You make a hay box from a crate or a cardboard box packed with insulation, with just enough room for your pan. You bring your dish briefly to the boil on the hob, put the pan in the box with the lid on, and let it finish cooking on its own retained heat. Not one extra second of gas or electricity. Depending on the dish and the quantity, you save roughly 20 to 80% of the cooking energy on the simmering part (Lowimpact.org). Ready made versions such as an eco cooker or a cooking bag usually claim around 30% (Wonderbag, Ecostoof).
Sounds like something from your grandmother’s day, and it is. During the war years the hay box was in almost every household, when gas and coal were scarce (AllardPierson). Now that energy prices are a sore subject again, it is one of the simplest ways to cut the gas you burn on cooking. I (Fred) built one myself from an old toolbox and two worn out blankets. In this article you will read exactly how to build one, what you can cook in it, and, something almost every other article skips, how to do it safely.
How does a hay box cooker actually work?
The idea is surprisingly simple: most of the energy in cooking goes into keeping something boiling, not into bringing it to the boil. Once something is boiling, all you need to do is keep it hot enough to cook through. Good insulation handles that warmth for you perfectly well, with no flame underneath.
So you heat your dish thoroughly, then let it finish cooking in its own retained heat (Milieu Centraal). The pan cools down very slowly inside the box. A well insulated box loses so little heat that a stew keeps cooking for hours. There is a reason the English name for it is retained heat cooking or fireless cooking (Lowimpact.org).
The trade off is that cooking takes longer than on the hob. An old rule of thumb from 1901 says: reckon on roughly three times as long as normal cooking (Martine Wittop Koning, via AllardPierson). But during that time you do nothing, and it costs you nothing. You put the pan in the box in the morning and your stew is ready in the evening.
Why a hay box and not a slow cooker?
This is the big difference a lot of people do not know: a slow cooker uses electricity the whole time. It keeps heating on a low setting, hour after hour. A hay box uses no energy at all after that first boil, only the heat already inside it (De Vegetarische Carnivoor).
For the environment and your energy bill, the hay box is the greener cousin of the slow cooker. The result is comparable: slow, gentle cooking at a low temperature. You just do not pay for it.
The downside: a hay box cannot get up to temperature on its own. You always need a hob (or a wood fire, or a camping stove) to bring the dish to the boil first. The box does the rest.
What do you need to make a hay box cooker?
The nice part is that you almost certainly have everything already. You need three things: a box, insulation, and a pan with a well fitting lid.
| Part | What you can use | Watch out for |
|---|---|---|
| The box | Wooden crate, sturdy cardboard box, old cool box, wicker basket | Roomy enough for your pan plus 5 to 10 cm of insulation all round |
| The insulation | Hay, straw, sheep’s wool, old blankets, a duvet, sleeping bag, newspaper, towels | Sheep’s wool and wool hold heat the longest |
| The pan | Thick walled pan or cast iron casserole with a well fitting lid | Thick walled and heavy holds heat longer |
A few practical points:
- Insulation thickness. Allow at least 5 to 10 cm of insulation all round and underneath the pan, plus a thick cushion on top. The thicker it is, the longer the heat stays in.
- Sheep’s wool wins. Wool insulates superbly and holds heat the longest, longer than towels (Jessica Koomen). Old blankets and a duvet work perfectly well too.
- A thick pan is better. A heavy, thick walled or cast iron pan stores more heat and releases it more slowly. That extends the cooking time in the box. Think of it as a bigger battery for heat.
- Careful with synthetics. A hot pan in a synthetic sleeping bag can damage the material or melt it. Towels and wool are safer (Be-Prepared.be).
How do you make a hay box cooker step by step?
Here is how to build a working cooking box in about half an hour:
- Pick a box sized to your pan. Your pan has to fit inside with room for a thick layer of insulation on every side. Too much empty space is actually bad: that air cools down.
- Build an insulating base. Put a thick layer of hay, wool or a folded blanket in the bottom. This is what your pan will sit on.
- Shape a nest for the pan. Press the insulation down so a hollow forms that your pan fits into snugly. The less air space, the better.
- Make a thick lid cushion. Fill a pillowcase or a cloth with hay or wool, big enough to cover the whole top of the pan. This also stops insulation falling into your food.
- Test it empty. Put a pan of boiling water in, lid on, box closed, and check the temperature after a few hours. That tells you how well your box insulates before you really cook in it.
Want something smarter? Build a wooden crate with a hinged lid and line the inside with wool. But a sturdy box with blankets works just as well.
What can you cook in a hay box, and for how long?
Anything that benefits from long, gentle cooking does well: rice, porridge, soup, stews, pulses, apple sauce, even yoghurt (holding it at fermentation temperature). Fast, crispy things like fried fish or a steak have no place in there.
Important first: the times below are guidelines, not exact laws. They depend heavily on your pan, your insulation and the quantity. Sources contradict each other quite a bit (one says “boil rice for 1 minute”, another says “5 minutes”). So take your time on the first go and check that it is cooked through.
| Dish | Brief boil on the hob | Then in the box |
|---|---|---|
| Rice | about 5 min | 1 to 1.5 hours |
| Potatoes | about 5 min | 1 to 2 hours |
| Soup / stock | about 10 min | 2 to 3 hours |
| Lentils | about 10 min | 3 to 4 hours |
| Dried beans (pre soaked) | about 15 min | 4 to 6 hours |
| Stew / braised meat | about 10 to 15 min | 3 to 5 hours |
Source for the box times: Lowimpact.org and Dutch cooking sources. They vary, so treat them as a range.
The method is always the same: bring the dish to a proper boil, let it boil for the stated time, and put the pan straight into the box (lid on, no peeking). Cover it with your cushion, close the box, and leave it alone.
How much energy and money do you really save?
Honest answer: it depends on what you cook. For a dish that would normally simmer for an hour, the gain is biggest. For a three minute egg it is pointless.
The figures from reliable sources:
- 20 to 80% of the cooking energy saved, depending on the dish and quantity (Lowimpact.org). You get the biggest saving on dishes that would otherwise need a long simmer.
- Around 30% fuel saving for the ready made Wonderbag cooking bag (Wikipedia, Wonderbag). Dutch eco cookers claim a comparable saving of up to 30%.
Putting a hard figure on it in pounds or euros is difficult, because it depends on your gas price and how often you cook this way. But on a dish that normally simmers for an hour, you easily skip most of that hour’s gas. If you often cook stews, soup or pulses, that adds up nicely over a year. And the box itself costs you virtually nothing.
Is cooking in a hay box safe?
This is the question almost every other article skips, and it is the most important one. Because you are leaving food to stand for hours at a falling temperature, and that calls for care.
Bacteria grow fastest between roughly 4 and 60 °C, the so called danger zone (USDA FSIS). If your food stays in that range too long, bacteria can multiply. The whole point of a good hay box is that your dish stays above that 60 °C for a long time, so it cooks safely instead of spoiling.
Stick to these three rules and you are fine (Aprovecho Research Centre, via Lowimpact.org):
- Boil hard for at least 5 minutes before the pan goes into the box. That gets everything hot right through.
- Keep the lid on. Every time you look, heat escapes and the temperature drops faster. So no peeking.
- Always reheat meat and chicken dishes until piping hot before you serve.
Why are meat and chicken riskier than vegetables? Because with meat the dish really does need to stay above 60 °C long enough, and if your insulation underperforms the temperature drops too fast. Several Dutch sources therefore advise against meat and fish in a simple homemade box (De Duurzame Kaart). A well insulated, full pan helps: it cools down more slowly. In a practical test, a full pan dropped only about 10 °C per hour and was still at around 60 °C after 3.5 hours (Lowimpact.org).
Two rules of thumb worth remembering: the fuller the pan, the longer it stays hot, and if you have any doubt about chicken or meat, always reheat it properly before it goes on the table.
Build your own or buy a ready made cooking box?
Building your own costs you next to nothing: a box, some old blankets or wool, done. For most people that is enough. If you want something neater or easier, ready made versions exist.
| Option | What it is | Ballpark price |
|---|---|---|
| Homemade hay box | Box/crate plus blankets, wool, newspaper | Practically free |
| Cooking bag / eco cooker / insulated cosy | Insulated bag filled with (sheep’s) wool | 40 to 100 euros |
| Thermal cooker (stainless steel) | Double walled, vacuum insulated pan in a pan | 100 to 200+ euros |
The eco cooker and the insulated cosy are really just modern hay boxes: a big insulated cosy or bag filled with wool (Milieu Centraal). A thermal cooker is the neatest option, with a double walled stainless steel outer pot and a removable inner pan, suitable for any hob. Pricier, but handy and easy to clean.
My advice: just start by making your own. If it works and you like it, you can always buy or build a tidier version later.
What are the common mistakes?
The traps people fall into (me too, at the start):
- Lifting the lid all the time. Every time you lose heat and the temperature slides towards the danger zone. Lid on, and leave it be.
- Too little insulation or too big a box. Empty air space cools down. Allow 5 to 10 cm of insulation all round and a pan that sits snugly in its nest.
- A half full pan. Less content means less stored heat and therefore faster cooling. Better to cook a full pan.
- Boiling for too short a time. Less than 5 minutes of hard boiling is not enough to get everything properly hot. Take the time.
- Treating meat and chicken like vegetables. Always reheat them before serving, or stick to plant based dishes.
- Leaving delicate dishes too long. Rice and pasta can overcook. Set a timer.
Frequently asked questions about the hay box
Do I really have to put hay in it?
No. Hay was the original filling, which is where the name comes from, but old blankets, a duvet, sheep’s wool or even scrunched up newspaper work just as well. Wool holds the heat the longest.
Can I cook meat or chicken in it?
You can, but be careful. Use a well insulated, full pan and always reheat meat and chicken dishes until piping hot before serving (Lowimpact.org). Not sure about your insulation? Then stick to vegetables, rice, soup and stews.
How much do I save with a hay box?
On the simmering part, roughly 20 to 80% of the cooking energy, depending on the dish and quantity (Lowimpact.org). Ready made cooking bags claim around 30% (Wonderbag). The gain is biggest on dishes that would otherwise need a long simmer.
What is the difference from a slow cooker?
A slow cooker uses electricity the whole time to stay hot. A hay box uses no energy after that first boil, only retained heat (De Vegetarische Carnivoor). Same sort of result, without the plug.
Will my food stay hot enough to be safe?
In a well insulated, full pan the temperature stays above 60 °C for hours. In one test a full pan dropped only about 10 °C per hour (Lowimpact.org). Boil for at least 5 minutes before the pan goes into the box, and keep the lid on.
Does a cardboard box work too, or does it have to be a wooden crate?
A sturdy cardboard box with thick insulation works fine. It is about the insulation, not the box itself. A wooden crate is simply stronger and lasts longer.
A hay box is really just smart insulation applied to your pan. And that is what saving energy at home comes down to: not letting the heat you already have leak away. Want to apply that principle to your whole house and genuinely bring your energy bill down? Then take a look at Energie Verpulveraar. There we show you step by step how to insulate and save energy yourself, without an expensive renovation. Small effort, big difference.