Hi, we have an extension to our house built on a slab, and the ground floor room has developed a strong mildew smell for the first time since we moved here in 2006. It started last weekend, after a few days of heavy rain, and by Monday was overwhelming. There are no water pipes under or in the extension part of the house. The gutter is above the 2nd floor room (above the mildew smell room) and there is no smell or moisture in that room. My first guess was that groundwater was not draining under the slab room and coming up into the foundation, since I have occasionally seen dark areas of the stone-tiled floor (which my wife recalls being listed as granite), and the smell seems the strongest at the front of the room where those tiles are. Also the drainage on our block is terrible, the water stays curb high for hours after a heavy rain. It would have to be a lot of water to reach the tiles though, as the flooring is raised up above the foundation. I ran a dehumidifier for the day but it didn't collect much water or reduce the smell. Tuesday the weather was cooler and drier so I opened all the windows; the odor dissipated, but didn't disappear completely. Yesterday (Saturday) the weather was back to hot and humid, the odor was strong again, and there were dark areas in the floor tiles (pictured), even though it hadn't rained in days. The outside foundation is visible from 2 sides but doesn't look wet. I removed a board from the deck right outside the front of the room, I did smell the mildew but the ground was dry. Since hot temps and humidity seem to aggravate the problem, is it possible that room air moisture is being absorbed by the tiles and activating mildew (or mold) that has grown under them? OR if it is groundwater, could it take days for it to flow from surrounding areas to under my slab room?