Gas diffusion - a question or two

[portyforty]

This forum may not be the place to ask this question, but there are some extremely knowledgeable folks here, so here goes:

I'm planning a little DIY double-glazing project.

This will involve several opening windows (we call them 'sashes' in NZ) which currently have a single layer of 4mm (3\16"?) glass. Each window is about...um, 40" H x 20" W.

I intend to leave the outer glass intact and add a layer of 6mm (1\4") perspex to the inside. These panels will be fixed to the sashes by screws at regular intervals around the perimeter, over a strip of regular foam plastic draught seal, around 3\8" x 3\16". Distance between the glass and perspex will likely be about 35mm (1 3\8")

I'll bore an extra couple of holes through the perspex at top and bottom of the panel, through which I intend to introduce argon gas, after which I'll plug the holes by rubber grommets or some-such.

My question: The argon will only be at atmospheric pressure. How much, if any, diffusion is likely to occur between the argon in the cavity and the air in the room? Do the two gases eventually merge? If they, how long might it take? Days? Months? Years?

As to why I'd use argon, I understand it would prevent fogging. I'm not so concerned about the thermal aspect, as our temperatures here aren't arctic. The idea of the double glazing is mostly for a sound barrier.

I've already tested this aspect with a loosely fitted perspex panel on one window, and monitored the result with a stethoscope. There's a definite improvement to the sound deadening quality of such a panel, compared to one with only glass, so I'm optimistic about a useful result in that area.

However, I'm uncertain as to how long the argon will stay put. BTW, I have access to the gas, from a local aluminum welder.

Thanks for reading.

 

[(*steve*)]

The argon will be actively pumped out of the window. As the temperature cycles, the pressure inside will raise and lower. Your seals are unlikely to be airtight. Thus the loss of argon will be much faster than if it was by diffusion alone.

Would some quantity of dessicant in the bottom of each window assist in reducing fogging? You'll still have the problem of the air in the gap being replaced, but at least the moisture content will be keep lower.

 

[duke37]

For optimum heat resistance I believe that 3/4 inch is best. Less than this there is less gas insulation and more than this there is gas circulation - convection.

Wider spacing is better for sound insulation and I would think that heavy glass would be better than lightweight perspex.

To minimise internal condensation the gap should be connected to the cold outside. I have internal secondary glazing with some ventilation to the outside by the original windows. I do not get condensation. The gap is a couple of inches and the inner glass has a rubber seal. I fitted it about 50years ago and there has been no trouble unlike sealed??? double glazing.

I doubt if you can seal the gap and a few weather fronts will change the internal gas.

 

[portyforty]

Thanks, guys.
Steve, I take your point about the expansion of the argon from external heat (sun).
But I wonder if the expansion might be countered by the ability of the perspex to bend (bulge)?
Re desiccant, once it had absorbed its maximum moisture, wouldn't it be effectively neutralized?

Duke37: You say that "To minimise internal condensation the gap should be connected to the cold outside."
Okay, that could be done fairly easily by drilling some vertical holes in the bottom rail of the sash, which would emerge into the newly formed cavity. They'd also be weather protected. Might get spiders crawling in but a tiny piece of tea-strainer gauze would stop that.

 

[dave9]

There is no need for it to prevent fogging. If your seal is air tight, as it would have to be to keep the gas in, you merely need to seal it with air inside that is low humidity. One way that insulating windows achieve this is a channel at the bottom that has a desiccant in it, then it can be sealed with normal humidity air and the dessicant will keep it dry enough until the seal fails.

As long as you have robust seals, air expansion from temperature change is not a problem. All glass insulating windows achieve this without any significant bulging of the glass.

 

[portyforty]

dave9, thanks for that. It sounds like an easier solution than I've so far proposed. The inner ledge of the sash is about 28mm (1 1/8" ) wide, so I could easily run a router along that and carve out a nice clean channel (maybe 3/8" x 3/8") and fill it with desiccant.

It's pretty dry in our house as we have a (firebox) fire going most of the time, so I'd only need to attach the perspex panel to a window and the gap would automatically contain air that's less humid than outside. As I type this, at 3:15pm on an overcast day in our Spring, the inside humidity is 36% and outside is 55%.

Good suggestion, thanks again

 

[duke37]

I doubt if you can make a reliable seal using wood which is permeable.

Do not confuse humidity with moisture content, temperature also comes into it. Grain is dryed with only a few degrees increase in the drying air. Your house will be dry inside because the temperature is higher than outside but the moisture content will also be higher (do you breathe?) and this will cause condensation in the window gap when the temperature is lowered.

 

[dave9]

You can seal wood with grout sealant, the old school type which is PVC based, but even old school methods of wood sealant like linseed oil, should suffice if it is allowed to cure before installation. Paint will do that too, just depends on the lifespan needed.

Double/triple pane windows exist. They aren't magical. Follow their lead and duplicate that result. All you need is a good seal on an air chamber that is relatively dry.

 

[kellys_eye]

Get to your local glaziers and simply get them to make up sealed-unit double glazing for the size you want and replace the existing single panes. They do the gas filling, they seal them.

Do it cheap, do it twice.

 

[duke37]

Get to your local glaziers and simply get them to make up sealed-unit double glazing for the size you want and replace the existing single panes. They do the gas filling, they seal them.

Do it cheap, do it twice.

Or add a double glazed pane inside the existing window with a largish gap for sound proofing.

I did it cheap 50 years ago, will consider doing it again in another 50 years.

 

[kellys_eye]

Fogging is the OPs concern hence the gas filling which is fairly standard in DG units these days as well as providing additional heat resistance.

Of course just hingeing the secondary pane would make access possible....

 

[VenomBallistics]

I had a summer job in a window factory.
what I gather so far.
The OP wishes to improve the insulating properties of the windows he has...
The OP wishes to achieve this while avoiding complications from condensation.
If I assume that the product described is more or less equal to plexiglass ... yeah, you have the first part licked.
the second part will require a different approach.
In an industrial setting, argon filled panes required some specialized equipment.
Air and moisture has to be purged from a glass / channel / glass sandwich assembly and promptly sealed up tight.
when I say sealed ... I mean sealed. Its a polymerized rubber compound that's slathered in great whacking gobs around the perimeter. This assembly is then fitted with small feed and purge tubes to facilitate the argon charge.
I think the home gamer could pull it off if you were out to fabricate this same style of assembly.
That is not the case here, and you will have foam, wood and screw holes to leak through.
You need another option.
Without getting into the reasons, some product lines had to have the ability to breathe.
while the assembly and much of the process was the same but breather tubes were installed. These would act most like the OP's solution.
To combat the condensation issues in these, we'd fill the channel ( hollow square tube steel with perforations) with a desiccant.
Your mission is to figure out how to stash some kind of desiccant in your assembly where it can do its thing without being an eye hammer