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I saw some double walled glass mugs like this one and I am wondering if that really works in keeping the liquid warm.

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If not, is it just for the looks and marketing spin?

Is there a vacuum between the two walls? Does it help in keeping the drink hot for longer?

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Whether or not the double walls and/or vacuum exist, pouring hot water into the mug for about five minutes (I usually do it twice, as the coldish mug quickly lowers the temp of the first water batch)helps keep the liquid warmer for longer. –  MargeGunderson Oct 17 '12 at 16:22

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Since I don't know this product, I can only answer with (my) common sense:

Air should be a much better insulator than glass, so even if there's no (good) vacuum, the insulation should work pretty well.

One thing to keep in mind is what is mentioned on the Wikipedia article on vacuum flasks

Heat transfer by thermal radiation may be minimized by silvering flask surfaces

and observing that a normal vacuum bottle isn't translucent. That means that I would expect some loss due to radiation, since the glass isn't covered in any way.

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We have two coffee press pots. One with a double wall and one without. The double walled one keeps the coffee warmer far longer. –  Rob Oct 17 '12 at 13:58

I use a double walled stainless steel mug. It works very well and keeps 500 mls of water hot for more than an hour, and yet still handles like a normal mug. The walls are separated by air, there is no vacuum

I would expect there to be more thermal losses with glass, but I would still imagine it to perform well. I have noticed over recent years the popularity of double walled glass in China. They like to walk around with their tea all day, so it must work well enough

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Yes, they work.

The reason they work for keeping liquid warm is because the air pocket slows down the transfer of heat from the liquid to the glass to your hand. Air has a lower thermal conductivity than glass does, which means that it slows down the loss of heat from your drink. (The thermal conductivity of air is 0.024 W/m/°C, while the thermal conductivity of glass is anywhere from 0.96 - 1.3 W/m/°C, depending on the type of glass.) It's the same reason that windows in modern homes are usually double-paned.

The space between the layers of glass are not a vacuum.

That being said, glass double-walled thermoses are popular mostly because they look cool (they give an optical illusion of the liquid being suspended in air). If your primary concern is keeping your beverage warm for as long as possible, it's better to go with a double-walled stainless steel thermos, which will retain heat much longer than a glass one. MargeGunderson is right, too - preheating the thermos, regardless of material, with hot/boiling water for a few minutes before putting your drink in it will be even better than pouring your tea into a room temperature thermos.

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The thermal conductivity of air probably isn't the right thing to compare to - I expect convection might be dominant - but the principle here is of course right. –  Jefromi Oct 17 '12 at 18:25
    
@Jefromi You're right, if there is a vacuum between the walls, like in a proper Thermos brand thermos. :) I don't think there's a vacuum in glass-walled insulated cups, but you've got me wondering now and I can't find a definitive answer at the moment. –  Laura Oct 17 '12 at 20:44
    
I'm fairly certain there's air, and that's what I'm saying - if since there's air there, convection is likely dominant over conduction, because air is generally much better at convection than conduction. So the thermal conductivity of air isn't necessarily pertinent, but even air convection will be much less heat transfer than conduction through glass. –  Jefromi Oct 17 '12 at 20:49
    
@Jefromi Yes, you are correct; my jetlagged brain is not using words properly today. We are on the same page, I think. If you would like to edit my answer to explain better, feel free. –  Laura Oct 17 '12 at 20:56

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