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installing a nail fin window over self-adhered membrane

Installing a Nail-Fin Window Over Peel-and-Stick Housewrap

In this studio mock-up, you will learn how to install a Pella flanged window in a wall with a fully adhered membrane.


Constructing The Opening

In this studio mock-up, you will learn how to install a Pella flanged window in a wall with a fully adhered membrane.

The first step in the assembly is to cut the sill flush and the head flush on the sides. Make an estimation for what that wall thickness is and cut vertically. Do the same on the other side. When you are done cutting, you should be able to turn the flap inward and press it to the jamb.

The next step is waterproofing. Create a positive slope sill in the unit. The contractor in the video uses cedar bevel siding. You could also do this by rough framing the sill with a 5-degree pitch. In this case you are just going to be attaching one piece of siding. This will give you positive drainage to the outside. Continue with the tapes.

Each manufacturer’s spec might be different. The manufacturer from the video recommends turning the tape six inches up the jamb, going all the way across, then six inches up the other jam. The tape they use in the video is only about three and a half inches wide, so they do it in two runs.

Pre-cut the tape to length and start from the outside. The tape used in the video has a pressure-sensitive adhesive, so the contractor sets the top edge before making any cuts or turns. Cut vertically at the corner, then turn the tape down towards the floor. Trim back any extra. Cut the same thing on the other side. Go vertically from the corner and turn the tape down.

After gently pressing the tape in place by hand, use a roller to fully adhere the product. The tape has a pressure-sensitive adhesive so it will not stay long-term unless you apply pressure with the roller. After rolling the entire unit, add a bowtie to cover the pinholes on both sides.

To cut a bowtie, fold your piece of tape in half. Make two cuts to create the bow tie shape. Once created, press the bowtie into the opening and turn the corner over the edge. That will give the window the ability to shed water. Roll the bowtie in place and repeat these steps for the other side. Do one more run of tape further into the opening to push past where the window will be.

Adding the second piece of tape allows water to shingle over and wear to the outside. The manufacturer in the video recommends a bead of sealant along the top edge to seal it off from any water that might find its way down the jamb. Do this step with a sausage gun.

A quick tip with the sausage tube: It is unnecessary to cut all the way around the tube with your knife. If you poke the tube in two or three places, it will be enough to get it to start coming out.

Finally, take a piece of that cedar bevel siding that you cut as a shim and place it in the reverse orientation. That will give you a level point for the jamb to sit on both ends of the window.

Putting In The Window

Use a double stack of two by fours beneath the window so that it has somewhere to rest without making the flange fold back down.

There are three common ways that a flange window will ship:

  1. Sometimes it will be integrated, but need to be folded
  2. Sometimes it will be in the 90-degree orientation
  3. Sometimes it will be completely separate and you will have to install it yourself. If this is the case, lay out a bead of sealant, then tap it in place

Next, dry fit the window. Dry fitting the window gives an interior carpenter the opportunity to help fit it for center so that it will maintain thermal and air break at final install. Set it in and trace the flange once you are happy with its location. Once you have that reference line, you know where to lay out the bead of sealant.

The contractors in the video are using a Henry sealant. Check with each manufacturer and make sure that whatever you are using to bed the window in is something that works with their product. The other good aspect of using the manufacturer’s sealant is you have one manufacturer carrying one warranty, so if you ever have an issue or a question, you only have one rep that you need to call.

Now, bring your window back in for final install. Once the window is tacked in place, check for level plumb and square. If you discover anything is off at that time, make any final adjustments before fastening the window. Once your unit appears to be in the correct position, fill every pre-punched hole from the manufacturer. Be sure to check the specs from each manufacturer for each window because this may not always be the case.

The contractors in the video use a modified truss screw which is an approved fastener from the manufacturer. They could also use a galvanized roofing nail. While fastening the entire sill, do not flash the sill any further than what it already was because you will want to leave it open for drainage. Once the sill is fastened, you can move on to side flashing.

Applying Flash to the Window

In this application you will use a piece of the actual blue skin that you just cut to size for window tape. This ensures you are using the same manufacturer and that it is bridging from the sheathing on the wall, all the way to the edge of the frame, and across the flange connection. Performing this step will make sure the flange is waterproof.

Make a cut at the bottom so the flange will lay flat. The head flashing detail with the fully applied membrane will be two steps.

  1. Have a piece just like the jams on the sides
  2. Detail the top of the piece with a little bit of sealant

You should be trying to bridge from that frame all the way to the WRB again, while making sure that the head jamb extends beyond both vertical legs. The final step is adding a little bit of the sealant across the entire head. Tool it in place.

At this point the mock-ups complete on the outside. If you wanted to add a cap flashing, you could use the same method.

Air and Thermal

Now that the exterior is watertight you will address air and thermal.

Use a spray foam application around the perimeter of the window. This method will actually apply no matter what your WRB is on the outside. The goal  is not to completely fill the void; just try to fill the first inch or so from the inside towards the outside.  It will leave an open area on the outside of the window so that if we ever have any moisture in that assembly it's able to migrate out.

Take special care to use the correct foam in this assembly. This is a formulation made for windows and doors which means it's not going to cause jams to bow or bind in any way.

Now that the foam is installed let it dry. Trim back what needs to be cut away, then there is one more air sealing detail. Once the foam has had a few moments to skin over apply a sealant over any shim.  That will bridge from foam across the shim and back to the foam, making one continuous air seal.  With those last beads of sealant in place the interior is detailed, the exterior is detailed and this is going to be a leak-free installation.

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