How to Install New Windows and Flash Them

Updated: Mar. 19, 2024

A comprehensive guide to window installation from a Marvin Windows expert

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Two men placing a window into place | Construction Pro Tips
Construction Pro Tips

Back in the day, most window installations went something like this: Set, level, square, nail, spray foam, rinse, repeat. But there are a few more steps these days, and not all of them have to do with preventing water from getting inside. In fact, many modern window installation steps are designed to allow water out, if it ever does find a way in.

To make sure we got it right, we asked our local Tyvek rep Lori Gunderson and Marvin Window installation guru Eric Klein to walk us through the process. In addition to proper procedures, Eric shared a bunch of installation pro tips, hints, and tricks.

In this story we demonstrate how to install a Marvin double hung window. Many of these steps are common among window manufacturers, but not all of them. When installing a window, always follow the specific instruction intended for the brand you are working with.

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Man cutting a hole in the Tyvek | Construction Pro Tips

Cut Out the Opening

  • Roll the house wrap, or WRB (Weather Resistant Barrier), over the entire window opening.
  • Then slice down the center of the WRB and cut it flush at the top and bottom of the opening.
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Man cutting angles in the corner of Tyvek | Construction Pro Tips

Angle-Cut the Corners

  • Cut at an angle about 6-in. up and away from all four corners.
  • Note: The bottom angle cuts are not required by all window manufacturers.
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Man rolling Tyvek up and under | Construction Pro Tips

Fold Back the Flaps

  • Fold back the side-flaps and hold them in place temporarily with seam seal or house wrap tape, not staples.
    • Pro Tip: Use a small piece of tape so it doesn’t tear the WRB when it’s removed.
  • Patch any tears that do occur with seam seal tape. If there are no fasteners in the way, fold the WRB back under itself.
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Placing cedar siding in the window sill | Construction Pro Tips

Slope the Sill With Siding

Even with a perfect install, water can still find its way into the opening. It’s a good idea to slope the sill in the rough opening to help unwanted water find its way out again. Achieve the slope by ripping down and installing a beveled piece of weather resistant siding.

  • Plan ahead and build the rough openings about 1/2-in. taller to account for the width of the siding.
  • Shim under the siding if the opening is super out of level, but avoid raising the window higher than other windows nearby. The difference in height will be noticeable and unsightly.
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Man installing window pan flashing on window sill | Construction Pro Tips

Install the Sill Pan Flashing

We’re installing Dupont FlexWrap on this window. There are other acceptable products on the market, but FlexWrap has a great track record and is recommended by the folks at Marvin.

  • Tear the 6-in. backing off the flashing tape and lay the tape down over the sill flush with the interior framing.
  • Run the tape a minimum of six inches up the sides of the opening.
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Rolling out the flashing with a rolling tool | Construction Pro Tips

Roll Out the Flashing

  • Remove the smaller strip of backing and push the tape down onto the wall. This flashing is pressure sensitive and needs to be pressed in place with a roller.
      • Pro Tip: On super cold days the stretched corners of the FlexWrap can curl back just a bit. Hold them down in place with a strip of seam seal tape.
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Man setting shims before placing the window | Construction Pro Tips

Install Shims on the Sill

    • Set the lower shims (preferably composite) in place before setting the window.
      • Pro Tip: Eric dabs a little sealant under the shims. That helps keep them in place when setting the window but keeps them loose enough, so they can be moved for minor adjustments later.
    • Space the shims near the edges, in the center, and for larger windows one no more than 14-in. apart.
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Man folding in the sides of the Tyvek | Construction Pro Tips

Fold in Sides

  • Wrap the two side WRB flaps inside, around, and on to the inside framing.
  • Staple or tape it in place.
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Sealing the sides and tops of the window with sealant | Construction Pro Tips

Seal the Sides and Top Before Setting the Window

    • Run a 3/8-in. bead of sealant about one-half inch in from the edge of the opening.
    • Leave a 3/4-in. gap on each side of all four corners. Leaving the corners free of caulk ensures the corner gaskets have a clean, smooth surface to adhere to.
    • Don’t caulk under the bottom nailing flange. Most exterior grade “Window, Door, and Siding” sealant will work.
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The Jamb and the Window Frame Line Up | Construction Pro Tips

Align the Inside of the Window

There is no reason to plumb the window because if the wall is out of plumb, so goes the window. But do the trim carpenters a favor and double check that the window jambs are as even with the framing as possible. If one side of the window protrudes in further than another, installing casing can be a difficult proposition. Only make minor adjustments. Over-twisting the window could cause the sashes to jam or stick instead of operating smoothly.

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Man leveling and fastening the bottom of a window | Construction Pro Tips

Level and Fasten the Bottom

  • Have a helper on the inside center the window in the opening.
  • Adjust the shims so that the bottom of the window is level and so that each one is in contact with the window.
  • Install two 2-in. exterior grade fasteners on each side near the bottom of the window, and then check the bottom again with a level.
    • Pro Tip: Eric prefers screws to nails because it’s easier to readjust the window if something gets out of whack. GRK cabinet screws are his favorite.
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Man squaring and fastening the window with a measuring tape | Construction Pro Tips

Square and Fasten the Top

  • Check that the window is square by measuring diagonally both ways.
  • Adjust the top of the window one way or the other until the measurements are the same.
    • Pro Tip: To ensure an accurate measurement, always hooks the tape under the vinyl drip cap. The drip cap also helps hold his tape measure in place.
  • Install two fasteners at the top near the corner, and then check for square again.
  • Don’t install any more fasteners until the window is shimmed and sealed on the inside.
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A cutout view of a window from the side | Construction Pro Tips

Seal up the Inside

This next section will walk you through the process of sealing around the window. Pictured above is what it should look like when you are all done. Note: Again, this is an approved method for installing Marvin windows. Other manufacturer’s methods will vary.

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Man installing backer rod in window cavities | Construction Pro Tips

Install Backer Rod on the Sides and Top

    • Before installing any more shims insert a backer rod on both sides and the top, but not the bottom.
    • Push the backer rod tight up against the back of the nailing flange.
      • Pro Tip: Eric uses his speed/rafter square to make several passes pushing in only a couple inches at a time.
    • The idea behind this backer rod is to eliminate thermal bridging and water infiltration. It prevents the side shims from butting up against the nailing flange, which could channel heat and water.
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Expert straightening the jambs of a window | Construction Pro Tips

Straighten the Jambs

There are three ways to check that the jambs are straight before installing the side shims:

  • Inspect the reveals (space) between the jambs and the sashes. They should be even.
  • The distance between the two side jambs should be equal at the top, center, and bottom. This can be checked simply by measuring. PRO TIP: Instead of measuring, Eric uses the top stop as a story pole and compares the gaps between the stop and the jamb at the top, bottom, and center. Create your own story pole if the window you’re installing does not have a top stop, and you have a bunch of them to install.
  • Open and close both sashes to ensure they operate smoothly.
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Man using an air shim to install a window | Construction Pro Tips

Install the Side Shims

  • Install shims on the sides of the window in the center, and about 4 inches down from the top and up from the bottom.
  • Some windows have dedicated holes to fasten the jambs to the framing (see next page). Place shims in those locations.
  • Often, the center of the side jambs will have to be pushed in a bit.
    • Pro Tip: Eric nudges the jambs over with an Air Shim bag to the exact position he wants, and then installs the shims.
  • Don’t shim the top of the window. If the building settles, the window could get compressed, which will likely cause the window to stop working properly and could even result in broken panes of glass.
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Window being held in place by a screw | Construction Pro Tips

Install the Inside Screws

In addition to the fasteners in the nailing flange, this window is held in place with screws run through a bracket called the receiver block. It’s located in the checkrail and can be accessed by removing the lower sash. The shims should be located at these locations.

    • Pro Tip: A long bid holder will make it easier to avoid scuffing the window with the drill chuck.
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Using low-expanding foam to foam up the sides of a window | Construction Pro Tips

Foam the Sides and Top

  • Run a bead of low-expanding foam in the gap along side the backer rod. Don’t over do it. Just dispense enough foam to create a 1 or 1-1/2-in. bead that bridges the entire gap between the window and the framing.
  • Run the bead out a little ways along each side of the shim. Don’t fill the whole space in because even low-expanding foam can expand enough to distort the jambs.
    • Pro Tip: Lay out a practice bead on a piece of cardboard, so you can adjust the gun, and gauge how fast to move the tip along inside the gap.
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Pushing in Backer Rod with a Square | Construction Pro Tips

Install the Bottom Backer Rod

  • Starting at one of the lower shims, push in a backer rod around the bottom and up to the other shim.
  • Don’t push it all the way in like you did with the other backer rod. Just push in almost to the point where the jamb meets the window unit.
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Create a back dam with window sealant | Construction Pro Tips

Create a Back Dam

  • Fill the gap from the lower backer rod almost all the way to the inside edge of the framing.
  • Tool in the sealant with your finger or a small chunk of backer rod to ensure that the whole gap gets filled. This back dam will prevent any water that gets past the other lines of defense from getting inside the house.
  • If the window is installed properly there should be nothing stopping the water from escaping back to the great outdoors.
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Adding insulation to the side cavity of a window | Construction Pro Tips

Add Insulation

  • Fill in the rest of the gap on the sides (above the lower shims) and the top with fiberglass or rock wool insulation.
  • Fill the whole gap, but keep it loose and fluffy. Insulation loses its effectiveness the more tightly packed it gets.
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Adding corner gaskets during a window install | Construction Pro Tips

Add Corner Gaskets

  • If the window you’re installing requires corner gaskets, now’s the time to install them.
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Man taping the flanges of a window down during install | Construction Pro Tips

Tape the Flanges

Some windows have a nailing flange that is an integral part of the window (usually vinyl windows), but this window does not. So in order to seal the area where the flange meets the window, the flashing tape needs to be run up onto the side of the window at least 1/4-in.

  • Install the tape on the sides first, about four inches past the bottom of the window and about an inch higher than the upper nailing flange.
  • Run the flashing tape at the top at least six inches past the window on each side.
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Taping the top flaps of Tyvek down after window installation | Construction Pro Tips

Tape the Top Flap

  • Fold down the top flap of WRB and cover the two angled cuts with seam seal tape.
  • Don’t tape the entire bottom of the flap. Instead, just use a few small strips of tape to hold it in place. The gaps between the tape are there to create an escape route for any water that gets behind the WRB above the window opening.
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That’s all there is too it! Okay, we admit that window installation is not as simple and easy as used to be…but neither are water damage and callbacks. We want to thank our two experts for stopping by and walking us through the entire process:

Meet the experts

Lori Gunderson

Lori has been a Tyvek Specialist for over 12 years. She works closely with Architects, General Contractors, Builders, and installers to help build a quality, energy efficient building. Before becoming a Tyvek Specialist, she worked for a national builder for over 10 years where she managed single family and multifamily projects.

Eric Klein, Senior Installation and Service Instructor

Eric has been employed by Marvin Windows and Doors since March of 1984 and has trained thousands of installers in the facilities at Warroad, MN and on his travels that have taken him across the globe. He is an Accredited Instructor and certifies installers in the Residential, Light Commercial, and Commercial window and door installation. Eric also works with several associates and vendors on how to better serve the installers, contractors, and service techs with better tips and tricks, and installation knowhow.

More resources: