Installing a Nail-Fin Window Over a Fluid-Applied WRB

This studio mock-up shows how to install a pellet flange window in a wall with a fluid applied membrane. The first step is going to be installing a sloped sill. The reason for installing this is so water migrates out if there is ever moisture inside the wall. There are two ways to accomplish this:

1. Use a piece of cedar bevel siding the second way
2. Cut the Jack studs at a 5 degree bevel

Start by installing the sill to flash the opening. In the video, they use a joint and seam product. You can see that it is already poking out where the manufacturer detailed the nail heads and seams in the sheathing. This product allows you to have one membrane from the inside of the jamb onto the face of the WRB.

Lay out the bead in a way that will ensure even coverage when spreading it with a tool. Adding extra bead between the sub sill and the cedar sill ensures the connection is made. Once it is laid down, spread the product from the inside of the opening out and onto the face of the WRB, covering the substrate. The joint seam product works in conjunction with the product that was already rolled onto the wall. Use a product that has more body and fiber to make things easier to work and connect any spots where it is a little thin. A good rule of thumb is you want the downturn to extend beyond the window flange.

The final step before test fitting the window is using a couple pieces of cedar shim and placing them in opposite directions of our sill. This allows you to check for level across the opening.

The contractor in the video starts by using a double stack of two by fours beneath the unit. This will protect the fin while turning it in the 90-degree orientation.

There are several possibilities when it comes to how flange windows may be shipped. Some fins will be integrated but folded down. Some will come with a 90-degree orientation, and some may come separate. If the fin comes separate, you will need to set it in with a bead of silicone and tap it in place.

You will lift the window into the unit with the carpenter on the inside to test fit it. This gives the ability to center the fin in the opening, ensuring even thermal and air coverage on the inside. Trace the nailing flange all the way around the two sides on the top. This gives a positive location for the bead of sealant when you set the window the final time. Now that there is a reference line in place, lay down a bead of the sealant, used to seal the rough opening. Put the bead on both sides and the top. The bead is not required by every manufacturer but is a belt-and-suspenders approach.

Once the window is tacked, check for level plumb and square. If you discover that anything is off, make final adjustments before fastening the window. The contractor in the video fills every fastener location of the window once he is happy with the unit’s placement. That might not be the case for your windows, check the manufacturer specs.

The final step in fastening the window will be the bottom flange. Due to the fact they used fluid applied membrane, they add a synthetic horseshoe shim. This will keep the bottom flange from bonding to the tackiness of the fluid applied membrane. They also use a modified truss head screw. The other option would be a galvanized roofing nail. Once it is fully fastened, you can finalize the flashing with our sides and our jam.

In the video, the liquid applied sealant bridges from the wall to the jamb of the window. It must go across the flange and the flange to jamb connection. Consider masking it off to protect the window. Once you do that, apply the final bead of sealant.

The product used in the video is the fast flash product. They also make a joint and seam product that is for larger gaps. Look for continuous coverage while working with the sealant.

Next, move up and treat the head. After the exterior sealant is applied, let it dry and peel the tape. If you were going to do a cap flashing, you would apply it in the same way that you treated the flange.

Now that the exterior is watertight, move inside to deal with air and thermal. Use a spray foam application around the perimeter of the window. This method will apply no matter what your WRB is on the outside. The goal is to not completely fill the void, but to 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.

It is important to use the correct foam in the assembly. The one in the video is a formulation made for windows and doors which means it is not going to cause the jams to bow or bind.

Once the foam is installed, let it dry and trim back any that needs to be cut away. After you have given the foam a few moments to skin over, apply a sealant over any shim. This will bridge from foam, across the shim, and back to the foam, making one continuous air seal.

With the last bead of sealing in place, the interior and exterior are now detailed. We are confident that this will be a leak-free installation for you.