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installing nail fin over housewrap

Installing a Nail-Fin Window Over Housewrap

In this studio mock-up you will install a Pella flange window over drainable house wrap.


Create a Rough Opening

In this studio mock-up you will install a Pella flange window over drainable house wrap. Start by cutting the window opening. This will be an eye cut, so cut completely across horizontally at the head and at the sill. Split the unit up the middle, then make more diagonal cuts for the head from there. Once it is all cut open, you can trim it back and fold it in. The next assembly step is to pull the tabs to the inside and fasten them. Then trim them to a specific point so they do not protrude beyond the interior side of the window.

You want to connect the window to the actual rough framing and not to the house wrap, that way you have one solid connection of air and thermal barrier. The manufacturer of the house wrap used in the video allows for staples, cap nails, or just roofing nails. For that particular house wrap, it is recommended to use cap nails if you're over 40 feet in the air or on a commercial project.

When cutting at the corners, the contractor in the video makes a diagonal cut. This gives the ability to fold the house wrap out of the way, waterproof the window to the wall, then fold it back down at final installation. This will create a shingle effect where everything above lays over what is below. For now, use a piece of tape to hold it out of the way during installation. The next step is a sloped sill.

Create a sloped sill by using a piece of cedar bevel siding or cut your framing at a 5-degree bevel. This creates a positive slope that water can drain from if it finds its way in. After our sill is sloped, go straight on to Hydra flash tape.

The first thing you are going to do for the sill is create a bow tie to bridge the corner. This will help you transition from the face to the inside and the corner. Cut a 6-inch piece of their six-inch tape, leaving roughly an inch in the middle of the material. Cut a V inward. Seat the first piece in the corner inside the opening, then turn the corner and pull it out. That will protect the pinhole that will be created when you have two straight pieces of tape, bridging that turn. Create another bow tie for the other side. Our next step is to apply the last piece of tape to the sill.

Your last piece of tape will go from the backside of the cedar out and down over the face of the drainable house wrap. The nice thing about the tape is it has a split back so you can peel one half at a time and work just part of the tape. Go about six inches up the wall, peel and work your way across the last little bit. Pull the tape off all the way and make sure to get it tucked into the corner and then turn up the edge. Now it is as simple as peel the second layer off and start to turn the corner. Since you added a bowtie beforehand, you can cut the tape straight out and fold it around the corner, not having to worry about creating a pinhole. The contractor in the video uses the Hydra gap drainable house wrap and their tape. This gives them one manufacturer to call if there are ever issues. Now that the sill tape is installed, the next step is to add cedar shims.

The shims are cut from the same sloped cedar bevel siding that you installed earlier. Put them in the opposite direction, creating a level platform for the jams to rest on. Doing this also allows you to shim the opening if there are any errors. It will also help you maintain an even gap all the way around the window so when it comes time to air seal and insulate on the inside, you will be equally insulated all the way around. Now it is time to prep the window for test fit.

Putting the Window In Place

Fold up your fourth and final flange. Some windows will come with the flange lying flat, which is mainly for shipping purposes. Some may also come with the flange already at the ninety-degree point or uninstalled and you will have to install it yourself. You will be laying out a bead of sealant and then snapping the flange in place. Once you have all the flanges folded out, test fit the window.

Center the window in the opening with the help of an interior carpenter, then trace the flange. Tracing the flange will give you a good line to apply the bead of silicone all the way around the two sides and the top of the opening. You will not trace the bottom because it will be left open for draining. The contractor in the video uses 100% silicone in his application because he knows that it works with his window manufacturer and drainable house wrap. Once your silicone is installed, place the window. Once the window is tacked in place, check for level plumb and square and make any needed adjustments. If you find that your window does not need any adjustments, file each one of the pre-punched holes from the manufacturer to fasten the window. Always understand the specs for your windows because this procedure will not always be the case. Now that your window is anchored, it is time to flash the sides.

Apply Flashing Tape to the Window

Side flashing tape bridges the gap between the frame of the window out and on to the WRB. Make sure that you get past the joint where the flange is connected to the frame and all the way onto the frame. Then, make a simple cut where the tape protrudes beyond the bottom of the window. This cut will allow the bottom to lay flat. Once you are done flashing the sides, do the same thing to the head.

After the head flashing is in place, this would be the time to install a drip cap if you wanted to. The drip cap would get flashed underneath as well. They do not install a drip cap in the mock-up, so they move on to folding the flange down and detail the end. This piece of tape simply ensures that the flap is going to adhere down, be out of the way, and allow everything to shingle drain over top of the window. Now that your exterior water manage is done, move inside for air and thermal management.

Sealing and Insulating the Window

Now that the exterior is watertight, you are inside and going to deal with air and thermal. Accomplish this by using a spray foam application around the perimeter of the window. This method will apply no matter what your WRB is on the outside. You should not be completely filling the void. Try to only fill the first inch or so from your side towards the outside. This leaves an open area on the outside of the window that allows moisture to migrate out of the assembly. Take special care to be using the correct foam in your assembly. The formulation in the video is made for windows and doors. This means it will not cause jams to bow or bind in any way. Once your foam is installed, let it dry and trim back any that needs to be cut away. There is only one more ceiling detail to complete.

Once you have given the foam a few moments to skin over, apply a sealant over any shim that will bridge from the foam, across the shim, and back to the foam making one continuous air seal.

After your last beads of sealing are in place, the interior and exterior will officially be detailed and you can be confident that it will be a leak-free installation.

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