When you're out in the woods, every calorie counts and so does every bit of heat you can save. One of the most useful bushcraft tricks I have found is turning an ordinary vacuum flask into a thermal cooker. With a little preparation, this simple method lets you slow-cook meals while you're on the move, without burning extra fuel.
Why cook with a thermal flask?
Cooking with a thermal flask sometimes called a "flask cooker" or "thermal cooker" uses the principle of retained heat cooking. Instead of boiling your food over a fire for hours, you bring it briefly to a boil, then seal it in a well-insulated container. The heat trapped inside keeps cooking your food slowly while you walk, set up camp, or gather firewood.
It's great if you're pushed for time as there’s no need to stir or watch your pot and uses far less firewood or stove fuel.
Plus it's portable, so your food can be cooking while you are enroute.
Choosing your flask
A wide-mouthed vacuum flask (sometimes sold for soups) works best, as it's easier to clean and to get food in and out. Aim for one that’s tough, well-sealed, and keeps liquids hot for at least 8-12 hours. I bought a cover for my flask, which you can see has done some serious miles! Make sure that the cover is bigger than the flask and has plenty of room to add your insulation.
Adapting your flask
Get together some insulating materials. I chose some insulating foil and an old yoga mat just because they were available and no longer used.
I started by making a 'jacket' for my flask from the mat by cutting it to size. I then used an upholstery needle and strong thread to stitch it closed with a basic blanket stitch. Be sure not to stitch it too tightly as you’ll need to be able to remove it to clean the flask.
For extra insulation I used the insulating foil to line the flask cover by just making a roll so I can remove it and put the cover in the wash (which it clearly needs after its last outing).
So the cooker doesn't lose heat out of the top (the lid is long gone) I made two discs out of the mat and foil and glued them together.
That's it, you're done!
Tip for cooking with your thermal flask
- Prep your ingredients: Cut food into smaller pieces to help them cook through.
- Preheat the flask: Fill it with boiling water, close the lid, and let it stand for a few minutes. This warms the walls so you lose less heat.
- Bring food to a rolling boil: Use your campfire or stove to boil your meal for a few minutes. For rice or lentils, boil for around 5 minutes; for tougher grains or beans, boil longer.
- Fill the flask quickly: Empty the preheating water, pour in your boiling food (including the hot liquid), and seal the flask immediately.
- Let it cook: Depending on what you're cooking, it might take 1–3 hours. Rice and lentils cook faster; chunkier stews need more time.
- Don't overfill the flask — leave a bit of space so you can close it tightly.
- Avoid dairy or foods that spoil quickly if you plan to carry the flask for many hours.
- Add hot water again to reheat if your meal cools before you're ready to eat.
- Keep the flask clean to prevent odours or bacteria build-up.
I find that my flask is most useful when I use it for cooking dehydrated foods (see my last blog) as all you need is boiling water and it can be cooked in as little as 10 minutes.
Next time you're in the woods, give it a try. You might be surprised just how much time and fuel you save.