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Sloped aluminum sill over exterior mineral wool

user-4405197 | Posted in Green Building Techniques on

I just wanted to see if anyone had a good suggestion for an exterior sloped sill for our windows. I recognize we probably made a major mistake in installing these, as we should have either left a larger space below the windows or set them an inch or more out from the plane of the sheathing. As it stands, they are flush with the sheathing. We are placing 2 layers of 1.5 inch mineral wool plus vertical wooden furring strips for a rain screen. My contractors plan was to use white aluminum to make a sloped sill to divert water from going behind the mineral wool. We thought by leaving a slight depression in the tape we’d have enough room to tuck the aluminum. But it doesn’t look like there will be.

I suppose we could use some of the Prosoco joint and seam filler or other caulk to seal the seam. I also have left over rubber gasket material from conservation technology that we could possibly use. I also thought of using some plumbers putty but worried it may become dry and brittle. Cutting the bottom piece of tape to give room to tuck the aluminum under it was also thought about. I hesitate to do this because I don’t want want to mess with the air and water seal the tape provided.

I thought that in the worst case scenario if water did get behind the mineral wool, the zip system sheathing would offer protection, and it could eventually dry out though the mineral wool. But obviously we want to avoid that in the first place .

The Windows are flangless from Klearwall and were installed on setting blocks. Hanno Band expanding foam strips were placed along the outside edges of the window and rough opening and then we used Siga Fentrim to tape the outside edges. The weep holes didn’t give us much room for tape. The plan on the interior is to use backer rod and Prosoco air dam.

The first picture is my window. The second is what we were trying to do. It’s a pic from one of hammer and hand’s projects.

Thanks for any suggestions.

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  1. GBA Editor
    Martin Holladay | | #1

    I don't have any suggestions, other than "next time, plan ahead" and "you probably don't have to worry."

    You wrote, "We probably made a major mistake in installing these, as we should have ... left a larger space below the windows." You certainly made a mistake in failing to plan for a sill. But let's hope it's not a major mistake.

    You wrote, "I thought that in the worst-case scenario, if water did get behind the mineral wool, the Zip System sheathing would offer protection, and it could eventually dry out though the mineral wool." That's all very true, so I don't think you have to worry very much.

    You'll just have to butt the sill extension (presumably, metal flashing -- although GBA readers should know there are many options, including peel-and-stick flashing covered by Azek trim) as close to the window as you can manage, and install a thin bead of high-quality caulk at the seam between the window and the sill extension.

  2. Toyoland66 | | #2

    From looking at your picture I would think you could install a sill exteinsion with a wall attachment flange that bends down and screws to the wall from the underside. You would want to drill holes in this sill extension where your window weep holes are, this will mean that the weeps will drain behind the mineral wool, but at least your sill extension will prevent bulk water from entering the mineral wool. The joint between the extension and the window could then be caulked.

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