Hi rockalicious2013,
The membrane type mattress encasements like the same type of mattress protectors are breathable but only to a point. They allow water vapor to pass through but not moisture itself so they are “somewhat breathable” but not to the same degree as cotton or wool mattress protectors. They are also effective against dust mites and other allergens and bed bugs and don’t allow any particles to go through the membrane. I don’t think they would present an issue with mold or mildew but they can affect some people negatively in terms of temperature regulation because they don’t allow air to flow as effectively as fabrics and will restrict the ventilation of the materials over and under them to some degree. Their main benefit is that they don’t allow the passage of any particles and are waterproof and have less of an effect on the feel of the mattress than thicker materials. Some people seem to be affected by the “crinkly” feel of some of these while others don’t seem to notice it at all (even with the same brand) but if you go in this direction I would focus on the ones where the surface is more stretchy rather than just the skirt because they will be less “crinkly”. As you mentioned, using a wool/cotton mattress protector over these will help with both the “crinkly” feel (for those who notice this) and with moisture regulation although it will still not be as breathable as using a fabric mattress encasement which allows more airflow under the protector but will also allow more particles to go through and aren’t waterproof.
Many of the manufacturers of the “membrane type” protectors listed in post #89 here and the posts it links to also manufacture mattress encasements that are similar except they are 6 sided and enclose the mattress and have a zipper instead of being 5 sided and fitting on the mattress like a fitted sheet. London Linens & Rugs Australia also makes some thin “membrane type” encasements that can be custom sized with different thicknesses including thinner encasements that would be suitable for using to encase a mattress topper (see post #9 here … thanks Maryanimal).
If you are willing to give up the waterproof qualities of a membrane type encasement … then a fabric encasement with a tight weave and small pore size will be more breathable than the membrane type because the pores are larger. If you use a mattress protector like the St Dormeir over it then this would provide you with water resistance without restricting airflow or temperature control.
Most mattress encasements (vs mattress protectors) are the “allergy” type and are made with a pore size of about 5 - 6 microns or less which prevents most allergens from penetrating the fabric. The most common of these are dust mites which are fairly large on a micron scale but dist mite feces, particles, and larva can be 10 microns or smaller and dander can be 5 - 6 microns or smaller and some other allergens can be smaller yet so the smaller the pore size the more complete the allergy protection (see here and here for some common allergen sizes). For most people 5 - 6 microns will be enough to prevent most allergens. These are available in natural fabrics such as cotton which are usually in the 4 - 6 micron range and made so that the pore sizes stay consistent with washing. An example of a cotton blend is the Allersoft here and 100% cotton is here. There are also many others that use synthetic fibers (such as here) where the pore sizes are smaller yet which may offer more complete protection for smaller particles but may not be as effective for ventilation or at absorbing and wicking away moisture (which is part of temperature control) which is why natural fibers tend to sleep cooler than synthetic fibers (which don’t absorb moisture). Rayon type “semi synthetic” fibers that are made from cellulosic materials such as bamboo, wood, or eucalyptus also absorb moisture and are similar to natural fibers in their ability to absorb and wick moisture. Some information about various allergy mattress encasements is here and there are also some comparisons of micron sizes between several common brands here.
The cost of these depends on the quality of the fabric with thicker and higher quality cotton being more costly and organic cotton being more costly yet.
There is more about dust mites and allergies in post #2 here.
Controlling bed bugs is another matter though and a mattress encasement that is good for allergens may not be completely suitable for bed bugs. An encasement that is designed to protect against bed bugs also needs to have small pore sizes (bed bugs are considerably larger than dust mites but the nymph stages are also mobile and also feed on blood and are smaller than the adults although still much larger than dust mites) or use a membrane that has no pores at all. In addition to this the zipper needs to lock and/or still be able to prevent creating an opening for bed bugs to enter or leave the mattress if a few teeth are open and the fabric needs to be thick enough that the bed bugs can’t bite through it. Most of the membrane type mattress encasements that are also suitable for bed bugs will say so on the description so they would be a combination mattress protector and bed bug encasement.
If you are concerned about bed bugs then it would also be important to make sure that you encase your foundation or box spring and this would also be part of an effective approach to managing a bedbug infestation.
If you have a mattress allergy encasement that has pore sizes that are small enough for allergens (or no pores at all), is thick enough to prevent bedbugs from biting through it, and the zipper can lock and prevent bedbugs from entering or escaping the mattress and there are no seams or openings or tears that would allow access for bed bugs or their nymphs, then it would generally provide good protection against bed bugs getting inside or escaping the mattress. Again … most manufacturers will specifically mention that their mattress encasements have been tested and are suitable for bed bugs (with locking zippers) and that don’t allow the bed bugs to bite through them. Most of the encasements that protect against bed bugs are the “semi breathable” membrane type
There is some great information here about bedbugs and they have more information about encasements here (including some brand specific testing) and they also have a very active and informative bed bug forum here. There is also a good chart here that shows some of the more likely places where bed bugs can hide.
If I didn’t already have a bed bug infestation I would personally be comfortable with a breathable cotton mattress encasement (the quality of the cotton and whether it was organic or not would be a personal preference) with a pore size less than 5 - 6 microns (so it could protect from allergens as well) such as the Allersoft or the other similar options available and making sure that the zipper was secure and then using a more water resistant protector (like the St Dormeir) on top of it to protect against the liquids that we release each night and from accidents and that can be easily removed and washed.
If you use a membrane type encasement that protects against bed bugs (and most of the encasements that are most effective against bed bugs have a semi breathable membrane) then it would be a good idea to have an additional protector on top of the mattress that could be easily removed and cleaned but it wouldn’t need to be waterproof since the encasement would already be waterproof.
The benefit of more expensive cotton encasements would include the quality and thickness of the material, whether it was organic, and how “stretchy”, thick, and “comfortable” it is and that it doesn’t affect the “feel” of the mattress although you may still need a water resistant or waterproof mattress protector or a wool puddle pad in addition to a “non membrane” encasement to provide water resistant or waterproof protection for the top surface of your mattress.
There is also more about mattress encasements and how they can be used in conjunction with a mattress protector in post #5 here and post #2 here.
Phoenix