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rigid foam insulation

How To Install Nail Fin Windows Over Rigid Foam Insulation

Jake Burton teaches you how to install a nail-fin window over rigid foam insulation through a series of videos.


Construct The Opening

In this studio mock-up you will learn how to install a flange window in a wall with exterior insulation. The foam shown in the video will not actually be in place on your window. It shows two inches of foam and a buck so that you can see where you will end.

To start, waterproof from the substrate behind the foam, around the buck, and to the inside. This will create a weather tight window installation. Once the foam is moved away, you will utilize the zip system sheathing as your WRB in the assembly. That means that you need to connect the WRP to the inside of the opening. There are a couple different ways that you can do that.

  1. Use their stretch tape
  2. Use their straight tape with a bow tie in the corners

The contractor in the video finds it easier to use the liquid flash to make the transitions, turns, and connections in one piece.

Liquid flash application starts with a few layout lines around the buck. This will ensure solid coverage all the way around the window. Then, make three passes on the outside of each unit, spreading it to make sure that you have even coverage. Try to push your spreader into the opening to ensure you are sharing it with the corner. Share it with the joint, covering both the WRB and the framing. Be careful not to jam it into the square corners. There should be enough room around the insulation that you can make the transition from the framing buck to the WRV without it having to be struck hard in the corner. The goal is to make one connection from the WRB of the zip sheathing down, into the opening. Make sure to take special care when turning corners and moving between different types of material/different sections of the material.

Don’t worry about getting it in one pass, the cure time for liquid flash is between 20 and 40 minutes depending on temperature. You have plenty of time to work the product into place.

The liquid flash is a moisture cure product so temperature and humidity will affect it. If you are working in low temperatures with low humidity, it is going to take longer to set up. If you are working on a hot day with high humidity it is going to set up quicker.

The liquid flash should bridge across the horizontal tape seam. Be careful when using the tape because it is shipped without a backing. This means that the tape has a release agent on the side to keep the outer run of tape and the roll from sticking to it. Take acetone and wipe down the face of the tape to get adhesion from our liquid flash to our tape.

When applying the liquid flash, start at the head then work one side. Move to the other side next and finish on the bottom. The reason to apply it in this order is because the product will skin over. So, if you started on the bottom and worked your way around, you would end at the place that you started. It is more likely to have skin on it or starting to cure at that point than it would be if you started at the top, worked down, then do the bottom.

Once the sides and head are flashed, add a piece of cedar bubble siding to the sill. This will give you a positive slope to the outside if you ever have moisture. When you are working on the bottom sill, start at the bottom and work your way around. That way, even if you are dragging horizontally, you will need to turn the corner. You can be pulling the material uphill rather than trying to drag downhill and cause drips.

Now you should be detailed to the backside of the cedar bevel siding. You do not have to go beyond because you will not have water getting that far back and the cedar extends beyond where the inside of the window will be. Now, you are ready to let the liquid flash skin over, then install the window.

Inserting The Window

Now that the jam and buck are both waterproof from the liquid flash, you are able to add one shim on each side of the window. These are just. Shims are a piece of cedar bubble siding that you were using before. You will put the shims in the reverse orientation of the piece below them, creating a level plane for our jams to sit on. Stand the last few of the nailing fins up.

There are a few possibilities as to how you will receive a flange window.

  1. They may be integrated, but folded over for shipping purposes
  2. They might already be in a 90-degree orientation
  3. They might come separate, leaving you to be responsible for beating it in silicone and then tapping it in place

Once all the fins are up, you will be ready to test fit the window. It will help to have someone on the inside of the building to help center the window in the opening and maintain the insulation line all around the window.

Take a permanent marker and draw a line around the flange. This will be a reference point for where you want to run the bead of sealant. Now, lay out a bead of the liquid flash behind the flange. It's not required by all manufacturers but will give you a belt-and-suspenders approach because you'll seal over top of that flange after the window is installed.

After placing your window in position using the reference marks, tack it in and check if it is level plumb and square. Once you know the unit is level, fasten the fin/nailing flange. The only thing that's different on this assembly is you're using tape and since you're using this liquid flash that has a slight tack to it, you will put horseshoe shims underneath the bottom flange to keep it from sticking. The shims give a clear path for any water that might find its way in to migrate out.

The contractor in the video likes horseshoe shims because they hang on the fastener when putting it through the supplied opening. Synthetics shims make it so you do not have to worry about swelling due to water like you would with a cedar shim.

Finally, you’ll fasten every bassinger hole. Check with your window manufacturer on the recommended method for your window.

Applying Liquid Flash to the Window

Before installing the last layer of liquid flash, take the time to mask off the side of the window. Liquid flash is going to roll past where the fin connects and onto the main substrate of the window. We want to protect that from being covered in black liquid flash at final installation, so do the sealing process in two beads. Use more of a squiggly shaped pattern on the fin than on the edge of the window because you will need a bit more material for that bridge. You are connecting the side of the window frame to the original framing buck. Take special care to ensure you cover everything. It is okay to wipe a little bit more out of the corner because it is more about the connection between the silver flange and the color of the window. This completes your exterior water management.

Now all that is left in the process is removing the tape. Wait until the liquid flash is dry, so it does not get on the glass or window. Take the seam tape and detail out the seams. Leave the top open in case any water ever migrates in above our window. This will create a place for the water to get out. You will do the same at the bottom. Once the masking tape is removed, add a head flashing.

Sealing and Insulating

Now that the exterior is watertight, it is time to deal with air and thermal. You will be 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 will not need to completely fill the void, only the first inch from your side towards the outside. It leaves an open area on the outside of the window that if you ever have any moisture in that assembly it can migrate out.

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

Once the foam is installed, let it dry. Trim back any that needs to be cut away, leaving one more ceiling detail. After giving the foam a few moments to skin over, apply a sealant over the shims that will bridge from foam, across the shim, and back to the foam making one continuous air seal.

Put the last beads of sealing in place, leaving the interior and exterior detailed. We are confident that this is going to be a leak-free installation.

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