It is the furniture tarp that is causing the mildew because the dampness is trapped allowing mold and mildew to flourish.

I don't understand why you thought an indoor sofa, regardless of the Sunbrella fabric, would survive on your screened in porch.

It is the "stuffing" of the cushions which is important. There are specific foam fillers that withstand outdoor environment.

I agree a screened in porch is still open to the elemnets so a proper outdoor sofa should have been the choice. I do also agree covering it up with a tarp traps all the moisture but really you need proper sofa for the use.All the store told you is the fabric was fine in the porch I bet no where did they say the sofa was made for outdoors