We have tons of info but will try to keep this shorter to encourage replies. We purchased a 12 yr. old home last month in Tennessee. Has crawlspace, first floor living space, second floor unfinished walk-in attic, roughly 1600 sq ft footprint with brick foundation and vinyl siding. House is on a sloped hill with crawlspace 6'-10' high, great access with lighting and electric down there.
HVAC is all electric with air handler inside crawlspace, same age as house. Home smelled musty when we purchased but it was vacant and carpets were dirty so we assumed it was just from being closed up for a few months. We had carpet and pad replaced in whole house, and also installed vinyl in bathrooms which were previously carpeted. We then moved in, aired the house out with windows open and fans on for a few days, and then closed up the house and ran the AC for a week or so. The musty smell returned and I had breathing trouble when home, possibly from an allergy to mold.
Inspected the crawlspace and found that clothes dryer was vented to crawlspace and also no vapor barrier at all. All vents were open year round. Ducts were reasonably clean inside and out but return ducts are interior lined with fiberglass and appeared dirty, hard to tell without removing. Not much mold visible in crawlspace but some white powdery mold sprinkled on joists near dryer vent and surrounding area. We cleaned all debris from crawlspace, vented dryer to outside, sprayed joists with 50% bleach and water mist, then dried out with fans. Recently shut all vents and installed dehumidifier down there with moisture outlet discharged to outside. Is much dryer down there already after just a few days, but we still have the smell when we run AC unit. Had numerous HVAC contractors and mold contractors inspect and make recommendations. Solutions were many but fall into three main categories:
1 - install sealed vapor barrier to crawlspace floor and walls, clean out existing air handler with biocide
2 - all of 1 plus clean and seal supply ducts, replace the interior lined returns with exterior lined ducts to remove potential mold build up or moisture in returns, install a UV light and filter kit at the air handler to kill future mold spores
3 - all of 1 with option for 2, plus connect a new exterior air vent and filter to the return line to supply some fresh air mixed with house air, seal the leaks in the house such as plumbing, door, and register openings, and create a positive pressure in the home to prevent mold, dirt, or other items from entering the home
Total project costs range from $3,000 to $10,000 depending on the contractors and options selected. I have done a TON of research and reading plus spoken to many contractors but am still at a loss for which option is best. Am leaning toward item 1 and 3 right now, then could possibly do 2 later if problem is still not solved. Any ideas or recommendations? I can supply more detail if needed, just let me know your questions.