Why flannel is so warm and marble so cold

You would not expect these answers

Flannel’s fuzz traps air in little pockets.
  • Flannel’s fuzz traps air in little pockets.
  • Image by Rick Geary

Dear Matthew Alice: The question I’ve always been meaning to ask: Why are flannel sheets warmer than cotton sheets? They aren’t just thicker and keep you warmer eventually, they feel warmer to the touch at the same temperature. The only thing I can think of is the fuzziness, but I fail to see how that helps. — Matt Bonner, E-mail from Rancho Bernardo

Dear Matthew Alice: Why is the brick on our fireplace always warmer than the marble hearth? In fact, why is real marble always colder than everything else around it? — Wondering, Talmadge

The question from our wonderer in Talmadge inspires the more basic question, “Is marble colder than everything else around it?” Negatory, my friend. Marble just feels colder to the touch than wood, brick, plastic, and the like. But if you measure the actual temperature of your bricks, your marble, your rug, bookcase, everything’s going to be just about the same. “So, hey, Matthew,” I can hear you saying, “why does marble just seem to be colder?”

Odds are, everything in the room is cooler than your fingertips, which hover in the mid-80s most of the time. Anything even a degree or two or three below finger temp will feel quite cool. With marble, though, the effect is enhanced by its unique conductivity. It is one substance that tends to suck heat out of things very quickly (unlike the low conductivity of plastic or wood, for example). So the marble is actually cooling your fingertip. This also explains why an expensive marble floor in your bathroom looks nifty but feels awful when you step out of a hot shower. Marble also has a high specific heat, that is, it takes a great deal of heat to raise its temperature. That’s one reason it’s a practical material for a hearth.

On a cold night, sleeping with flannel sheets will warm you up faster than cotton because of the ever-useful fuzz. (Flannel is cotton or wool or a combination brushed to raise a hairy nap surface.) Flannel’s fuzz traps air in little pockets, and the air acts as insulation to keep body heat in, cold air out. The fuzzier the fabric, the more effective the insulation. Pound for pound, mohair and cashmere are the warmest natural fibers because of their high fuzz factor and light weight. Make sure you don’t put some heavy horse blanket on top of your flannel or you’ll squash down the nap and negate the insulating effect. It occurs to me that a large fuzzy dog might work just as well as flannel sheets.

Cotton isn’t “cooler,” it just feels cooler over the long haul because it doesn’t trap radiated heat as well and it absorbs perspiration, which has a cooling effect on your body.

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