Join Date: Aug 2009
Bikes: 2016 Masi cxgr, 2011 Felt F3 Ltd, 2010 Trek 2.1, 2009 KHS Flite 220, 1991 Bianchi Osprey
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I've had VERY variable results with chemical foot warmers. It may be brand dependent, but I haven't done enough research to tell you which ones work best.
It is true that they need access to air to work, but there should always be enough air in your boots to allow them to work - I don't believe that any of us wear boots tight enough to keep a minimum of oxygen from circulating into them.
On the other hand, I have wondered if TOO much air can limit their useful life - the basic reaction is oxidation of metal (iron powder, I think?), and surely oxidation goes faster in a well ventilated system as compared to one with comparatively stagnant air - (i.e. think blowing on a fire....). If you've got ventilation holes on the bottom of your Sidis, you may be blowing on the pads with a 15 MPH bellows. (or 20? you get the idea).
FWIW, I've been wearing these this winter:
(with powergrips, rather than with a cleted pedal). They're rated to -25 °F or so, but I don't think that the rating translates to cycling. Still, with these at 20 °F, my feet are absolutely comfortable for hours. At 10 °F, they start to get a little cold after maybe 90 minutes.