Calculating R-value for Vacuum Insulated Glazing

The vacuum is not the issue. It is only a very small drop in pressure. The reason for the small drop in pressure is so that the two panes of glass don't start bowing inwards. Some research is being done on putting small transparent spacers between panes but it is in the early stages. The conductivity depends on the gas used which is either air or argon or krypton. The last two noble gases have a lower conductivity. The gap should remain small (12 to 16 mm) so that convection currents do not start which would reduce the effective insulation value. A good thing to google is "all about glass" and I shall try to find the PDF location for you.

Just to clarify, I don't plan to use argon or krypton, I plan to use vacuum.

Googling "all about glass" gave me 94,000 hits--can you narrow it down a little ;-)

For general info, search for Insulated Glazing Units (IGU). For specific info, search for Vacuum Insulated Glazing (VIG) or Evacuated Glazing.

This generally does not work well if the temp difference is large between the two faces (that is to say, works better when cooling the space than heating the space). Another general rule is the lites cannot be very large.

Thanks, those search terms are helping somewhat.

I haven't found a straightforward equation so far, but I have now seen some more tidbits that might help me:

• the ideal (air) gap between panes is typically in the range of 12 to 20 mm (but varies depending on what is in the air gap)
• the thermal conductivity of argon is 67% that of air

Jumping to a (tentative) conclusion, I'm guessing that if I can achieve and maintain a vacuum between panes around -12 psi that I'd have achieved a reduction in thermal conductivity on the same order as using argon for the fill gas. (Just a guess, but if I have 1/5 the material in the gap, conduction has to be reduced, maybe even to as low as 20% that of air at atmospheric pressure.)

The other factor is how much heat is transferred by convection. General statements related to that say the thicker the gap, the greater the loss due to convection (IIUC, with a thinner gap, it is harder to achieve air currents to support convection).

A reduction to -12 psi is about as good as I expect to achieve and maintain with the approach I have in mind. But, it sounds like that is enough to do some good.

So, for now I'll do without a formula. Maybe in the reasonably near future I'll make up a test window and actually do some (simple) testing. (Or maybe I'll keep googling and find appropriate formulae.)

Oops, wait, this page: Insulated Glazing has some formulae that might be the ones I'm looking for--I'll have to dig into them, but maybe not right now.

When you say vacuum, that only means pressure level drop to that level and this pressure can be due to little Argon or Krypton then it works much better so what is the problem? If you have suction then also have one fill in channel for replacing air. You only have to replace air and then create vacuum. Let over Argon is less harmful for temperature changes.

the first result i got for "all about glass" + pdf gave me the result.

http://www.yourglass.com/agc-glass-europe/gb/en/toolbox/pocket/2/All-about-glass.pdf

Thanks very much--great resource!

I think the formulas there will be useful to me, but so far I haven't seen anything on vacuum insulated glazing.

try registering at

http://www.glassfiles.com

you can get access to their library of articles and then search on that site for vacuum. you get a good set of results which are probably relevant to your question.

With a large area, and a good vacuum, the load pressing the glass panes together is about 23,000 pounds per square meter. If it breaks you have an implosion bomb.

That is why windows makers use argon. A very short cell, say 1/4 inch, will not allow for high speed convection cells to form.

They also use antireflective coatings.

http://www.google.ca/#hl=en&xhr=t&q=insulated+%2Bglazing+%2Bargon&cp=25&pf=p&sclient=psy&safe=off&aq=f&aqi=&aql=&oq=insulated+%2Bglazing+%2Bargon&pbx=1&fp=814250bde4529105

http://www.google.ca/#hl=en&xhr=t&q=%22anti+reflective%22+%2Bglazing+%2Bargon&cp=17&pf=p&sclient=psy&safe=off&aq=f&aqi=&aql=&oq=%22anti+reflective%22+%2Bglazing+%2Bargon&pbx=1&fp=814250bde4529105

In general, flat surfaces are weak at resisting pressure differences. An 80% vacuum is nearly 12 psid. Imagine a 12" square plate of glass. This would be 144 in² x 12 lb = 1728 lb. How thick is the glass?

Have you given any thought to the moisture content of the air/gas?

Most of the double glazed panes I have replaced were due to condensation
on the inner sides of the glass. (units usually outside a flimsy guarantee.)

Just a thought?

jt.

Just read the book "Lost in the Desert" by Mustapha Camel.

properly made sealed double/triple glazed panels are filled with either dry air or dry inert gas.The seals they use should last for decades. Glass is impermeable to moisture, obviously the seals used on those panels were bad or not filled with a dry medium.