Double-glazing question

Where can I find information on double glazing principles? Should the gap be vented (to the interior)? Double vented for circulation?
Would appreciate any advice.

 

The principal of double-glazing is to have a layer of still (non-moving) air as this is (after vacuum) the best insulation. So, no ventilation but airtight.

What's it for?

 

Thanks for the response
The double-glazing is for my 37 ft yacht which I have done now on the principle of them being totally sealed. However 1 of them leaked and had moisture penetration into the gap causing misting etc. As they are silicon sealed in, it is very difficult to seperate the layers, but easier to reseal the outside. This led me to thinking that if I had made ventilation holes (small) the gap could be eventually dried out... Hence my seeking expertise advice as I thought I might have got it wrong...

 

Normally double glazed windows are purged with a dry inert gas, usually Argon or Krypton. Using a gas with a high molucular weight reduces heat transfer and using a dry gas prevents condensation. In theory you could correct the problem by inserting a hypodermic needle through the silicone seal at the top and at the bottom. Attach a dry Argon or Krypton gas bottle to the bottom needle and allow the gas to flow through for awhile. Since Argon and Krypton are heavier than air, the air should rise to the top and escape out the top needle. When your done purging the gap gently pull the needles out and the silicone should close up behind it. Well that is the theory anyway.

If you have to redo one of the windows put a low-emissivity coating inside while you have them apart.

Regards;
Mike Schooley

 

Thanks for response
Agreed it should be dry (which is why I have a dessicant sachet inside as well). However, I wondered if a slow ventilated gap might deal with a possible future leak until I could reseal the outside...

 

If you have a gap you have a leak, so that doesn't make sence.
Wait for low humidity weather and try purging the space in between as described by Portager when you repair. If no gases available use compressed air, this is less "wet" because it has been compressed!

Good luck

 

Distinction

There is a distinction between double glazed windows and the evacuated "thermal" type windows. Not all double glazed windows are the inert gass filled ones, and if they're not, it's a bit of a waste to try to fill and seal them. The gas molecuels are small enough to escape through all but the properly designed seals. Helium, for example, eventually bleeds through a rubber baloon.

 

Interlude:

I am assuming you have a water tight but not air tight seal. This is common with silicone sealant since liquid water molecules have a high surface tension so they do not leak through very small gaps.

When the temperature drops the water vapor which is trapped inside will condense creating a partial vacuum. If the seal is not air tight the vacuum will draw air (which includes additional moisture) into the gap. When the temperature increases some of the condensation will evaporate, increasing the pressure and causing air to leak out. This process tends to accumulate moisture, literally out of thin air. I have seen sensor enclosures accumulate water over a period of time, especially if exposed to many freeze/thaw cycles.

Desiccants tend to create a similar phenomena until they become saturated and then have very little effect.

Since Argon or Krypton are heavier than water vapor, they tend to separate in a stagnant column. This means that if you fill from the bottom very slowly and allow the excess gas to vent from the top it will eventually purge the water vapor from the space. Once the water vapor is removed, then there is no condensation and the only vacuum that is created is due to the expansion and contraction of the gas due to temperature which is small.

When you have a seal that is not gas tight then, it will have a higher permeability to light molecules, like helium, and lower permeability to heavy molecules. Since Argon or Krypton are heavier than air and water vapors the air and water vapors will tend to leak out and the Argon or Krypton will tend to stay behind.

Regards;
Mike Schooley