Lightweight vacuum flasks / ways to carry hot liquids

Question

All the hiking gear is getting more and more lightweight and hi-tech... but vacuum flasks haven't changed much for the last fifty years, they're still extremely heavy pieces of steel! One litre bottle weighs 600-700 grams...

Generally I try to avoid using vacuum flasks, but there're some conditions when they're indispensable (winter hiking in restricted NP where you can't use any kind of stove, for example).

Using an insulated bottle (not a vacuum flask) is not enough; in fact, sometimes even the vacuum flasks get cold(-ish) by the end of a long winter day in the mountains!

I tried using flameless heaters but their temperature drops down significantly in winter (on one trip in balmy 32F/0C a heater made my tea just barely lukewarm!)

Is there any better/lighter way to carry hot liquids on such trips?

Answer 1

A smaller flask is of course lighter, but loses heat faster and holds less. Your whole drink strategy needs to be considered.

A litre is a lot though, unless sharing - there's plenty of choice of 500-600ml flasks; one I found easily weighs 300g (compared to 530g for their lightweight 1l model. Thermos, the brand I've linked, has been in the business a long time (since the vacuum part was always silvered glass, visible on the inside) and is synonymous with vacuum flasks in the UK. I wouldn't go smaller than about 500ml as the hold time tends to be too short.

The main loss is at the lid, which is merely insulated. A better quality flask will hold heat better (and may weigh a little less overall). The best lids I've found are those that allow you to unscrew slightly to pour, using all their volume for insulation. I would avoid those with a clever mechanism that traps the dregs of your drink in an inaccessible volume. This means less insulation and also, having opened one up destructively, I never want to drink from anything so filthy again. Regardless of the lid design, supplementing the insulation in this area (wrap it in spare clothing, use an insulated pouch, etc.) will help a bit.

The smaller the flask, the more important it is to preheat it well when you fill it (the surface area to volume ratio is worse, and the steel is the same thickness, so each drop of your drink is heating more steel).

The other important heat loss is opening the flask, letting out hot drink/air/steam and replacing it with cold air. Waiting as long as possible for the first drink helps here. If you're in a group, share out one flask at each stop, keeping the others closed. I'll sometimes save mine until we're nearly done, a habit from winter kayaking when a hot drink borders on being emergency kit if someone gets wet and cold.

Hiking I might be tempted to carry a tiny stove (you could do better than this 60g one I made) for actual emergencies, combined with a smaller flask (and a water bottle, possibly insulated), or, if you drive to your hike, leave a big flask in the car for the end - something else I've done when kayaking.

Answer 2

I can personally vouch for one of the latest Thermos flasks in terms of it's ability to keep drinks warm all day in winter conditions (even after you've emptied half of the bottle).

The Thermos Ultimate flask weighs in at 300 grams for a 500ml capacity, they also have a larger option of 900ml weighing 400 grams.

I bought one of these in 2019, it worked so well, I bought my dad one, and both of my grandparents - I wouldn't bother with a different brand or model now.