Sheep wool is an excellent insulator because of its “crimped nature”. There are millions of tiny pockets that trap air. Cotton is very similar. A big difference is that wool has a certain oil that repels water and keeps the air pockets full of air. Cotton, on the other hand, will absorb the water.
In terms of trapping air, I think it is very similar although Cotton is rated with a thermal resistance (R-value) of 3.4 per inch and wool is 3.5 per inch. Keep in mind that trapped air is an excellent insulator, so looking at the R-values and assuming other factors are very similar, wool traps more air.
The question can be answered in a couple ways:
A) Nature’s “purpose” to wool is to keep sheep warm. That happens partly on account of the fiber itself, but mostly on account of the large air spaces masses of wool fibers trap. (Or at least partly impede from escaping.)
B) Nature’s purpose for cotton isn’t to keep anything warm or to trap air. The URL below says it’s to “disperse the seeds”. Sounds reasonable. But for this discussion what’s relevant is that cotton’s ability to trap air is probably related to its ability to spread seeds. I.e., cotton catches the wind and makes the seed buoyant. But that’s not quite the same as halting the movement of slow-moving air, which is what wool does.
So far, the answer is wool, by a mile.
But there are other factors to consider. People think of cotton as some uniform product, but in fact types of cotton plants have short or long fibers. Durable and not-so-durable. The tough long fibers make excellent fabric.
And for wool, there’s very expensive, carefully cultivated wool, and poorer wool.
Why go into all this? Because the process has *just* begun. The fabrics that you wear go usually go through very lengthy processes to “tune” them, as it were, to their intended purpose. They may be coated, twisted, soaked, tinted, etc. Many of the natural aspects of the fibers may be stripped away. Not all of that improves the end fabric’s ability to trap air! Many of those changes, for example, are to make the wool or cotton better at absorbing dye!
Then there’s the weaving process. Both wool and cotton can be woven in a way that improves their warmth (their air retention).
So, in summary, we’ll have to allow that a very warm cotton could be warmer than a poor wool.
But overall, check fishermen and farmers in cold climates: They’re not wearing cotton on a cold winter day.
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