Step 8: Insulate against drafts

installed window with a gap
Photo: David Carmack
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Fit the sash into the window frame.

Inside the house, apply a single thin bead of minimally expanding polyurethane foam to the gap between the window and the framing. Allow the bead to expand and cure for one hour before adding more. Repeat until the cavity is completely filled.

When using fiberglass insulation, cut unfaced batting a little larger than the space between the window and the framing, and push the batting in with a putty knife. To prevent air leaks, cover the gap with aluminum tape.

If the gap is too narrow for either foam or fiberglass insulation, seal it with a bead of caulk.

TIP: Do not fill the gap between jamb and framing with too much foam too quickly. Otherwise, the jamb could bow and thereby bind the sash.

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    Tools List

    • hammer
      20-ounce hammer
    • two-foot level
      2-foot and 4-foot levels
    • tacker hammer
      Hammer-tacker,
      for stapling builder's felt
    • utility knife
      Utility knife
    • 16-foot tape measure
      Measuring tape
    • caulk gun
      Caulk gun
    • drill
      Optional: Drill-driver and 1 5/8-inch stainless screws,
      for attaching sill extender to bottom of narrow aluminum sill

    Shopping List

    1. WINDOW

    2. CAULK

    3. MINIMIALLY-EXPANDING FOAM INSULATION

    4. 15-POUND BUILDER'S FELT

    an alternative to membrane for flashing the rough opening



    5. WATERPROOF MEMBRANE

    6. METAL FLASHING

    7. FOAM INSULATION

    8. 1 1/2-INCH ROOFING NAILS

    for fastening nailing fins to the sheathing