Last edited: 8 August 2019
Heat-retention cooking saves cooking fuel because after food has been heated to cooking temperature using a conventional stove or solar cooker, it is placed into an insulated container where it continues to cook until it is done. Retained-heat cooking is often introduced along with solar cooking since it further reduces the use of traditional fuels such as firewood, and the use of this method allows much more food to be cooked each day in a solar cooker. This method of cooking is also known as retained-heat cooking, fireless cooking, haybox cooking, or wonder box cooking.
Heat-retention cooking is an age-old method that can be used to conserve energy not only during times of crisis, but anytime. Depending on the food item and amount cooked, the use of a haybox or insulated cooker saves between 20% and 80% of the energy normally needed to cook a food. The longer an item usually takes to cook normally, the more fuel is saved. For example, with a retained-heat cooker, five pots of long-cooking dry beans will use the same amount of fuel to cook to completion as just one pot cooked by traditional methods.
The principle of retained-heat cooking is simple. In conventional cooking, any heat applied to the pot after it reaches boiling temperature is merely replacing heat lost to the air by the pot. In retained-heat cooking, food is brought to a boil, simmered for a few minutes depending on the particle size (see below), then put into the retained-heat cooker to continue cooking. Since the insulated cooker prevents most of the heat in the food from escaping into the environment, no additional energy is needed to complete the cooking process. Food cooked in this way normally cooks within one to two times the normal stovetop cooking time. It can be left in the retained-heat cooker until ready to serve, and stays hot for hours allowing for cooking during sunny daytime hours and hot meals after sunset. “Timing” is much less important than in stovetop cooking: stick a pot of rice, beans, or stew in at lunch time, and it will be ready when you are, and steaming hot, at dinner time.
The retained-heat cooker itself is any kind of insulated container that can withstand cooking temperatures and fits relatively snugly around the pot. Hayboxes have been made using hay, straw, wool, feathers, cotton, rice hulls, cardboard, aluminum foil, newspaper, fiberglass, fur, rigid foam, and/or other suitable materials as insulation. The insulation is placed between the rigid walls of a box, within a double bag of material, or lining a hole in the ground. “Instant hayboxes” have been created by wrapping a sleeping bag, blankets, and/or pillows around a pot. The most effective insulating materials create many separate pockets of air, which slow down the movement of heat. Two to four inches of thickness (depending on the material) are necessary for good insulation. Some materials, such as aluminum foil or mylar, actually reflect heat back toward the pot. Important characteristics of any insulating material incorporated into a haybox include:
- It must withstand cooking temperatures (up to 100 °C (212 °F)) without melting.
- It does not release toxic fumes (any kind of foam insulation needs to be covered with aluminum foil or mylar) or dangerous fibers (fiberglass also needs to be covered).
- It can be fashioned to be as snug-fitting as possible around the pot. A little pot in a big box will not cook as effectively; it’s better to wrap pillows, towels, or blankets around it to fill up the space.
- It can be made to form a relatively tight seal, so that heat does not escape from the cooking cavity. Since hot air rises, a container designed to open at the base rather than the top will retain more heat.
- It is dry, and can be kept dry, since wet materials don’t insulate as well. An inner layer of aluminum foil or mylar helps keep cooking moisture from entering the walls of the box. Mylar, which can be salvaged from used food storage containers, balloons, etc., tends to be a more durable inner layer than aluminum foil.
- Cooking containers, too, should have tight-fitting lids, to prevent the escape of heat and moisture.
Since water is not lost in haybox cooking the way it is during extended stovetop simmering, the amount of water used to cook grains and beans is normally reduced by one-quarter. Instead of adding two cups of water per cup of dry rice, try adding one and a half. Also, the larger the amount cooked, the more effective haybox cooking is, since a full pot has more mass and therefore more heat storage capacity than a half-full pot. Haybox cooking is ideally suited for a family or large group, or anytime there’s a reason to cook in quantity. If you’re cooking alone, try cooking full pots of food using a haybox, then reheating small portions for individual meals – this too can conserve fuel.Retained-heat cooking has many other advantages in addition to energy and water conservation. As mentioned, it makes “timing” less critical, since it keeps meals hot until serving time. Once the initial boil-and-short-simmer stage is past, it also eliminates the danger of burning the food on the bottom of the pot (the sad fate of too many pots of grains, beans, or other foods left simmering too long without stirring on the stove). Hayboxed food can actually be better for you, and tastier, than food prepared exclusively on a stovetop, because most of the cooking takes place in the 82 °C (180 °F) to 100 °C (212 °F) range, rather than at a constant 100 °C (212 °F). (Lower temperatures preserve more flavor and nutrients, as they also do in crockpot cooking and solar cooking.)
If you want to prepare multiple items for a meal but have only a limited number of flame sources, hayboxes can also greatly facilitate the logistics of food preparation. For example, you can bring your beans to a boil, simmer them 15 minutes, and put them in a haybox; then bring your rice to a boil, simmer it five minutes, and put it in another haybox; then prepare your vegetable stir-fry or soup, etc. At the end, you’ll have a uniformly hot, unburnt, multi-dish meal, all off a single flame, probably consuming less total fuel than you would have used simply to cook the longest-cooking item alone without a haybox. You’ll also have used one-quarter less of your drinkable water supply in preparing the food.
Presoaking and draining beans always makes them easier to cook, as well as to digest. A few particularly long-cooking foods, such as garbanzo beans, may need reboiling partway through the cooking process. For health reasons, meat dishes should always be reboiled before serving.
Hayboxes are second only to solar cookers (which, however, are dependent on sunshine) in their potential to conserve resources. They’re easy to build, easy to use, and have many other advantages.
[The original text above was copied from http://www.lostvalley.org/haybox1.html.]
Heat-retention cooking is increasingly seen as a valuable component of the Integrated Cooking Method, by helping to save fuel resources and improving the air quality for the users and their families. The method is simple. Use a solar cooker when the sun is available, use a clean cookstove when it is not, and use heat-retention cooking to supplement both cooking approaches. As with promoting solar cooking, it is important to understand who the potential users will be, the typical foods in their diet, and any unique cultural factors that are part of their usual cooking methods. Alternative cooking methods are often introduced into the rural areas of developing countries by nonprofit organizations who provide training and help underwrite the expense of solar cookers, clean cookstoves, and heat-retention baskets. It has also been shown that involving the local population with the fabrication and assembly of the cooking appliances can greatly increase the chances of their continued use after the training workshops have ended. Also, training a few local residents to lead workshops themselves and follow-up with users is important.
Often these nonprofit programs target those most in need of alternative cooking methods. Either cooking fuel represents too large a portion of their family budget, smoky conditions are creating health problems, and dwindling local forests are likely being cut down for fuel. But evidence has shown there is also a middle-class market for alternative cooking technologies. These people usually have access to radios, and are concerned with their social status in their communities. Radio advertising of demonstration workshops have drawn good size groups to investigate the technology. If a local celebrity can be involved in the presentation, potential users will see this cooking option as improving their social status, and not just a cooking solution for the poor.
The NGO Practical Action makes these suggestions for the promotion of heat-retention cookers:
- Any manufactured cooker needs to be marketed as an aspirational product, not as a product for the poor.
- These should be designed as a middle- to upper-income product, but enable access at a lower-income level.
- Demonstrators should be encouraged to cook a variety of food items, as users tend to associate the technology only with those food items used in the demonstration.
- It's more important to focus on what the user sees as the advantages of this technology, not what the developer thinks are its advantages.
- Building and selling heat-retention cookers can be a source of income for poor families.
- The seller should be the user of the technology.
- Participants in a construction class should bring their own materials for building a heat-retention cooker to make the point that these can be built by the local people using only the materials that they have.
See Promoting solar cooking for more information on promotion in general.
Using a solar box cooker as a retained-heat cooker
When combining retained-heat and solar cooking, if food has gotten thoroughly hot in a solar cooker, but clouds arrive before the food is finished cooking, a switch from solar to retained-heat cooking should be made before the oven temperature drops below the boiling point. For large recipes this may be accomplished by simply closing the reflective lid (if using a solar box cooker on the pots of cooking foods. For smaller recipes, the solar oven is opened, taking care not to allow steam to escape from under the lids, pots are pushed close together along with any heated additional mass. Insulating pads or soft cushions are tucked closely around the pots and well-heated mass. The lid is then closed. This effectively makes the transition from solar to retained-heat cooking. The cooker lid remains closed until shortly before serving time, when the food is tested. If not completely done, a very little conventional fuel will usually finish the job.
Usually solar/retained-heat cooking is done right where the solar cooker is located. However, a lightweight portable cooker can be moved temporarily indoors for its retained-heat cooking time if the sun clouds over or if it rains. It may also be brought inside more or less permanently during the off season or at night and function as an insulated box for retained-heat cooking. Used in this way the cooker continues to save fuel rather than simply being stored until conditions are right for solar cooking. Combining solar cooking and heat-retention cooking has led to greater acceptance.
Tips and Tricks
- Keep the steam in the pot. The more steam you start with, the longer your food will stay hot. Do not lift the lid to check right before hayboxing!
- Wrap the pot(s) tightly in aluminum foil if possible, two pieces for each pot unless the pot is very small. Use heavy-duty foil if you can get it. Then re-use the foil each time (wipe and dry, if needed). The more you use it, the more crinkled it will get, creating thousands of small air pockets that help to insulate the heat the foil reflects back to the food. This can really make a difference in how long your food stays hot enough to keep cooking.
- If possible, wrap the foil-wrapped pot(s) tightly in thin cloth before putting in the haybox and insulating with thicker materials.
- Terrycloth towels can be good insulation to pack around foods in a retained heat cooker.
- If you cook in a box oven and eat after dark, get some black cloths and towels to pack around your foods before the sun gets too low (also use extra rocks in the cooker to soak up more heat, if there is room for them). Wrap and cover the food with the black cloths and leave the cooker open for the last of the good sun before closing, so even your insulation will start out warm to keep your food hot longer.
- After cooking the evening meal, use the heat-retention cooker to keep water warm for morning use.
- November 2014: Comprehensive report on heat-retention cooking prepared by Dieter and Imma Seifert: Notes on Fireless Cooking - Cooking With Retained Heat - Dieter and Imma Seifert
- July 2014: The blog Resilience has passed along a comprehensive article on heat-retention cooking with wonderful illustrations originally written by Kris De Decker for Low-tech Magazine. It can be read here: If We Insulate Our Houses, Why Not Our Cooking Pots?
- November 2007: At the Solar Cookers and Food Processing International Conference (2006) in Spain, Wietske Jongbloed of the KoZon Foundation learned a useful trick from David and Ruth Whitfield: a heat-retention cooker can be made from empty rice bags. The Whitfields advised that two large rice bags, with insulation such as wool or cotton in between, make a sack that will keep a pot of food at cooking temperatures long after it is removed from a heat source. KoZon introduced the idea in Tahoua, Niger, where cooking pots are typically very large — 12 liters (3.1 gal.) to 16 liters (4.2 gal.). For these pots, four rice bags are needed to surround the pot, and a fifth bag stuffed with insulation covers the top. Of this five-bag system, Jongbloed says, “It is named ‘Idadafoua’ and works marvelously.” Jongbloed writes that women in the Sahel cook meals for families of 10 to 12 or more people. The KoZon project in Tahoua teaches the use of three cooking devices — fuel-efficient wood stoves, heat-retention cookers, and solar CooKits. CooKits are used on sunny days for meat or fish sauces, eggs, sweet potatoes, and groundnuts, and for cakes that are sold by the slice to earn money.
Articles in the media
- December 2017: Thermos bag: A masterstroke in saving energy - NewsDay
- April 2017: Kenya learns to cook with solar power – even when the sun doesn’t shine - Reuters
- February 2016: Thermos Nissan Thermal Cooker - The Boat Galley
- July 2104: If We Insulate Our Houses, Why Not Our Cooking Pots? - Low-tech Magazine
- April 2014: The Slow Cooker That Requires No Electricity - The Atlantic
- March 2009: Pot Cooking - Odettes Recipes from Zimbabwe
NGOs employing heat-retention cookers
- January 2017: Solar and Fireless Basket Cooking in Kithuia Village, Kenya - Nicholas Nzola Kithembe
- June 2016: How to insulate a hay basket cooker - Solar Cooking The Netherlands - KoZon
- March 2015: Cook fireless with enthalpy, not fuel - NAREWAMA
- November 2014: Notes on Fireless Cooking - Cooking With Retained Heat - Dieter and Imma Seifert (Comprehensive report on heat-retention cooking)
- Fireless Cookers Datasheet - Practical Action
- Guide to Designing Retained Heat Cookers (English), (Spanish) - HELPS International
- Fueling the Future - Maximizing fuel conservation with the Integrated Cooking System - Patricia McArdle
- Engaging Communities in Alleviating Smoke, using heat-retention cooking - Practical Action
The Fireless Cook Book
The Fireless Cook Book - Margaret J. Mitchell (Full text available online)
Retained Heat Cooking ... the Wave of the Future Again
by Leslie Romano
Discover how easy it is to make and use your own off-the-grid cook box to cook uncommonly good food of all kinds. This is a frugal, time-honored method of cooking that saves time, space, money, resources, nutrition, and electricity. Includes sections on the history and science of retained-heat cooking; how to make and use your own cook box; tips and suggestions based upon personal experience; recipes; homeschooling ideas; and ten incredible advantages to cooking highly nutritious, perfectly cooked food with the natural, easy-to-implement retained-heat cooking method.
Audio and video
- July 2016:
- April 2013:
- February 2010:
- February 2009:
- May 2009:
- November 2007:
- June 2008:
- Integrated Cooking Method
- HotBag Project
- Using rice bags to build a heat-retention cooker
- Kapok Cooking Basket
- KotoThermosCooker Scout
- Guide to Designing Retained Heat Cookers
- Heat storage
- HELPS International
- Pot-in-pot cooler
- How to make a heat retention box
- Practical Action's information sheet on "Fireless Cookers"
- Solar, Off Grid, Green & DIY Videos
- Thermal Cooking Weblog: Fireless Cookers
- An audio interview with Mike Bridgwater about his work in combining solar cooker and heat-retention cooking in Tanzania.
- Heat Retention Cooking vs. Solar Cooking - Mike Bridgwater
- A Wikipedia article on Vacuum flask cooking
- The Heat Retention Solar Oven by Ronald L. Conte Jr.
- Guide to Designing Heat Retained Cookers - Don O'Neal, HELPS International
- Guía para el Diseño de cocedoras de calor retenido - Don O'Neal, HELPS International'
- The Fireless Cookbook - Margaret Johnes Mitchell (Kindle version available for USD 0.99)