How to Replace a Basement Window

Basement windows are usually out of sight and out of mind, but if your old steel-framed, cast-in-place units are starting to show signs of corrosion, or the single-pane glass and lack of gasketing means leaks and drafts, then it’s time to tackle a replacement. In this video, host Justin Fink visits Fine Homebuilding veteran Mike Guertin to see his method for removing an old basement window and replacing it with a modern, energy-efficient vinyl window.

Step 1 – Remove old window and frame

Start by removing the old window sash from the metal frame. Then, using an angle grinder with a metal cutting wheel, cut through the bottom of the frame, going all the way from the inside to the outside edge. Cut completely through the sash stop and the concrete within.

Repeat the process across the top of the frame. But if the exterior-side of the window has a top flange that is covered by wall sheathing and siding, leave this spot for now. That will be easier to access after the next step.

With the top and bottom cut, pry each side toward the center making an ‘X’ shape from the severed metal window frame. Prying the window out of the opening may be quick, or it may be a major struggle. The difficulty depends totally on the thickness of the metal; and the oldest windows were built from the heaviest gauge of steel.

After the top of the frame is payed down in the middle, use a reciprocating saw to cut the outermost portion of the top flange. I wasn’t kidding about the “major struggle.” In this case, Mike had to use his largest pry bars and even a sledge hammer. But with the old window out of the way, the hardest part of the job is behind you.

Step 2 – Prep the opening for the new window

Next, chisel away the remaining concrete left by the hollow sash stops on the bottom and sides of the opening. A pneumatic chisel makes quick work of cleaning up the opening, but a hammer and masonry chisel works fine, too.

After sweeping the concrete clean, coat the surfaces of the opening generously with siloxane masonry sealer until the concrete is saturated and won’t absorb any more sealant (several brands are available). Wipe up excess and let the surface dry before moving on to the sill.

Mike likes to set the new windows atop a 2×8 pressure-treated sill, which he rips with a 5- to 10-degree slope on one edge to help water drain away. Cut correctly, the sill should snuggly and require a few taps of a hammer to seat it in place in the opening. It should project between 1/4 in. and 1/2 in. beyond the exterior face of the wall. Once you’ve got it located, mark its position. If the height of your opening doesn’t permit the use of a full-thickness 2x sill, you can re-saw or plane the pressure-treated stock to be thinner.

The sill will be secured in the opening with a combination of masonry screws and adhesive sealant. After boring pilot holes through the wood, switch to a masonry bit to bore holes into the concrete. Then, remove the sill. Apply adhesive sealant across the bottom of the opening, including a thick bead on each of the two ends. Then fasten the sill in place.

Step 3 – Making the bucks

Next, dry-fit the window in the center of the opening and measure the gap on both sides. If the concrete opening is out of square by more than 1/4 in., the buck pieces may need to be tapered. In some cases, like on this installation, there won’t be room or need for a buck above the window. That’s okay. The window can be fastened to the mud sill.

Cut the side buck pieces to match the gap between window and concrete, but subtract about 1/16 in. to account for the sealant that you’ll apply. The bucks should fit snugly, and Mike fastens them using trim head screws driven at a steep angle into the mud sill above and the window sill below.

Step 4 – Stops and sealing

Next come stops for the front edge of the window to seal against, which Mike cuts from 5/4 cedar. Which he then rounds over with a trim router for a cleaner look. These stops are cut to length and fastened with screws to the side bucks and mud sill.

After applying sealant to the inside face of the stop pieces, slide the window into the opening. If it is fitting too snugly between bucks, don’t force it. A block plane can be used to shave the side channels of the vinyl frame to give the window a bit more wiggle room.

Once in place, screw through the sides of the window and into the side buck pieces. Choose screws that are long enough to bit into the wood, but not so long that they’ll bottom out on the concrete beyond.

The last step is to add a bead of sealant around all four edges of the interior-side of the window, and another bead across the top and two sides of the exterior. Never seal along the outside of the window sill, which should be left open for drainage.

Read the companion article:

Replacing a Basement Window

More on basement windows:

How to Trim a Basement Window — When trimming windows in thick walls, you’ll need wide extension jambs and thick stools. Here’s how.

Can a Deck Ledger Span a Basement Window? — A new deck ledger will cross two cast-in-place basement windows. Glen Mathewson covers the code implications.

How to Make an Egress Window Well Not Look like a Window Well — A FHB reader describes how he solved an aesthetic problem with a new basement window.

Egress Windows: Understanding ‘Net-Clear’ Opening Requirements — Code specialist Lynn Underwood responds to a reader who wants to put a bedroom in a basement.