If you close your eyes and imagine a flannel shirt, there's a better-than-fair chance that the first thing that comes to mind is a lumberjack standing in the middle of a frozen forest, axe in one hand and a mug of warm coffee in the other. Or perhaps you're more likely to picture a hiker cresting the peak of a leaf-strewn hill just before a perfect autumn sunset. What all of these elaborate fictional scenarios have in common is that flannel is being worn to keep the wearer warm.
So, the short answer to the question "are flannel shirts warm?" would be an emphatic "yes," followed by multiple exclamation points. But rather than ending things there, we're going to take a deeper dive into precisely what flannel is and how it manages to keep wearers feeling warm and cozy.
Contrary to popular opinion, flannel doesn't refer to a single type of fabric. Flannel fabrics can be wool or cotton, although a flannel shirt is far more likely to be made from the latter.
What makes flannel stand out from other fabrics made of wool or cotton is the same thing that gives it its warmth. Flannel fabric has a "nap," meaning that the yarns that have been spun into the fabric are slightly raised, which is what gives flannel its highly textural feeling and soft hand. This nap is typically created via the use of a fine metal brush, which will be run along the thick fabric to raise its fibers. Some flannels are brushed on just one side, but the flannels that rank among the warmest are more often brushed on both sides.
Think of the role that insulation plays in keeping a house warm. When insulation is placed between the walls of a home, it makes it less likely for warm air to leave. With more heat trapped inside of the home, the living space becomes warmer.
Flannel's brushed quality has a similar effect in regards to warmth. The heat that your body naturally generates is forever escaping, but the raised fibers found in a flannel shirt will trap that heat, keeping it closer to your body.
Then there's the quality of the fabric's weight. Because more substantial fabrics make a better candidate for brushing, most flannel shirt fabrics are thicker and heavier to begin with. Both of these qualities will also play a role in trapping heat and warding off chills.
We've just explained the natural factors that make flannel shirts warm, but that doesn't mean that additional ways to up the heat factor of a flannel shirt or flannel robe don't exist. One surefire way to create more insulation is to line a long flannel robe with Sherpa fleece. Sherpa fleece functions in a similar way to flannel, relying on its raised, fuzzy texture to trap heat, the difference being that Sherpa fleece is a synthetic fabric rather than being made from cotton or wool (no matter what the name may imply).
We could fill an entire book with suggestions for this factor, but for the sake of brevity we'll concentrate on a few favorites. During the shoulder season, a flannel shirt can be worn as a versatile layer of outerwear over a simple tee or blouse. When temperatures begin to dip, think of wearing your flannel shirt under a fleece sweater or fleece jacket. And when the thermometer really drops, you can use it as a base layer under a turtleneck or shawl collar cardigans.