Barrier cloth protectors are the most effective in controlling dustmites and offgassing.
The opinion that encasements and barrier cloths work better when made with synthetic materials is not convincing. Untreated 100% cotton barrier cloths work as well as synthetics against dustmites and odors and produce no offgassing of their own.
These natural fiber cloths were originally developed for the medical industry. Thick fibers are woven together so tightly that even tiny dustmite particles can’t easily find their way through.
Synthetic barrier cloths are sometimes made from polyester and often made from vinyl. If you are chemically sensitive, you may be troubled by offgassing of these synthetic products. In addition, life-cycle management of vinyl has not yet proven itself sustainable.
Protective encasements are especially recommended for featherbeds in order to keep them cleaner longer. Standard sizes can be purchased, or there are several sources for custom sewing from organic cotton to any size, with or without a barrier cloth.