How to Install Replacement Windows

Plastic Shims Work Great to Hang Replacement Windows

Installing replacement windows can be a great DIY way to improve the insulation and value of your home. These instructions are for situations where the window frames are in good enough condition that you don’t need to strip the window down to the studs. If the frames are in good condition, you can simply install a slightly smaller window into the existing frame, making this an easier DIY than a full frame installation. This article assumes you’ve already removed the old window and prepped the opening. Here’s how to install replacement windows:

Order Your New Windows

You’ll need to measure the opening of your existing frame to determine what size window you should order. Open the window and measure from the lowest point to the top of the frame. Because things move over time, measure it on either side and the middle, measure the width too, top, middle, and bottom. The smallest measurement is the size you should order for your windows, otherwise it might not fit. Order the window ½ to ¾  inches smaller than the opening, this allows enough room for adjustments, caulk, insulation, etc. 

How to Install Replacement Windows

  • Clean the opening: Remove any debris, using a shop vacuum cleaner to remove any dust, or wipe it down with a slightly damp cloth if you don’t have a shop vac.
  • Prep the Sill: Take flashing tape and cut it the length of the window sill. Place it so that it covers the bottom ledge of the interior side of the window, this is important for waterproofing. Carefully place it so that it is fully flush in the corners. Using a level check that the sill is level. If it bows use hard/rigid Wobble Wedge® plastic shims to bring the sill up to level. Once the plastic shims are in place and you’ve verified it’s level, screw each plastic shim in place (you can use the pre-drilled holes in Wobble Wedges®) and use more flashing tape to hold the plastic shims in place.
  • Dry fit the window: there should be a small opening all the way around the window.
  • Caulk it: Using an elastomeric caulk (it will be flexible and expand/contract with the seasons giving you a good seal) apply a ⅜-inch bead all the way around the exposed inner face of the exterior casings. Apply two beads of caulk along the windowsill (bottom).
  • Put the window in place: First put the bottom of the window in position and then tilt the top of the window into place. Press it firmly against the caulk. Hold it in place with hard/rigid Wobble Wedge® plastic shims at all of the pre-drilled frame screw holes. Screw through the pre-drilled holes, and through the shims. Don’t tighten the screws all the way so that you can do the next step.
Hold the window in place with plastic shims.
  • Check the position: Make sure that the sashes sit evenly in the frame. If there are uneven gaps, position a hard/rigid Wobble Wedge® plastic shim at the meeting rails (where the sashes meet when the window is closed) to make the spacing even. Verify that the window is plumb, level and square (be sure to measure diagonally to check for square). Also make sure that the window opens and closes properly. Adjust around the edges of the window with hard/rigid plastic shims. After all measurements are right, score and snap the Wobble Wedge® plastic shims to remove excess. Finish tightening the screws.
  • Fill the gaps: From the outside, gaps around the window that are less than ¼ inch wide can be filled with the same elastomeric caulk used earlier. If the gaps are larger than ¼ inch, prefill them with backer rod weatherstripping, and then caulk. For gaps inside you can use a minimally expanding foam or the elastomeric caulk to fill the gaps.
  • Last steps: Reinstall the stops or add new ones. Finally you’re ready to prime and paint or stain the window sash and frame! You’ve now learned how to install replacement windows! Congrats!
Plastic shims