing. The gap was around 2.5 inches. Although the system was sort-of draft proof, it certainly wasn't airtight enough to stop flies getting in between. We eventually took the plunge and got modern double glazing, but I don't reckon it's anyway near as effective as the old stuff.

[Dave Plowman (News)]

Different argument. I was asking for figures for ordinary double glazing both sealed and open. Of course no one here actually knows.

[Dave Plowman (News)]

Then I ask again why they are made sealed - and more expensive varieties use other than plain air inside.

Seems a lot of effort to go to if it makes no difference.

But then I'd not expect you to know the facts about anything.

[Capitol]

I think you are nit picking. All double glazing units have some degree of breathing. That's why silica gel is used in the seals. You can't get a gas tight soft seal that's going to last 25years on edges up to 8M long. Filling with other gases gives a short term improvement, but units will mist when the seals break down. A small hole will have very little effect on a large panel as the heat exchange relies on air circulation within the panel, not from outside air being introduced.

[Chris Hogg]

As pointed out in my last post, there's a good description of factors affecting the heat transfer through double glazing here:

Argon is used, because it offers some advantage over air. But the size of the gap is also important, and the optimum gap differs for the two gases. Increasing the gap size reduces the U-value. For air, and a gap of 0.125", the U-value is 0.6 BTU/hr/ft/deg.F, whereas it's about half that at a gap of 0.5". This is the optimum gap for air, whereas for argon the optimum gap is 0.438" giving a U-value of 0.26 BTU/hr/ft/deg.F, a saving of some 16%. Not a massive saving, but presumably worth it.

The gap width also influences the amount of convection that takes place in the gap, and hence the heat transfer and loss from the inner pane to the outer, due to that convection. For Argon, there is virtually no convection with a gap width of 0.25", and a U-value of

0.35 BTU/hr/ft/deg.F, rising to about 41% of the heat loss when the gap is 0.75" when the U-value is 0.28 BTU/hr/ft/deg.F.

DG units are sealed, partly to keep the argon in, and partly to keep moisture out. When the seal fails and moisture gets in by capilliary action, it cannot easily escape the same way, so remains in the gap and will condense on the glass, obscuring the visibility. A small hole allows slow exchange with the outside air and for the gap the breathe and the condensation to clear. If the hole is small, heat loss through it by exchange of warm air for cold, outside air, will surely be negligible.

I suppose the question remains as to why, in the days before argon was introduced into DG units on a regular basis, all units didn't have a small hole to the outside to allow any condensation to clear. Maybe I'll have the answer to that in the depths of winter!

[The Natural Philosopher]

A lot were vaccuum filled

And of course its not JUST air that gets in its mould spores and lichens..

[Tim+]

A philosophical question, can you "fill" something with a vacuum?

A practical question, if any DG units were ever "vacuum filled" how thick would the glass have to be to prevent significant concave distortion on a typical large domestic picture window?

I have a sneaking feeling that they never were "vacuum filled" but I'm happy to be proved wrong.

Tim

[Huge]

YMYA. At 15psi, air pressure would squash the panels completely flat.

[Tim Watts]

Vacuum DG is a new thing as far as I know:

[The Natural Philosopher]

You can see such distortion on many windows.

Mm.

in that design they are using micro spacers

Other designs like curved windows with bigger vacuum gaps can get away without them.

The fact is a vacuum is FAR better than an air gap. Or an argon gap.