Mold spores are damn near everywhere and they are nigh impossible to get rid of. Also, mold needs two things to grow: moisture and food. Mold feeds on damn near anything, so the best way to prevent mold is to prevent moisture.
But where is the moisture in a bed coming from? At first I thought it may have been residual 'chemical' moisture from when it was new and we may not have allowed it to 'dry' enough before covering it in all our sheets. On further thought, it seems much more likely that the moisture is coming from our own bodies via transpiration and/or perspiration (water vapor and sweat from the skin).
Now, why is the mold on the bottom and not on the top? My guess is that this is similar to condensation on window-sills during the winter. When air hits a cold surface such as the window glass, it may reach the dewpoint allowing the water vapor to condense into liquid water. A similar phenomenon may be happening with a bed because the bed absorbs water vapor moisture from your body at night and it encounters the cooler surface, maybe reaching the dewpoint, at the bottom of the bed.
Another point to note is that a waterproof mattress covering will not prevent the bed from absorbing water vapor from your body. Water vapor freely passes through breathable waterproof materials.
Alright, so what can you do to prevent this from happening?
- Cover the top or the entirety of your mattress with a gas-impermeable mattress protector. This would prevent water vapor from absorbing into the mattress. Here is a good example that even advertises that it prevents mold and mildew.
- If you don't have a gas-impermeable mattress protector, then don't sleep in a cold environment. Optimal sleep temps are 60-68 deg fahrenheit. A colder sleep environment may promote more moisture condensation on the underside of your bed.
- If you don't have a gas-impermeable mattress protector, then you need to keep your bed well ventilated. For instance, keep it on box-springs and a metal frame without any bedskirt or anything underneath the bed to inhibit airflow. Sleep with your window cracked open or your bedroom door open to allow airflow. Uncover your bed during the day to allow it to breathe (there's some evidence to suggest this may prevent dust mites as well).
- I called Sealy and asked them how to prevent mold growth on memory foam and they could not give me any meaningful advice.
- House plants are great mold breeders. Consider getting rid of them, especially if you have a problem with mold gnats.
- If you have a bathroom that connects close to or directly to your bedroom, make sure it is well ventilated with a roof fan. Imagine the amount of moisture that could come from a poorly ventilated bathroom if you have just two people showering every day.
- Basements, even daylight basements, are notorious for mold because moisture is drawn in from the ground through cracks, joints, and porous materials. If you have a memory foam bed in a basement, then definitely get a dehumidifier!
- Check your home for other sources of mold and water-leaks/moisture. Check under carpets, behind dressers, inside dressers, all surfaces inside cupboards, etc. If you want to take it a step further and spend some money, you can test your air for mold to see how bad the problem is and what types of mold are present in your home.