World's Cheapest, Lightest, Best Foot Warmer

This is so effective it's just stupid. Almost weightless, costless and super-tough, it works well alone or under booties. In thinking about materials used in booties I have to say, they are very poorly chosen. Neoprene? Are you kidding me? Very heavy, not very windproof, and because of the density of rubber, a poor insulator.

Once wet, neoprene is as cold as cotton. Worse, it stays wet and conducts, evaporates and convects your feet into icicles. (It takes at least 8hrs to dry a 7mm wetsuit on a warm day) It has an R value ~ 0.5 per inch of thickness. Thinsulate under this stuff would be pretty good, which is why it's the material of choice for SCUBA dry-suits.

Here's a table I found for the R value of Thinsulate Ultra. As you can see, it would take over 5 inches of neoprene to match Ultra 200's R value. Need I say more about how poorly the industry is being served by its product designers? (the correct thickness of US150 is .64")

I would recommend a good wool knee-sock to wick the moisture up out of the shoe. There is plenty of airflow, even under tights, to accomplish this. They also do a great job of keeping your calves warm - especially the snowboarding kind with extra padding in the back for heel-side pressures.

PS: I was very surprised that a Google search for 'R-Value' turned up this blog post first and foremost. Given this I was a little uncomfortable with my source for this info, so took it upon myself to do a few hours of research. I found two sources I found credible,  cited at the bottom of this Excel spreadsheet. The first was from an educational physics page, and the second was a Wikipedia page on R-Values. As you can see, I looked for materials the two tables had in common and then tried to come up with a conversion factor.

I ended up favoring foam board as reference materials, as they too are purposely designed to insulate. Of interest, felt, though heavier, would be a good alternative to Thinsulate. Please note, the table R-Values are for 1" thicknesses, while the R-Value for Thinsulate is for the material's stated thickness. At the end of the day, it looks like it would take 3-6" of neoprene to match the insulation provided by the best Thinsulate. (A layer of US100 and one of  US150 is ~ 1" and R 3.32)

The materials` weights are wildly out of proportion to each other. It should also be noted that these values are for conduction losses only, which assumes they are in an air-tight environment and all radiant losses have been eliminated. (both good assumptions under SpaceBlanket, but uncovered, far, far from bicycle shoe cover conditions)