Mulch is a balancing act, and you need to understand what you are trying to do.
For weed control, mulch has to be deep enough to block light. It won't be too much of a surprise that dark materials don't require as much depth as light materials to do this.
For moisture retention, the mulch needs to have small enough spacing to slow down wind, and be made of a material that doesn't wick the water from the soil to the air.
Now that I have you confused:
Mulches I use on a routine basis:
1. Used carpet. Use upside down. water goes through it, weeds don't seem to be able to.
2. Straw flakes. Break up a bale, and the straw is in flakes. two to three layers of flakes, a flake being 1/4 to 1/2 inch thick.
3. duck weed. (Ok, not everyone has a pond, but I do. Go ahead, turn green with envy.) Skim with a coke crate, bread tray, or old screen. Apply as 1/4" thick layer, with good overlaps.
4. Grass. I only grass around established trees that are too tall to interest slugs. Apply in 4-6" thick layers
5. Plastic. Only used in strips around shelterbelt trees, and as weed control in dogwood and willow stooling beds. The laying in complicated by the need to get water through it.
Sherwood Botsford Sherwood's Forests Warburg, Alberta T0C 2T0 http://www.sherwoods-forests.com 780-848-2548