Hi booyyah,
You can read more about allergy encasements and dust mites in post #2 here and the posts it links to.
If your main concern is to use a 6 sided mattress encasement as part of a strategy to protect against dust mites and allergens then I would generally suggest one of the more breathable encasements without the membrane because these are meant to remain around the mattress on a semi permanent basis and will allow better airflow and then you can use a waterproof or water resistant mattress protector (see post #89 here) that can be more easily removed and cleaned on a regular basis and will only affect the airflow on the top surface of the mattress instead of all around the mattress (in the case of a waterproof membrane type protector).
Phoenix