So I go backpacking, often in winter. After a while you start to get interested in weight (since you are carrying everything on your back). I bought a kitchen scale and weighed everything, this showed some surprising results. I was thinking though, and if I am comparing two shirts and one is 10% heavier, but twice as warm than the heavier shirt is better, even though it's heavier. I decided I need to attach some sort of thermal insulation number to the items so I can compare weight/warmth ratios. This is going to be harder than simply buying a scale, but I think I came up with a process to test this, and I'm looking for any input on what I can do differently before I invest the time to do it. What I am going to do is go in my basement, where the temperature is fairly constant (56-60F). Use a microwave to heat a liter water bottle (http://images.google.com/images?q=nalgene") to 100F (approximate body temp). Then put the bottle in an item and after a certain period of time test the temperature. Here are some of the thoughts I had. First while 60F is pretty warm (I wouldn't be wearing any cold weather gear in 60F), it's still enough of a difference that I should be able to measure the cooling. Next, originally I had thought to measure time it would take to reach a certain temperature (say 90F). The problem with this is I don't have any temperature probe that would allow me to measure without taking the bottle out. I think the measuring after a set time will be more accurate. I know that when you are sleeping you lose most your heat to the ground, thus I want to avoid having the items sitting on anything. I plan on putting them on hangers, with some way to suspend the bottle inside. I plan on testing everything 3 times, doing everything once, then starting over and doing it all again. This way the item won't still have warmth from the last test, plus I'll be able to see if the slight changes in ambient temperature affect things. Any suggestions, or comments? Can anyone tell me the formulas for estimating how long it will take to heat a liter of water in a microwave of X watts? Also how long it should take for a liter to cool down from 100F to 90F in 60F ambient air?