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Wikipedia claims, without attribution, that a tea cosy:

... insulates a teapot, keeping the contents warm

The purveyor's of these teapot covers follow a similar vein:

Pretty, original and practical, any one of our tea cosies will keep the pot perfectly warm while lending a cheerful air to the tea table.

and even The Independent newspaper's got in on the action (albeit on what seems to be a piece written solely to push the things):

And if you're going to have a drop ("go on!"), you'll be wanting a nice tea cosy to keep it warm.

I find it hard to believe that a standard knitted/cloth tea cosy, with a bit of air to help out, will keep a pot of boiled water "warm". Do tea cosies make any perceptible difference to the amount of time a teapot remains warm?

Really Wild Tea Cosies, Loani Prior

The definition of warm is, of course, up for grabs.

  • 1
    Let me get this straight. You 'assume that tea cosies "work"', but not that 'that a standard knitted/cloth tea cosy [...] will keep a pot of boiled water "warm"' and allow that 'The definition of warm is [...] up for grabs'. OK. Is there any possible answer which could be wrong under those limits? – dmckee Sep 8 '12 at 19:15
  • That is, I guess, a fair point and my own fault for trying to be fair. I apparently can't edit the post atm but please feel free to remove the sentence about them "working" – Ben Sep 8 '12 at 19:25
  • You're still down as the author so you should be able to edit... Maybe a suggested edit was pending. – dmckee Sep 8 '12 at 19:49
  • I wonder if an analogy to a much thinner, cotton t-shirt will help. It won't protect you against a blizzard, but it does keep you warmer... – Oddthinking Sep 9 '12 at 2:35
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    Force is a word with special meaning in physics. It is unrelated here. A t-shirt stops heat escaping (and provides modesty). Its effect on external forces is minor. – Oddthinking Sep 9 '12 at 20:46
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TL;DR Yes, of course it will keep your tea warmer...

Tho following tea cosy is the most popular tea cosy on Amazon.co.uk. I'll assume it as the typical cosy.

Tea Cosy

It is made of 100% cotton.

Technical Details

  • Keep Calm and Drink Tea Tea Cosy
  • 100% Cotton
  • Blue with white trim

Amazon.co.uk – Keep Calm and Drink Tea

Now, cotton is a relatively good insulator, with an R-value of 3.7:

Cotton insulation is increasing in popularity as an environmentally preferable option for insulation. It has an R-value of around 3.7 (RSI-0.65), a higher value than most fiberglass batts.

source

Now, what is, exactly an R-value?

The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux through it.

source

Basically, it measures how much an insulator "slows down" the temperature decrease.

Thus, by definition of r-value, and by showing that cotton has a positive one, and that a typical tea cosy is 100% cotton, I have shown that the tea will be kept warmer than it would normally be.

Now, how much warmer? Probably not much. Also, hugely variable:

  • Does the cosy fit snugly?
  • What is the tea pot made of?
  • What is the shape of the teapot?
  • What is the external temperature?
  • How long later will you measure?

Then again, the definition of warm is quite subjective.

I assume, of course, that you didn't mean that the tea would be kept warm indefinitely! That's not what is being claimed, clearly.

  • i like this answer. it is actually very British in its vagueness:) – Rory Alsop Sep 9 '12 at 0:29
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    I suspect that, like double glazing, its the prevention of convection heat losses that really make a difference. The actual insulating properties of the cozy are a secondary effect. – DJClayworth Sep 9 '12 at 3:43
  • Actually, @DJClayworth, the cotton would (mostly) prevent conduction as a vacuum is needed to prevent convection. See how a Thermos works for more info: clearly, a cosy does not change a tea pot in a thermos! – Sklivvz Sep 9 '12 at 14:08
  • I think you have it the wrong way round. Convection is thermal transfer by the movement of fluids. Anything that prevents the movement of fluids (a tea cosy, a pane of glass, even a very thin plastic sheet) prevents convection. See: en.wikipedia.org/wiki/Thermal_convection – DJClayworth Sep 9 '12 at 16:40
  • @DJClayworth, If the answer is correct wouldn't it work similarly to a wetsuit? – Ben Sep 9 '12 at 20:19

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