I'm living in an apartment at school with my fiancee. The walls are nice and thick but the ceiling isn't unfortunately but there's less we can do about that than the temperature.
The floor, as far as I can tell, is just a concrete slab with linoleum tile over it. (maybe some intermediary? never dealt with it so I dunno) the point is that it's darn cold in the winter. (winter, ha, there's a joke. it's been in the high 30s/low 40s all week) My IR thermometer read 10 degrees cooler near the edge of the apt than in the middle and 5 degrees colder there than the walls/ceiling. not a huge difference I know but that combined with it being a hard thermally conductive surface makes your feet FREEZING. a month ago we were fidgeting with the thermostat 6 times a day because if your feet were on the floor, they were cold and you're freezing even moving around doing stuff. sit down with your feet up for a while, then the heat kicks in and you're sweating burning almost no calories.
Now, with the carpet in, the thermostats set almost 10* cooler for the same perceived air temp/comfort, it's comfortable all day, the temperature doesn't seem to fluctuate as much like before (big furnace in little apartment with un-sensitive thermostat), nor does the furnace seem to run even 1/4 as often.
We'll see how summer goes, long spans in the 90s are frequent and the humidity is fairly high. By then we may want the cool linoleum floors again. Either way the carpets not nailed down or anything and it was FREE. When my fiancee's home flooded a number of years back, they did the cleaning themselves and got new carpeting etc themselves so the insurance check had gobs left over. well, her dad didn't measure out the house beforehand and got more than double what he needed so there was a whole 12x60 roll still in plastic as well as leftovers from the other roll he didn't finish.
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My guess is that the carpet pad (if used) and the carpet together are helping insulate and also helping slow down the cold from radiating to the surface of the floor and also pulling heat from your room temperature. I've noticed the linoleum in our kitchen and bathrooms are also much cooler than the carpet floors in the bedrooms and living room.