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How To Install A Vinyl Window

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In this article we will be discussing the installation of a vinyl window with nailing flanges. There are some different methods for installing windows and we will explain how we normally do it. Always read the instructions that come with the window. If the window is not installed according to the instructions provided by the manufacturer, they may not honor their warranty. The method we use normally fits within the manufacturer’s instructions.

Things you will need:

  • 2” or longer screws or other approved fasteners
  • caulk and caulking gun
  • 3/16” cedar lath
  • hammer tacker stapler with 1/2” staples
  • 6” Moistop window flashing


The following instructions assume that the window framing is already completed and the framed opening is true, level, and square. The framed opening should be about 1/2” larger than the window frame. So, for example, if the framed opening is 36” wide, then the window should be 35 1/2” wide. When you put the window in the hole it will have a 1/4” gap between the window and the framed opening on both sides of the window. The same is true for the measurements going up and down.

Cut the lath to the length of the bottom of the window frame.  Place the lath on the window sill so that the window will sit directly on it.  Staple the lath down to the framing with the hammer tacker.  The purpose of the lath is to raise the window up while keeping continuous bearing underneath it.  You can also raise the framing below the window 1/4" making the framed opening 1/4" smaller.  Then you install the window sitting directly on the sill framing and omit the lath. 

Windows used to be installed with an open gap across the bottom to match the sides and top.  Many manufacturers have decided that the windows need to have continuous support under the bottom of the window frame.  We use the lath to provide continuous support while raising the window.  We want to raise the window so that the amount of window frame showing after sheetrock and trim is about the same on the bottom as on the sides and top.  We try to use 3/16” lath because the 1/4" lath is often a little bit too thick.

Cut the Moistop window flashing the length of the bottom of the window plus 18”.  The Moistop is like a giant roll of tape but only half of the back is sticky.  You will stick the Moistop on the outside of the wall centered across the bottom of the framed window opening so the ends extend 9” past the edge of the opening on both sides.  The sticky half needs to be the top so that the building paper that goes under the siding can be tucked under the bottom half of the Moistop.  Some people put the Moistop on the sides of the framed opening also but we prefer to do the sides after the window is installed.  Both methods are considered acceptable.

The window may have blocks across the bottom or foam pads to protect the windows during shipping.  Remove them now and then lean the window against a wall with the inside of the window facing you.  If the window is very big you may want to remove the sash so the window is lighter when you are trying to lift it into the framed opening.  The sash is the part of the window that slides or moves.

Run a heavy bead of caulking across the nailing holes on the nailing flange at the sides and top of the window.  You can also turn the window on its side and caulk across the bottom, but we normally caulk the outside of the framing where the bottom nailing flange will go.  It takes less effort this way, and there is a risk of breaking the corners of the nailing flanges when you tip the window on its side.  If you do break the nailing flange, you can still use the window, but you will need to be more careful sealing the window.  The caulking that we prefer for window installation is called Quad and is made by OSI.  It has excellent adhesion, stays pliable, and is very durable.  Quad is very difficult to clean up so be careful with it.

Lift the window and carefully set the bottom of the window on the lath that is on the sill of the framed opening. Set the bottom of the window in the hole first and then tip the top into place.  If you are installing the window by yourself, you can drive a nail above the window and bend it over the nailing flange to hold the window in place while you make sure the window is centered in the hole.  Make sure the window is not going to fall before you let go of it.

If the window is a fixed glass window then all you need to do is center the window in the framed opening so the space between the window frame and the framed opening is the same on both sides.  The space should be 1/4".  A carpenter’s pencil is about 1/4" so you can use one to judge the space on both sides to make sure they are the same.  The top and bottom should be right. You are ready to fasten the window to the framing according to the manufacturer’s instructions.

If the window slides up and down then you need to center the bottom, middle, and top separately.  As long as the framing is straight, this is all the adjusting you need to do. If you are not sure about the framing then put a level or straight edge on one side of the window to make sure it is not bowed.   You are ready to fasten the window to the framing.

If the window is a slider you will need to put the sash back in the window frame before continuing.  Slide the sash almost closed, so there is about an 1/8” gap between the sash and the window frame.  This will show you if the window is adjusted correctly.  If the gap is within 1/8” different at the top and the bottom, then you can fasten the fixed side and the bottom of the window.  The frame on the sash side moves easily.  Push the middle in or out to make the gap 1/8” from the top to the bottom and fasten the middle.  Finish fastening the rest of the window.

If the manufacturer’s instructions do not specify what fasteners to use, or how often to place them, use at least a 2” fastener and place them about every 12” apart.  The corner of the window is the most likely place for the nailing flange to break so don’t put a fastener closer than about 4” from the corner of the window frame.

If the manufacturer's instructions don't prevent it, screws are the best fastener to us to install windows because they can be removed without damaging the window if you ever need to pull the window back out.  Some people like to use nail guns to install windows because it is a much faster method.  Be very careful if you choose this method.  If not done properly you can break the nailing flange or damage other parts of the window.  If you want to use a nail gun I recommend using a Paslode cordless nail gun with the smallest nail accepted by the window manufacturer’s installation instructions.  Some people prefer to use roofing nails because the nail head is larger.  The finished product with roofing nails is excellent but they are harder to install and much harder to pull out if you ever need to remove the window.

That is how we install windows at  Now you need to decide if you have the skills, talents, and physical abilities to do it yourself.  The most difficult part is lifting the window.  If it is a small window, or you have help lifting a larger window, the rest is just a matter of following these steps.  If you have any questions about these instructions please Ask Eli.  Good luck with your project.

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