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While renovating a bathroom in their 22-year-old home, the owners were shocked to find wet insulation, severely decayed framing and sheathing, and live carpenter ants behind the tiled shower stall (Figure 1). Tucked into an outside corner, two sides of the stall were against exterior walls. The walls consisted of floor-to-ceiling ceramic tiles set on mortar and metal lath, standard gypsum wallboard, unfaced fiberglass batt insulation between 2x4 studs, plywood sheathing, and wood shingles. The bathroom had a window, but no exhaust fan. The shower pan and drain tested leakfree, and the owners recalled that the tiles and grout had only a few cracks. This told me that the cause of the problem was not liquid water leaking into