The insole foot warmers are thin, so as not to make your hiking boot or shoes too cramped. I could see the potential for the insole foot warmer to cause your hiking boot or shoe to become cramped if you already had a tight fit, but for most hikers, I don't see it being a problem.
Overall, Ashley and I were very impressed with the ToastiToes Product. The adhesive on the product allows you to place the foot warmer for optimal comfort and they provide quality heat. The insole foot warmers are very unique as compared to other products we have seen and they work very well. Some other potential uses for the product would be placing them in your sleeping bag to warm it up while out camping or backpacking.
To find out more about this product please visit http://www.footwarmers.org/