Mold spores, (seeds) are everywhere. A dust particle lands on a liquid or damp surface surface that has organic nutrients. A mold spore lands on the dust particles. The near wet condition continues for + 24 hours, the mold spore sprouts and grows. After a couple days the growth is large enough to be visible with the naked eye. It will happen in any green grass climate during much of the year.
It is all about humidity control at surfaces contain organic material, which you are aware of.
<55%RH is about as good as you are going to do during the grass growing seasons. Your a/c setup ideally should maintain <50%RH during significant sensible cooling loads. But during evenings and rainy days, the a/c does not run enough to remove much moisture.
Cool surfaces like concrete slabs under carpeting and wood floors very sensitive to this problem. Parts of you a/c exposed to the humidity in home also have problem. The source of the moisture is from the occupants and the much needed fresh outside air needed to purge indoor pollutants and renew oxygen.
The easy way out is not be concerned until mold spots appear or the musty odor is detected. Next clean the mold spots with soap and water. Insulate the surfaces prone to mold to warm them above the dew point of the air.
Homes are usually not comfortable when the indoor %RH rises to the point of condensation on surfaces.
The ultimate fix is to add a small whole house dehumidifier to the a/c to remove 2-6 lbs. of moisture per hour to maintain <50%RH during when the outdoor dew points are +55^F and there is low/no sensible cooling loads.
New homes are usually air tight enough during calm winds, that I include fresh, filtered fresh air option provide fresh air ventilation when the home is occupied.
That will get you indoor air quality and comfort that your home capable of providing.
Keep us posted about your opinion of solution. Many of the knowledgeable home builders are including option as part of the systems.
Ultra-Aire has been providing these systems to homes since 1995. The early users where occupants that were sensitive to allergens from dust mites and mold. This last year there were about 100,000 systems installed in the U.S. from various manufactures.
Regards Teddy Bear.
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
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