MDF won't hold up well in wet areas, or steam/humidity areas.

Yeah...it's Humid too.

No matter what you are doing with this house all it takes is the right temperature and humidity if you did not kill everything.

Though San Diego has mild weather, it still gets into the high 70's and low 80's sometimes, and the closeness to the ocean causes some humidity, which without HVAC it is uncomfortable, same with heating, it's a necessity.

Are you in a climate that allows one to open a window and cool the interior or get fresh air, or would opening the window allow too much wind or humidity?

When people start moving here in droves it feels like a flood and has affected the housing stock accordingly with our inventory down to 1 month.I suspect many will also come from other parts of the country especially parts of the country were they dont think twice of heat humidity thunderstorms tornados etc.. all things the SF bay area has none of  LOL. 

The technical reason you should leave the heat on low is that a lot of the materials used to build home don't do well at low temperatures and/or low humidity.

First week I made $25, carrying a 25 pound case door to door in 95 degrees and 80% humidity in Louisiana. 

I moved in last summer and it keeps cool even on the hottest/humid of days. 

Your home is in an area with excess humidity and dust/salt can provide the food source.