R-value for insulating a vaulted ceiling?
The numbers I found for your area are actually R30-R60, so more than R30 would be ideal as air conditioning is expensive and the one-time cost of better insulation can easily pay for itself.
How deep are your rafters?
R values of common materials can be found in this chart from greenbuildermedia.com:

To this chart, I'll just add that if you're wealthy there is also Aerogel to consider, with an R-value of R-20 per inch.  It's still insanely expensive so the time needed for it to pay for itself might exceed the life of a normal building.
Thanks to a correction from Lee Sam, according to (See ICC R806.1.2 in the Residential Code) you need a 1&quot; air gap between insulation and sheathing so the following numbers are adjusted accordingly:
Say you have a 6 inch deep rafter, you divide the total R-value you need by 5 inches.  The result shows for 30 to 60 R-value you need a material with an R-Value per inch of 6-12, so you'd need Polyurethane spray foam or Foil-faced batts of Polyisocyanurate.  You would actually need aerogel if you wanted to hit a total R-value of 60.
If you have an 8 inch rafter you need an R-Value of 4.28 to 8.57 per inch depth.
For a 10 inch rafter you need an R-Value of 3.33 to 6.66 per inch depth.
For a 12 inch rafter you need an R-Value of 2.72 to 5.45 per inch depth, so anything other than blown fiberglass or Vermiculite will do the trick.  11 inches of Aerogel batting from a reputable source will give you an R-value of 220 at a cost of $1031.25 per square foot.
If cost is an issue to you, still think about the long term cost of AC since you live in Texas, but use these numbers to check which of the materials that work are available to you.  Calculate the cost per R-Value per square foot one inch depth for each material and then look at the total R value you get if you completely fill the space with each material.  It might be worth spending 10-50% more per volume on a material if it lets you increase the total R value.