Thermos Bottles and Cold Weather Climbing

A vacuum flask (also known as a Dewar flask, Dewar bottle or Thermos) is an insulating storage vessel that greatly lengthens the time over which its contents remain hotter or cooler than the flask's surroundings.

The vacuum flask consists of two flasks, placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a near vacuum, which prevents heat transfer by conduction or convection. (

When there is a potential for cold weather and winter like temps, especially in high altitude environments, I consider my vacuum “thermo” bottle part of my med kit, mainly because there is so much one can do with it when it heat or water are needed.  Personally I have used it as a to keep water hot so that it can be used as a source of heat to aid in melting snow and creating additional drinkable water, warming up my body’s core, hands and feet, or dry out my gloves, inner boot liners and socks after they got wet by just putting some hot water from my thermos in to one of my ½ or 1 liter HDPE water bottles, and of course if I want a hot drink I can do that too.

When I’m guiding in cold conditions I keep only boiled pure water in my thermo bottle.  Mainly because pure water is more polyvalent (multifunctional) and it has more practical applications than having coffee, tea, or hot cocoa, since caffeine or milk contain artificial flavors/smells, etc., which can compromise someone’s well being if they have some sort of allergy or rejection to some or all of the hot drinks components. This reduces the chances of a medical distress situation among other reasons.

You may start wondering by now what kind of factors make up a good vacuum bottle for the outdoors. Other than just the cost and resistant materials it is made out of, and of course other than how it suppose to make me look and feel, cool or hot ;-) it has to be able to maintain hot water hot for a long period of time, this period of time will depend on what kind of activity you are thinking to use the bottle for.

Any vacuum bottle can cut the mustard when it comes to keep things warmer for about 4, 6, or 12 hours. But what it takes to make the real difference is when some bottles can maintain that warmer temperature range for longer period of times like 18, or 24+ hours. Now we talking eh!

Today’s days Stainless Steel with a grade of 18/8 is the preferred material to build vacuum bottles, mainly because the interior and the exterior of the bottle will last longer by holding up to the use and abuse.  

Ideally the main factors I would like to see on Vacuum “thermo” bottles are: The capacity to keep water hotter than 98.6oF/37oC (human core body temperature) for a long period of time (24hrs+), that it will take up the least amount of volume in my backpack and withhold the maximum useable volume or capacity of drinkable water for myself (not less than .75L/25.4fl.oz. and not more than 1.2L/40.5fl.oz.).

Not long ago I did a little experiment trying to figure out what would be the relevant factors that would make the perfect Denali trip vacuum bottle. I did find out that the simpler the interior top lid is the less heat will be lost (avoid push button tops, go with simple screw tops), the less metal the bottle has, it will be lighter weight-wise, and keep liquids hotter for a longer period of time. The volume capacity vs. the total weight of the bottle did make a great difference at the end of the day in my experiment.

Left: Push Top Lid Right: Screw on Lid
Left: Push Top Lid Right: Screw on Lid

At the same time, if you build a closed cell foam parka to cover the bottom and the main bottle body, you will prolong even more the time that the vacuum bottle will maintain the heat of the water. This is mainly because once again, the vacuum bottle consists of two flasks, placed one within the other and joined at the neck. This joining of the interior flask with the exterior flask is the single spot from where the water’s heat will be transferred from the interior flask to the exterior part of the bottle by direct conduction. By building the closed cell foam parka you will delay even more the heat loss by just diminishing the temperature gradient with the exterior wall, at the same time by having thinner steel walls you will need less heat to warm up that metal and hopefully it will transfer less heat over a period of time to the bottle’s outer wall.

I’m sure by now more than one person is asking them self if I did find that perfect Denali Vacuum Bottle, Yes! Indeed I did, but, and this is a big BUT. The vacuum bottle that won my favor appears is not longer manufactured as I’m writing this article :-( Oh well maybe it will come back again at some point, for right now I will give the second and third bottles on my list a try.

Richard Riquelme

AAI Guide

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