The key to energy efficient flooring is in the efficiency of its insulation. The most common types of flooring are:
While beautiful and easy to maintain, wood flooring allows more heat to escape than carpet or vinyl. This is easily remedied by adding a layer of fiberglass or mineral wool insulation underneath.
A layer of insulation in itself is only more efficient by adding an extra layer underneath. To make carpet more sustainable, add either an insulating underlay or carpet padding.
The combination of tile and caulk makes it the least sustainable flooring option, hence the coolness to the touch of bare feet.
With all the possible designs (now in wood and stone) as well as its status on par with carpet in terms of energy efficiency, vinyl has become an increasingly popular option for flooring.
This US Department of Energy tool will give you zip code specific information on insulation: www.ornl.gov/~roofs/Zip/ZipHome.html
US Department of Energy tips on adding insulation to an existing home: www.ornl.gov/sci/roofs+walls/insulation/ins_06.html
City of Seattle Green Home Remodel Guide series www.seattle.gov/dpd/GreenBuilding/SingleFamilyResidential/Resources/RemodelingGuides/default.asp