It’s almost an inevitable part of any serious winter outdoors adventure; at some point, you’ll be stuck standing around in the snow, and your feet will get cold. Worse, on multi-day trips winter backpacking, your boots slowly get wetter and wetter…by day 3, you start your morning putting on a rock-hard, icy boot, and it sometimes seems like they’ll NEVER warm up. In our opinion, it’s the single worst part of winter, as it’s much easier to warm up cold hands, noses, ears, whatever.

Oh, there have been solutions, and we’ve tried them all. Boot blankets like the Arctic Shield Boot Insulators can be a big help while you’re standing around in camp, but they’re relatively bulky, difficult to walk any distance in, and won’t help you at all while you’re hiking. Disposable toe warmers add significant warmth, but have their own problems; if you put them on top of your toes a lot of the heat goes out through the boot, if you put them under your toes they can make your boots really uncomfortable, and they’re often so hot that they make your feet sweat, which means colder feet once they stop working.

A couple of years ago, we started seeing electric heated insoles appearing on the market. Great idea, with a couple of flaws. The first is that once they’re discharged, they’re pretty much just heavy insoles; that’s not a horrible thing when you’re back home for the night, but the tradeoff didn’t seem to work all that well for backpacking. The second is that they were both significantly thicker and stiffer than the insoles that came with any of our boots. Thicker meant that they wouldn’t work in a lot of footwear; stiffer meant that they made a lot of footwear uncomfortable. Certainly, they had some advantages over previous ideas, but were still a pretty expensive option with too many problems…we passed on them.

But, then out came the ThermaCELL ProFLEX, and things appeared to be looking up. We’ve had excellent results with ThermaCELL’s insect repellent devices, so it made sense to check out the company’s newest offering. They’ve done some very intelligent design work with these insoles, including using a newer, more flexible material that looks like it could alleviate the discomfort issue. Into our boots they went!

And, we found lots to like. The first thing is that, yes, they were much more flexible and comfortable. We tend to have pretty stiff winter boots to begin with, and adding a lot of stiffness means they start to feel like bricks on our feet; that didn’t happen with these. Generally, once testers had them in for a little while, they stopped even noticing them; score one for ThermaCELL! And while they still take up more space in our boots than the original insoles, that mostly wasn’t a problem (but do pay attention to your boots before you buy. If they’re already tight, these likely won’t work).

Second, the use of a removable, rechargeable battery. ThermaCELL makes ProFLEX Battery Packs available for a reasonable price; you’re going to want at least one extra set, for a reason to be discussed later. Many of us carry portable power packs now to keep various devices running; with a lightweight USB charging cradle, you now can re-juice batteries on the fly, dramatically increasing their utility in the field.

Third, the remote control. Remember, the goal is to keep your feet JUST warm, so they don’t sweat; stuffing a disposable in your boot and heading out on the trail is an overheating nightmare waiting to happen. You’ll sweat like crazy, and just about when you’re ready to quit, the disposable will stop putting out heat and all of that lovely sweat will turn to ice. Instead, hit the trail with the heat OFF; when you stop for a long break, THEN turn it on. And, ideally, remember to turn it off just before you start up again, balancing the dropping heat from the insoles with the increased heat from blood flow.

Speaking of that heat…don’t expect to “feel” it. ThermaCELL very intentionally kept the heat low, with 111 degrees produced on high and 100 on medium (there’s no low). That sounds pretty warm, but in reality you barely notice it if you aren’t paying careful attention. A lot of the heat simply goes to warming the boot around your foot, warding off chills; it isn’t like putting your stocking feet up by the fire when you come inside. In fact, the best way to notice what’s going on is to only charge one battery, then go out with one live/one dead. You’ll notice it then; the unheated foot will be noticeably cooler in cold weather. One tester was teaching a winter backpacking seminar at about 20 degrees; after 3 hours standing around on snow, he noticed that one foot was perfectly comfortable, while the other was chilled…not cold, just reaching the “I’d like to go inside for a while” stage. Mission accomplished!

Sounds pretty much perfect, doesn’t it? Well, close…there’s one fairly significant problem. Remember when we said you’d automatically want to buy another battery pack? Here’s the deal. As soon as the batteries are put into the insole, they’re “on”…not producing heat, but in circuit and using power. It’s a relatively small amount (according to their information, roughly 2%/hour), but it can add up. As it did for the editor who was all excited about them on a winter backpacking trip. Charged them up, put them in his boots, and off he went. Day 1, it stayed fairly warm…he didn’t need them. Day 2, same thing. Overnight, the temperature plummeted, and in the morning he turned the insoles on…nothing.

So, just don’t put the batteries in until you need them, right? Well…that leaves a big hole in the insole, right under your heel. You need the battery in, period. And, that’s why two pairs of batteries! Now, you can minimize the problem by simply taking the batteries out whenever the boots aren’t on your feet; that might have helped our guy out a lot over two nights. But, we don’t always remember to do that, so spare batteries are our best solution. We’ve encouraged ThermaCELL to come out with an “UNbattery,” or something of the sort; a plastic blank that’s cheap, light, and fills the hole until you need heat and put a real battery in.

That blind spot aside, we found the ThermaCELL ProFLEX insoles to be a worthwhile addition to our winter kits. They aren’t going to solve every problem, but at the very least we’d be thrilled to have them in our boots if we got hurt and were waiting for a rescue. And if you get stuck officiating or just watching at XC ski meets…well, you might just decide they’re your new best friend!