I need to get a mattress cover (the kind that zips on) to keep the allergens from my 13 yr old asthmatic daughter. They sell cheap ones that are all vinyl and non-breathable, and others that are quite expensive that have a "membrane" that breathes but prevents the dust mites from getting in or out. any opinions on what is best? I'm also thinking of getting her an air purifier for her bedroom and have to start researching that. Thanks.