We've had 3M's Thinsulate thermal insoles for a year. But thanks to last year's very mild winter in D.C., they were never worn ...

... until last week, when D.C. got coooooold.

Most shoe or boot soles aren't insulated, reps at 3M told me at last year's Outdoor Retailer show, and can be a huge source of heat loss. 3M's thermal insoles aim to fix that.

"Thinsulate thermal insoles combine 3M™ Thinsulate™ Insulation with energy-return performance foams to provide exceptional warmth and minimize shock in every step," the company says on its product page.

But would they work, and would they be comfortable?

Until recently, I ran in traditional road and trail shoes, ones with plenty of cushion and an average heel-to-toe drop. Shoes that — with a good pair of merino socks — had enough bulk to keep my feet fairly warm, even on snow or frozen ground. Frozen toes were still an issue, but once we got moving, my feet tended to warm up some, but were never actually warm.

Review: Icebreaker's Arctic water-resistant merino jacket will keep you dry, inside and out

Now I'm a minimalist convert, wearing Altra's zero-drop Intuitions on most of my road runs and the company's zero-drop Superiors on the trail. Each has some cushion, but I worried as the temperatures dipped into real winter — not last year's practice winter — that my new favorite kicks wouldn't be very warm. (Those of you zero/zero shoes such as New Balance's Minimus Zeros or Merrell's Trail Gloves should be worried about your toes freezing off, too.)

Update: My sources in the field (i.e. my training partners) tell me that these do not work so well in "glove-like" shoes. The insoles caused one tester's feet to sit up too high in his New Balance 110s, throwing off the fit.

December and most of January had been pretty mild again. Then winter happened and it got cold ... really cold.

I couldn't resist when the opportunity popped up to tag along for an early-January run in the Massanutten Mountains. Temperatures were forecast to be warm enough (between about 28 degrees and 40 degrees) but friends living near the trailhead warned the singletrack was covered in a couple inches of snow.

This seemed like a great time to try out the insoles. I feared running in the Superiors on hard-pack snow would feel the same as walking barefoot on ice.

For that run, I paired the insoles with a medium-weight pair of Point6 merino blend socks, and despite the colder-than-expected temperature and snow, my feet were never cold. When I used these insoles on a 45-degree day with ultralight Injinji toe socks, they were comfortable but on the warm side.

Last week, when temperatures crashed down into the teens (16 degrees Wednesday morning), I opted to move the insoles to the Intuitions for a couple quick — and freezing — road runs. I was curious about how warm my feet would be in such low temperatures, but I wasn't quite brave enough to wear them with just my ultralight Injinji socks. I put on the same pair of 3/4" Point 6 merino socks and headed out the door.

My ankles were cold (note to self: get longer tights) but I didn't think once about my feet. They stayed plenty warm, even during some strong wind gusts.

Pro: Warm and comfortable. As a bonus, these are also really comfortable. They're squishy enough to offer some nice cushion without sacrificing ground feel, and won't make your shoes feel snug. These insoles are like getting the warmth from your thickest ski socks and the feel of your thinnest micro-weight pair.

Con: Soggy. The one drawback is they do not dry quickly. Last week I waded through a cold (COLD) mountain stream until my shoes were saturated (research, not stupid ... ), threw my pair of Stuffits into the Superiors and walked away. Two days later when I went to retrieve them the shoes were dry, and the insoles were wet. I let them air-dry overnight before using them again.

Verdict: I may have to buy a second pair so the insoles can live in all of my shoes until the frostbite factor goes away. They're cheap enough that you can get away with multiple pairs.

Find more information here, and buy them at major outdoors stores such as Bass Pro, Cabela's and Dick's Sporting Goods for around $20.

Update: Insoles come in a wide selection of sizes, but if you have to size up they can be trimmed back down -- lengthwise -- to the correct size. I used my factory insoles as a template and cut the 3M insoles easily with normal scissors.

Sara Davidson is an ultrarunner and our resident women's gear destroyer. She fears frostbite.

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