Until a week ago I've been commuting in some uninsulated but waterproof and nearly windproof Lake MX255 mountain bike shoes with Superfeet Wintergreen cold weather insoles, Smartwool Extra Heavy Mountaineering socks, and Pearl Izumi Amfib mountain bike shoe covers. I've been comfortable throughout my 9.5 mile one way commute down to the low 20's and able to get by, feet getting cold after 5-6 miles, down to 16-18ºF.
The shoe/insole combination has worked with the standard Smartwool Heavy Trekking socks down to 30-35ºF and adding the mountaineering socks and shoes covers have each gotten me another 6-9ºF. I'm on a quest for the next 6-9ºF as that's where I think I'll hit my next barrier in the form of frozen hands or glasses icing over.
Anyhow, the cold weather insoles only got me about 5ºF when I put them in and I suspect the biggest part of the problem is losing heat through the cleats and the exposed sole of the shoe. The shoes were purchased a Euro size and a half large to make room for the extra thick socks. Even with the mountaineering socks I still have a small amount of space left but not a 1/4" worth so toasty feets are out along with the other basic 6mm felt insoles I can find.
I really like the added stiffness and really need the arch support of the Superfeet insoles so I've been looking for some 1/8" thick felt to cut to size and put beneath them for added warmth. I'm running out of patience in my quest to find the felt locally and friend suggested installing some 2-3mm neoprene instead, possibly with some foil or aluminized mylar underneath that.
What do you think? Would it get me another 6-9ºF? or would I be settling for less?
BTW, after riding 5000 miles clipless throughout the rest of the year, I can't stand riding any substantial distance on platform pedals. They just feel wrong.