What’s the best moldable liner for mountaineering boots?

Well, in general, you take the liner that comes with the boot. And I wouldn’t recommend doing anything otherwise. Putting a Brand X liner in a Brand Y boot could lead to some serious fit issues, possibly even compromising the insulation by squeezing your foot or creating voids where moisture and cold air could be trapped.

La Sportiva Nuptse boots

Nuptse mountaineering boot

That said, most liners have some combination of closed- and open-cell foam made of polyethylene or a material called Alveolit. The popular Koflach Arctis Expe (US$365; www.koflach.com), for instance, has a heat-moldable liner that comes with both materials. That’s ostensibly to help with durability, as the knock on Alveolit is that it doesn’t last as long as lesser (and admittedly not as warm) liners. That said, the Arctis Expe is a proven boot, good for temperatures down to minus-60-degrees Fahrenheit.

La Sportiva’s Nuptse mountaineering boots (US$525; www.sportiva.com) would be an interesting choice as well. The boot uppers are made with five-ply coated nylon, leatherette and leather, not plastic, for a softer feel. The liners are high-density foam with an aluminum coating that reflects heat. I haven’t tried these but they’re said to be exceedingly warm. But fit is very important on these boots. You can’t just buy a big boot and let your feet slop around, as you can with some plastics.

Scarpa’s Inverno (US$279; www.scarpa.com) remains a great buy in a good, all-purpose plastic mountaineering boot. Its standard liner is made from EVA foam (not heat-moldable). But for another $150 you can buy high-altitude liners made with Alveolit foam. Don’t expect too much use of them, however, as they simply don’t hold up that well.

It’s a little hard to say which of these would be “warmest” as there are so many variables—the temperature, snow or ice conditions, your condition, socks, and on and on. But the boots I mention, plus many others, are meant for intensely cold conditions. Combined with the right socks and overboots, there are very few places on the planet where you couldn’t go.

Check out this year’s more than 400 must-have gear items, including a comprehensive hiking boot section, in the 2006 Buyer’s Guide.