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used this to seal around the vents. Put a 4x8x16 solid block in where the vents were after closing them then pushed the backer rod into the gaps about 3/4 inch to seal off air flow then used the cement caulk. Didnt order enough so used a water noodle of the kids for the last 4 vents. You can improvise but backer rod is inexpensive and works great.