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Condensation between the panes is mostly a cosmetic issue.
Unlikely. Moisture getting in between the glass panes indicates failed seals. The seals fail because exterior air is allowed to reach the seals to degrade them. My timber windows and doors don't suffer from this. The glass units were sealed on the exterior by linseed putty or silicone. Exterior air never reaches the seals between the glass panes. The 30+ year old double glazing units remain perfectly fine and completely clear of condensation on the inside of the units. This also applies to a cheap unit I bought to do DIY repair on a windows broken by flying rock. The repair was sealed externally using basic B&Q silicone.

Putty grows mouldy but don't degrade like exterior rubber seals on PVC windows. Once the rubber seals degrade from time and UV, the exterior air is then let in to attack the seals between the glass panes.
 
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Unlikely.

Sealed glazing units are filled with air, usually. Logically - Unless that air is able to easily flow past the seals, the insulation properties of the unit will not be much worse that when it worked as a sealed unit. In practice, the moisture gets into sealed gazing units via tiny breaches in the seals, what then have is misted windows, which although they still work almost as well as new, they make it difficult to see through them.

Moisture getting in between the glass panes indicates failed seals.

Yep. Secondary glazing is not air or moisture tight, yet if done properly, it can work well - even though inner surface of the glass can steam up when the outside temperature falls.

The seals fail because exterior air is allowed to reach the seals to degrade them.

Yep.
 
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Sealed glazing units are filled with air,
Donno. I would assume vacuum or special gas.

even though inner surface of the glass can steam up when the outside temperature falls.
I have never seen steaming up inside the units on any of mine. Steaming up on the house interior, sure, during cooking.

All my units are now double sealed with the original putty/silicon seal plus a bead of hybrid polymer sealant. Done to protect the paint edge rather than the glass units. Also to block mould access to the putty.
 
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Argon gas

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Donno. I would assume vacuum or special gas.

No, just air, or in some cases a gas filled, argon gas - but the gas quite quickly leaks out to be replaced with air. My understanding is that it is really difficult to make a sealed unit completely sealed, they do all have some slight leakage. It's only when the leakage becomes excessive, that moisture gets in.

Likewise I have never seen any of mine windows steam up, even the the old ones which were 30 years old when I replaced them. They were only replaced because they were alloy framed, moisture used to collect and run down the frames, plus the hinges, catches etc. were worn out. They were also weird, in as much as they were made to fit into the original timber frames, and the timber frames were rotting. I fitted those.

I priced up the cost of buying and again fitting them myself, versus having them all done by a local fitter and it just wasn't worth the struggle/time for me to tackle it. 2/3 days and it was all done, without the struggle.
 
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No, just air, or in some cases a gas filled, argon gas - but the gas quite quickly leaks out to be replaced with air. My understanding is that it is really difficult to make a sealed unit completely sealed, they do all have some slight leakage. It's only when the leakage becomes excessive, that moisture gets in.

Here is an honest opinion on windows and the gas filling...

 
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Looks like it's a problem with the pvc windows. I'll stick with wood if my paint jobs can last. All my frames are fine. But limited rot on the windows, mostly on the wood beads. After 30+ years, not bad.
 
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What causes the seals to fail is not air but a combination of sun(heat), seals hardening off, expansion/contraction and vibration or movement, the seal will crack or break down allowing air and thus moisture into the cavity, the dessicant will do what it can but once saturated the moist air will condensate on the glass. In glass unit manufacture I'm not am expert - you need to ask @ronniecabers but my understanding is glass units have never had a vacuum, they're either sealed to normal atmospheric pressure or filled with argon or similar gas. I know this from old and from 30years of being in double glazing and the last 20 as a service engineer is that after 3 to 4 years the argon or whatever was pumped in at manufacture will be long gone and replaced by bog standard air, the dishonest industry will not tell you that, the honest will and I'll let Ronnie back me up on that and Ronnie manufactures DGU's.

Going back to what DiyNutJob says, then he must be a lucky one because its a well known industry fact that in timber windows where linseed putty is used its the linseed oil that attacks the seals, it was thought that after swapping to butyl based putty a solution was found but that also attacks the seal, even the chemicals in silicone will break the seals down. But of course in timber windows drainage is also a contributing factor in blown units as it was never incorparated into the frames like it is in pvc
 
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I will confirm everything @crank39 says. No vacuum in a sealed unit, just normal air, unless argon/krypton gas filled which WILL leak out over a period of years...usually by 5 years its all gone
 
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One of the most important factors in keeping a selaed unit clear, is the state of the dessicant in the spacer bars.
Timber double glazing has no desiccants, and doesn't need any. The glass unit seals are never exposed to outside air. The units are externally sealed by putty. I think PVC windows only hold the glass units in place and don't attempt to prevent outside air reaching the glass unit seals. The exterior rubber gaskets is to stop water getting in.
 
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I am afraid you are very much mistaken there @DiyNutJob . ALL sealed units have dessicant . Also very very few sealed units are sealed in wood with putty these days. No manufacturer of sealed units will warranty a unit puttied in. One , due to Linseed oil degrading seals and two, putty cracks over time and let's water in , and a lot of wooden older frames had no 'drainage ' ( as upvc does ) and thus this lead to those with cracked putty ' sitting ' in water
 
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Going back to the comment about 30 year old double glazed sealed unit lasting... that will be true ...in pvc and wood as back then the materials were of a better quality ( spacer bar for example was thicker , you couldn't bend it ) . These days a sealed has an expected life of 10-15 years. @DiyNutJob , if you replace any of those units you have now , they probably won't last anywhere near as long .. especially puttied in with Linseed based putty
 
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@DiyNutJob , if you replace any of those units you have now , they probably won't last anywhere near as long .. especially puttied in with Linseed based putty
I broke a couple. One from heat gun and one from line trimmer flicking a rock. Both were replaced with the cheapest units I could find at the time, from a small double glazing maker who made to measure. The DIY repairs were sealed with B&Q bulk silicon, I didn't see the point of putty. In performance, the replacements were indistinguishable from the original. The key is in the exterior seals. If outside air doesn't reach the unit seals, those units will last forever.

All the units I have that are puttied or siliconed, now have primer, paint, and then hybrid polymer sealant on top. My goal for them is to attain a lifetime performance.

I am quite sure my units have no desiccant, unless they are hiding inside the aluminium strips. I can't remembered if I took apart the broken units to see what was there. But I definitely remember not seeing anything else apart from the aluminium strips and the plastic corners. They had grey unit sealant.
 
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In theory yes... but as has been mentioned No Sealed unit will last for ever.
Basically a sealed unit is sealed using a hotmelt ...190 degress c plus and they start at one corner and work round to the unit to that corner .. as you get to the beginning it is no longer 190c and you end up with a ' hairline ' fault, very rarely visible to the naked eye. Now all the materials, wood/pvc/sealant/silicone/putty/beading even glass to a degree , contract and expand in the ever changing weather. This creates an unnoticeable reverse bellows effect and slowly draws any moisture in the air /wood / sealant etc I to the sealant unit. Now the dessicant ( actually to be technical it's molecular sieve ) will absorb the moisture and any chemicals until it is fully saturated and then you end up with moisture in the unit. Now this can happen quickly or slowly depending on various factors , how long the unit was left before it was sealed , quantity of water vapour in the air at time of sealing , whether there are any chemicals in the air...some will fail quickly, some slowly.....BUT they will all fail at some point that is a guarantee. South moving round to West facing will fail first due to most of our weather comes that way, regardless of wood or pvc. The industry started to refuse to guarantee into wood with putty due to issues previously mentioned... in fact some manufacturers won't guarantee into wood at all, some if only glazed certain ways but Putty and just silicone are not normally covered. As another thing if the units are going into wood they should not have the ' edges 'taped , as there is a potential to hold water within the tape.
I would put money on that the new units will not last as long as the originals , yes they may look the same but I can guarantee that from 30years ago , both the dessicant formula and the spacer bar material thickness have changed
 

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