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Window sills see a lot of water. This is largely because glass is very waterproof, so all the rain that hits a window will end up on the sill. All that water needs to be directed outwards. For that to happen, first sills need to be proper pitched, especially if windows are set deeper into walls to optimize Psi/thermal bridge values (Linear Thermal Conductivity in Btu/hr*ft*F or W/mK). Then, to assure that water is direct outwards, Pro Clima developed EXTOSEAL ENCORS, a very stretchy self-sealing sill tape with acrylic-modified butyl adhesive which works in winter down to -4°F. It can be applied to the exterior of the window frame (European window installation style) with the 3/4″ release strip - and up both sides of the opening for a continuous, one piece sill. It can also be folded down onto the face of the wall WRB, though preferably by no more than an inch so as not to block any outward drying potential.