Ask any gardener or landscaper for mulch recommendations and you will certainly get an answer with some conviction and seldom will opinions match. There are some universal truths to mulching though and general observation can tell us one universal truth.
Wood mulch that is applied year after year becomes rotten underneath and matted on top. This leads to water shedding; water is directed away from the root zone and aeration of the root zone is greatly impacted. Water simply can not penetrate a thick layer of matted wood mulch and therefore this is certainly a bad mulch practice.
Rubber mulch never mats or rots and is always permeable. Therefore, all water runs straight through with no water shed. Rubber also holds moisture in the ground. If we have a semi load of rubber mulch that is soaking wet and leave it on a concrete pad in the hot Florida sun the material will never dry out. The water will remain trapped in the rubber because rubber is an insulator. The top is warm and the middle is cool so the water doesn’t escape.
Years ago we were presented with an opportunity in Dubai. No matter what the customer tried they could not hold moisture in the ground until they top dressed with Everlast Rubber Mulch. The rubber mulch allowed all of the irrigation to flow through to the plants and held the water in the ground. The soil temperature was only 2 degrees warmer than the wood much beds due to the insulative nature of rubber mulch.
This is exactly why rubber mulch acts as weed barrier. The rubber stays dry so weed seed dehydrates before it reaches the soil. It is just that simple!