Although there are many ways to facilitate retained heat cooking, such as a Wonderbag, or haybox, there are only two types of stainless steel thermal cookers. One has foam insulation, the other is vacuum sealed. They both have pros and cons.
When buying either I would watch for quality of construction. With both types of insulation there are higher quality units available and lower quality units available. Make sure the base of the inner stainless steel pot is thick and sturdy. Look for any possible places that heat could escape (lids, handles, hinges etc) from the outer unit, minimize the possibilities. Make sure the seals are secure. The outside of the cooking unit should not be hot when the hot food is inside.
Vacuum sealed thermal cookers seem to retain heat longer with smaller sized units,a definite plus if you prefer to use small units. Note that all cookers need to be at least 75-80% full to maximize efficiency, no matter what volume they hold. If you do not want to fill them that full you will have to adjust the time the food is in the thermal unit.
What to watch for with a vacuum sealed unit. As listed above check for quality of construction, make sure that the lid seals well, and choose a size appropriate unit. The main warning I would have with these units is that if they are dropped and cracked the unit can become ineffective, so treat them kindly. A high quality vacuum sealed unit can be pretty pricey and are becoming harder to find and purchase.
Foam insulated units are less expensive then the vacuum sealed units. I have seen some units that are poorly made, so watch for quality. Make sure that heat does not escape around the lid. Although foam insulated units do not seem to hold the heat as long as the vacuumsealed units, it is most noticeable with smaller sized units. This is because there is less volume of food and will cool more quickly. To compensate for the size, when buying a smaller foam insulated unit makesure that it is built with more foam insulation inside like the Saratoga Jacks 5.5 L.
I have not found vacuum sealed units that guarantee to maintain the heat more that 8 hours, the same as with a high quality foam unit. Foam units are very durable, a definite plus, as I have dropped mine pretty hard. They remain effective unless the inner stainless steel lining is cracked, which is very hard to do. With any thermal cooker the more food inside the cooker, the longer it will remain hot. The hot food helps retain the heat.
Some thermal cookers hold a large volume of food. I would suggest buying a thermal cooker that has two pans inside, for those days when you do not need to fix as much food. By filling the extra pot with boiling water you will retain the effectiveness of the thermal unit.
For more information visit www.cindyssense.com, my facebook page,facebook group and pinterest.
I'm busy working on my blog posts. Watch this space!