Once you’ve found the right fibre, you then want to ensure that your yarn has been spun to the correct weight and twist. And the yarns to avoid are yarns that are too fine or too loose.
A thin, fine yarn will be very slow to weave with, and will put a lot of strain on the warp unless it’s woven with multiple strands at a time. A loosely- spun yarn will compress too much on the loom when you beat it down. It is also in danger of letting dirt and dust rub into the core of the yarn when it is underfoot.
The thinnest yarn I work with for a rug is the Balfour & Co hard-twist yarn, which can be as fine as 24 WPI. While this yarn is thin, it is very tightly spun. When you run it between your fingers it feels dense and hard, and it will hold its shape on the loom. The tight twist acts as a natural shield for the dirt and dust, which will brush off the surface of the yarn rather than sink in.
I would usually weave with 3-4 strands of this yarn for a rug, and 1-2 strands of the standard Balfour & Co rug yarn, which is approximately 13 WPI.
The Balfour & Co standard rug yarn is not as tightly spun as the hard twist, but it is still spun firmly and plied up from multiple strands. Again, dust and dirt brush off it rather than sinking in, and the coarse surface also prevents pilling.