Heat packs and cold packs are the two most misunderstood items in a live-insect box. People assume a heat pack sets the box to a temperature, that a cold pack is its opposite, and that more of either is safer. None of those is true. This guide explains what each one actually does, what its rating means, and why the packaging around it does most of the work. Our shipping page covers when we ship, weather holds and the live arrival terms for your own order.
What a shipping heat pack actually is
A disposable shipping warmer is the same technology as a disposable hand warmer. Inside the pouch is iron powder, salt, water, activated carbon and vermiculite. The pouch itself is microporous, so air can get in.
When the outer wrapper comes off, oxygen reaches the iron and the iron begins to oxidize. Rusting is an exothermic reaction: it gives off heat. The salt and water speed it up, the activated carbon distributes the heat and moderates the rate, and the vermiculite holds moisture so the reaction does not burn itself out in twenty minutes.
Three consequences follow from that, and all three matter:
- It needs air. A warmer sealed inside an airtight bag or an airtight box will underperform or stop. It is competing with the animals for the same oxygen.
- It cannot be switched off. Once the wrapper is off, the reaction runs to completion. Nothing about it is adjustable in transit.
- It takes time to start. These packs come up to output over a period of tens of minutes, not instantly, which is why they are activated before a box is closed rather than after.
What the number on the pack means, and what it does not
Shipping warmers are sold by duration: 20 hour, 40 hour, 72 hour. That number is a claim about how long the pack keeps producing usable heat. It is not a claim about temperature, and it is not a promise about the box.
What a warmer provides is a difference above the surrounding air, inside an insulated space. It does not hold the box at a fixed number. If the outside air drops, the inside drops with it, just less far and less fast. Two boxes with identical packs, one moving through 45°F and one through 10°F, do not arrive at the same internal temperature.
The duration is also the honest limit of the whole arrangement. A 40 hour warmer in a parcel that takes 60 hours protects the first 40 and nothing after. This is the real reason live-animal shippers care so much about transit speed. The pack sets a deadline.
Why the pack never touches the insects
Surface temperature on an air-activated warmer runs well above the temperature anything inside the box should experience. Standard practice across the live-animal trade is to keep the pack away from the primary container entirely: taped against a box wall or the underside of the lid, with insulation, paper or void fill between it and the animals.
Direct contact is the classic way to kill a shipment with the thing that was meant to save it. The insects nearest the pack cook while the ones at the far end of the container stay cold. The goal was never to warm the animals. It was to stop the box getting cold.
The same logic explains why doubling up is not a safety measure. Two warmers in a small box can overheat it, and in a poorly ventilated box they can also pull down the oxygen. More is not more careful.
What a cold pack does, and why it is not the mirror image
A gel pack works on a different principle. It is not producing anything. It is absorbing heat, mostly through a phase change: as the frozen gel melts, it takes in a large amount of energy without its own temperature climbing much. That is what keeps the surrounding air steady.
Three differences from a heat pack are worth understanding:
- It is finite in a harder way. Once the gel is fully melted and has warmed to the box temperature, it stops doing anything at all. There is no gradual tail-off.
- It can be too cold. A hard-frozen pack against a container of insects is a chilling risk in the middle of summer. Many live shipments use a pack that has been allowed to temper rather than one straight from a freezer.
- It makes water. A melting pack plus a warm box produces condensation, and damp is its own problem for dry-kept feeders like crickets and waxworms.
Side by side
| Heat pack | Cold pack | |
|---|---|---|
| Principle | Iron oxidation, produces heat | Phase change, absorbs heat |
| Needs | Oxygen, and insulation to keep the output in | Freezing or chilling before dispatch, and insulation to keep ambient heat out |
| Rated by | Hours of heat output | Weight of gel relative to the load |
| Runs out by | Reaction completing | Gel fully melting and equalizing |
| Main failure mode | Direct contact, too many packs, no air | Chilling the animals, condensation, too little gel for the distance |
| Used when | Cold weather at origin, route or destination | Hot weather at origin, route or destination |
The box does more of the work than the pack does
A thermal element in an uninsulated single-wall carton is close to useless. The heat leaves, or the outside heat arrives, almost as fast as the pack can compensate. What actually determines how long a box holds a survivable temperature is the combination of four things: the insulation, the internal volume, the amount of void fill, and how long the parcel is in transit.
Volume is the counterintuitive one. A large, half-empty insulated box is worse than a smaller, well-filled one, because there is more air to condition and more room for the container to slide around and end up against a cold wall or a warm pack. Void fill is not padding for its own sake. It is thermal mass and it is positioning.
Our guide on how live insects are shipped covers the full layer stack that sits around the pack.
The right answer is often neither
In mild weather, a well-insulated box with no thermal element at all is usually the correct choice. Adding a heat pack to a box traveling through 75°F weather does not add a safety margin. It adds a hazard.
This is the part that most often surprises people receiving a box. Finding no heat pack inside is not evidence of a corner cut. It can be the deliberate decision, and it can also be the reason a shipment moves on one day rather than another. When heat, cold or a storm on the route puts live bugs at risk, we may hold an order until it is safe to ship, and we tell you by email or text when we do. Our shipping page has those terms, and the seasonal picture is covered in shipping feeder insects in summer and winter.
What to do with the pack when the box arrives
Take the thermal element out before you do anything else with the container. A spent heat pack left sitting against a tub of feeders on a counter is still a warm object, and a melted gel pack is a wet one.
It is also worth noticing the state it is in. A heat pack that is stone cold on arrival tells you the parcel ran long, or the box was not sealed the way it was meant to be. A gel pack that is completely liquid and warm tells you the same thing about a summer shipment. That observation is useful information if the contents look poor. Our guide on what to do when your feeders arrive covers the unpacking sequence, and dead on arrival covers what to record before you disturb anything.
Where the terms live
Nothing on this page states what is packed with any particular order, or in what conditions. Our shipping page sets out when orders ship, when we may hold one for weather, where we ship (the 48 contiguous states and Washington, DC), and the live arrival guarantee that applies if a shipment arrives in poor condition.
If you are picking feeders partly on how well they cope with a hard journey, dubia roaches and worms are generally the more forgiving feeders, and crickets, which we do not sell, the least. Our guide to dubia roaches vs crickets compares the two once they are home.