Download PDF version (746.2k) Log In or Register to view the full article as a PDF document.


Flush-fin windows can be installed so that they lap over stucco siding, but sometimes they have to fit into a recessed opening and the flange has to be trimmed to fit the opening. After removing the glass, I measure the frame and double-check the diagonals to verify that it’s square. When I mark the flange with these measurements, I take an equal amount off each side so that the unit will be centered in the opening when it’s installed (see slideshow). I cut the flange down to size with a saw, then round the corners over to make fitting easier. While dry-fitting the unit, I use a sharp utility knife to cut back the vinyl flange and fine-tune the fit as necessary. If I’m installing a flush (not recessed) window that has fins, I mark the fin outline right on the wall with a pencil, so that I can accurately mask off the wall with tape.

Sealant. Sealant is another important ingredient in any retrofit window installation: You need to use the right amount of the right kind, and you need to use it in the right locations. Sealant should be durable and paintable, adhere well to the substrate, and have good elongation characteristics, which generally rules out silicone. I’ve had the best success with polyurethane and other sealants that meet ASTM C920 standards, like Moistop (800/773-4777,, OSI Quad or WinteQ TeQ:Seal (800/624-7767,, RainBuster 450 or 900 (800/473-1617,, and Schnee-Morehead 7100 (800/878-7876,

Once I’m satisfied with the dry fit, I pull out the unit and apply a fat 1/2-inch-diameter bead of sealant to the old window frame. Water can penetrate and travel behind the exterior cladding and may want to exit the wall cavity at the window frame, especially at the top, so I pump additional sealant upward into the upper frame corners of the old window. Sealant application around the frame is especially critical — I call it the “primary” seal — because it provides the first line of defense against water intrusion.

I also pump extra sealant into the lower corners, but I leave a pair of open gaps just below the weep holes of the old window as I apply the sealant to the sill. That way, any incidental water that gets behind the cladding and into the old window frame can drain out the weep holes and through the gaps in the sealant, rather than get trapped in the new window assembly.

Before installing the window, I mask off the fin with blue painter’s tape. On a flat (not recessed) wall installation, I also mask off the wall, using the marks I made when I dry-fit the unit as a guide. I install the tape about 3„4 inch outside of these marks. Then I apply another continuous 1/2-inch-diameter bead of sealant to the window fins, again leaving voids for the weeps at the base of the window.

Sealant needs compression and tooling to be effective. Many of the leakage problems I investigate are the result of sealant that doesn’t completely fill the gap between the window frame and the wall. When the window is installed, squeeze-out should be visible around the entire perimeter of the window (except at the two small locations under the weep holes of the old window). By masking off the fin and the wall (or trim) with tape, I’ll be able to tool the sealant after I’ve installed the window and still get clean, straight lines.

Fasteners. Once the window is fully compressed in the sealant (this is a two-man job), I check it a final time for plumb, level, and square, then drive fasteners through its frame into either the wall framing or the old window frame. I never drive fasteners through the sill, though, because this can damage the old window’s weep system. To keep the frame from distorting, I shim the window with plastic shims, which — unlike wood shims — won’t compress, decay, or hold water.

Some window manufacturers supply the screws required to hold the window in place, but when they don’t, I like to use 3-inch-long exterior-grade deck screws. If the window manufacturer specifies a fastening schedule, I use it; otherwise I space fasteners no more than 18 inches apart.

After the window is secured, I tool the sealant smooth with my finger, using a little water to smooth out and feather the bead over the tape. Then I remove the tape before the sealant cures, taking care not to mess with the beads. Painter’s tape seems to be equally effective at sealing off both smooth, painted trim and rougher stucco finishes, and the joint becomes almost invisible if a color-matched sealant like OSI Quad is used.


Before applying interior trim, I fill voids between the new frame and the wall with low-expansion foam. High-expansion foam shouldn’t be used here because it would distort the frames of most windows. I like OSI’s WinteQ TeQ:Foam. It expands completely in about 20 minutes, at which point the excess can be trimmed off. I also like the company’s foam gun, which has little plastic tips that can get into tiny spaces and around corners. The foam seals the air space between the window and the wall and provides additional support for interior trim around the window.

After the window has been installed, there’s always a little interior trimwork needed to finish up the job. Most window-replacement contractors opt for the quick and easy approach, and apply flat vinyl trim with adhesive backing around the unit. A good finish carpenter, however, can earn a little extra revenue by upgrading with new jamb extensions and casings. Many homeowners are willing to spend the extra cash for the look of wood trim.

Gene Summy is a contractor and building inspector in Laguna Niguel, Calif. His company, TLS Laboratories, specializes in rain-related problems.