Flanged Window Installation Tips | Step by Step Instructions

flanged window installation

Follow these flanged window installation tips for a leak-free result. These easy-to-follow, step-by-step instructions outline some of the things to consider with your flanged window installation project. While the process is pretty simple, there are a few things to keep in mind as a do-it-yourselfer that may save you time, money, and ensure success.

Measure Twice Cut Once

Where have you heard me say this before? Probably on just about every project we talk about! Carefully measure the width and height of the main body of the window. Meaning, “DO NOT” include the flange itself in your measurement. Then add about ½” to each dimension to determine the size of your framed opening. Not sure how to properly frame a window opening? Read our article on “constructing a window frame opening” detailing the proper steps to take in your design, so that you meet standard building codes. This is probably a good time to dry-fit your window and make sure it fits properly before you move on to the next steps.

Seal the Lower Corners

The area that is most susceptible to water intrusion is the bottom corners of the window opening. The best solution is to buy plastic corner seal brackets. Cut the bracket length to the overall thickness of the wall minus 1/8”. Apply a small bead of caulking to the lower corner of the window opening to not only seal the bracket, but to help hold it in place during the next few steps of the flanged window installation process.

Install the Lower Membrane Seal

Installing a membrane seal will help to protect wood if you happen to have a small leak. Precut two strips of the membrane approximately 6 inches longer than the window opening. Before you affix the membrane you should spray the area with an adhesive primer for a better bond. Install the first section of the membrane along the lower edge of the window opening. Lay the 2nd piece of the membrane over the first, overlapping it by about 1-1/2”. Cut a slit at each end of the second membrane adjacent to the edge of the window opening and then fold it back onto the topside of the window framing.

Caulk the Perimeter

Before you place the window in the opening, fill any gaps in the wood with caulking to prevent water from migrating through the cracks. Run a full bead of caulking across the top of the window opening. These 3 beads of caulk should be on the face of the opening approximately ¾” beyond the opening itself. Run a 4th bead along the bottom face of the opening, however you should leave 2 or 3 gaps that are 1” wide. The gaps would allow trapped moisture to escape.

Install the Window

Place a small block in the 2 lower corners of the opening. The space in between will be filled with insulation. Assuming that the lower frame is perfectly level the 2 blocks will be the same thickness. Otherwise, you can use this opportunity to level the window by using blocks of varying thicknesses. Apply a dab of silicone to the bottom of the block to hold it in place during the installation of the window. Set the window on the lower blocks, center it left to right in the opening, then gently press it back against the silicone.

Check the window for level then nail in place with flat head shingle nails. Make sure that the drain holes are located on the bottom edge. Otherwise your window is upside down and would need to be rotated 180 degrees. Double check for level after you install the first nail, then install the remaining nails.

Membrane Seal

Install the remaining membrane seal. Apply the vertical membrane strips by affixing them to the face of nailing flange. They should be long enough to overlap the lower membrane by 2 inches and extend above the top of the window by 4 inches. The top membrane should be placed over the vertical side membranes and on top of the upper window flange.

Flanged Window Installation Video

For a better understanding of the steps above, watch the video which will help to clarify the details. Make some popcorn and watch the flanged window installation video now:



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Have more questions about flanged window installation and what to look for? Contact One Source Renovation, LLC at WindowRepairGuy.com or give us a call at (815)-634-8922.