Is there such a thing as fabric with a zero or even negative R-rating?

R-Value is a measure of a materials resistance to heat flow; it's roughly the inverse of heat conductivity. So a negative value isn't possible, but of course some materials have a lower R-value than others.

We usually think of things with a high R-value as keeping you warm, but they can also keep you from getting too hot. Think about the insulation in your attic: in the winter, it helps keep you warm by preventing the heat in your home from escaping, and in the summer, it helps keep your home cool by preventing the heat from outside transferring to your cooler home. So a negative R-value, if such a thing were possible, would mean things work backwards: heat transfers from the side with lower energy/temperature to the side with higher energy/temperature, and of course that's not how the world works.

But I say, forget about this crack physics lesson and R-values, and if you're looking for a material that will keep you feeling cool in the summer, you may want to look at silk. You can find base layers, underwear, and sleeping bag lines made out of the material. I've got a silk sleeping bag liner from Cocoon (there are other brands, as well) that I don't usually use, but I do find nice on a very hot night. I'm of the type that needs something covering me when I sleep, no matter how hot it is, and silk works nicely for this.

The drawbacks of silk are that it is expensive (though silk base layers are usually half the price of wool) and delicate. In the summer, I've found a silk shirt makes a decent base layer to aid with sweat management, but you have to be careful with it. The shirt I had was also very sheer. It was a long sleeve shirt, and my plan was to wear it under another shirt as a wicking layer, and, if that was too hot, to wear the silk shirt along. Well, it was so see through that in doing so, I'd be showing off my manly, hairy chest more than I intend to. For the sake of my hiking partners, I kept the other shirt on.

For me, I've found I really don't need silk base layers, and I rarely use my sleeping bag liner for the purpose I original had in mind (to keep my bag/quilt clean in all seasons), but I do like the silk liner in very warm weather where it can substitute for a sleeping bag (and I've got a lightweight down jacket in case I do get chilly at night).

+1 on the R-value concept above.

Wicking qualites certanly vary. Try putting on your base layer top just after a shower (as in bath) when your skin is saturated. If it wicks well, you'll feel the "flash" cooling right away. If not, it feels sticky and will pretty much be the same way on the trail. Done right, your layers should keep your skin dry and move the moisture on out.

Adding a shell slows the process so cold wind doesn't take away that dry warm layer of air next to your skin and HOPEFULLY keep moving the moisture on out.

Wet cotton?

Plus 1 on wet cotton.

When I take off a wet cotton shirt I usually feel immediately warmer than I did when it was on.

I think Tacoma Mountain Rescue (or name something like that) used to have a torso (or maybe two) with one wearing a wet cotton t shirt. Anyway they demonstrated in some way that the one with the wet cotton was cooler than the one with nothing on it.

Sorry for the vague detail. This was 40 years ago.

R-rating is a 50 year old invention created by the insulation companies to make it simpler to sell and compare their products to eachother. What R value actually represents is the reciprocal of a material's U value (or rate of heat transfer). Because of it's relationship to a materials "U" value, R can only approach, but never be less than zero.

Regardless of the technical stuff, I believe what you are really asking about weather one can "feel" cooler while wearing a shirt, vs. not.

As others have said, a wet shirt will certainly offer the sensation feeling cooler, especially when the air is dry.

Personally, I habitually wear a lightweight long sleeved shirt whenever I am either doing yard work or hiking, even on the hottest and most humid days (I live in DC Metro Mid Atlantic swamp land.)

Not just any ole shirt, but one which is very good a wicking moisture away from my skin. Even when the air is completely saturated with moisture, I'd rather have my sweat get soaked and spread into a hydrophillic material than fester on the surface of my skin. And if there happens to be a slight poof of wind at some point, my shirt "cools" me off a little in a way that my sweat infused skin wont.

Besides, a shirt keeps both the bugs and sunburns away (I dip my shirts in permethrin once a year.)

hope this helps.

And for the curious about hydrophillic materials: http://www.ncbi.nlm.nih.gov/pubmed/7635133

A wet cotton shirt? It has a positive R-value, but it's small and overwhelmed by the evaporative heat loss.

Wearing no shirt still has a positive R-value. The boundary layer of air – that thin layer of air closet to an object that almost doesn't move – that boundary has an R-value of about 1, but depends on wind speed. In high wind, that boundary layer gets thinner and the R-value decreases.

If you wore a coat made of heat pumps, whether they were thermocouple-based ones or the more efficient refrigeration cycles based on freons, would pump heat "uphill", against the temperature gradient, and might be viewed as having negative R-value. Although other descriptions such as BTU/hour ratings are more useful.