How to frame a basement window in a concrete wall

Build the stud wall around the window with full studs on each side, a header and a rough sill. Then close the gap to the concrete with one of three finishes: a drywall return, a pressure-treated wood buck with jamb extensions, or a new window set in a treated buck.¹² Keep foam between the room and the concrete and seal every edge. Fix leaks and settle egress first.

TL;DR

• A drywall return keeps the most daylight and suits sloped block openings. A treated buck with jamb extensions gives a trimmed look that takes bumps. A new window in a treated buck also fixes an old, drafty frame.
• Use pressure-treated lumber where wood meets concrete, and seal the foam edges so warm room air never reaches the cold concrete.
• Keep the sash free to open fully. Under the 2021 International Residential Code, an egress opening needs at least 5.7 square feet of net clear opening in normal operation.³
• Replace an old single-pane metal hopper before drywall goes up. Swapping it later means tearing out the trim you just paid for.
• A leak at the window stops the job. Poor drainage around window wells can lead to water leaks.⁴

What is the best way to frame a basement window in concrete?

The best way to frame a basement window in concrete is to build the stud wall first, boxing the opening with king studs, a header and a rough sill. Then fill the space between that frame and the concrete with a treated buck or a drywall return. Seal and insulate at the concrete before the finish. It takes five steps.

1. Settle water and egress before you frame

Look at the window after a hard rain. A stain under the frame, rust on a metal hopper or damp concrete below the sill means water work comes first. Water ruins finished walls, and a window opening is one of the easiest places for it to get in.

Decide what the room will be, too. A bedroom needs an egress window that meets code, and that can mean cutting the opening bigger. Framing around a window you'll replace next year is wasted work.

2. Frame the wall around the opening

Build the stud wall the same way as the rest of your basement wall framing. At the window, run full-height king studs on each side. Add a header across the top and a rough sill across the bottom, with short cripple studs above and below.

Hold that framing back from the concrete edge. One carpenter on a DoItYourself.com thread about a sloped block opening advised framing 1/2 inch inside the entire opening, trimming the foam board back 1/2 inch and returning the drywall to the concrete face.¹ The small setback gives the drywall a clean edge to wrap around.

Two layouts need extra thought:

• Window tight to the joists. Some hopper windows have their top edge right at the bottom of the floor joists.² There's no room for a header, so add blocking between or under the joists. The top plate and the ceiling drywall both fasten to it.
• Sloped concrete sill. Some block basements have a concrete sill that angles down toward the glass. Squaring the framing to the bottom of the window cuts out too much light, so frame it as it lies.¹

3. Add a treated buck where trim needs a nailing surface

A buck is a frame of lumber fastened to the concrete inside the opening. It gives jamb extensions, casing and a sill something solid to nail into. Use pressure-treated lumber, because it sits against concrete that can stay damp. One homeowner who pulled out old metal hoppers framed the openings with pressure-treated 2x4s and set slightly smaller double-pane windows.²

Put a foam sill gasket or a strip of rigid foam behind the buck. It keeps the wood from wicking moisture out of the concrete. Fasten the buck with concrete screws, then seal the joint between buck and concrete.

4. Insulate and seal at the concrete

Rigid foam goes against the concrete wall and gets cut tight to the window opening. Seal every foam edge. In one forum thread, a homeowner covered an old window with foil-faced foam board and caulked the edges to stop air leakage, and said it improved how the basement felt.² The seal around an opening you're keeping matters as much as the foam itself.

Fill the gap between the buck and the window frame with a low-expansion window foam. Regular expanding foam can push on the frame and make the sash bind.

5. Return, sill and trim

Pick the finish from the options below. Whatever you choose, keep the stool, casing and any blind hardware clear of the sash swing and the crank.

Which return and sill option should you choose?

For a deep basement window, choose a drywall return for the most daylight and the least carpentry. Choose a treated buck with jamb extensions when you want trim that matches upstairs and takes bumps. Choose a new window in a treated buck when the old frame leaks, sticks or is single-pane. Whichever you pick, give the sill a solid surface.

Sill options

The sill takes the most abuse in a deep opening. Cold glass collects condensation that drips onto it, and it becomes a shelf for plants and phones.

• Cellular PVC stool. It won't rot or swell, and it paints well. It's a safe pick where condensation is likely.
• Sealed hardwood stool. It looks best with stained trim. Seal all six faces before it goes in.
• Drywall with corner bead. It's the cheapest and matches a drywall return. It's also the first to show dents and water stains.

MDF swells when it gets wet, so keep it for casing well above the sill.

Splayed returns

Angling the side returns outward lets more daylight spread into the room. It suits a drywall return best, because the angle is easy to form with corner bead. On a sloped block opening, following the existing slope keeps the light you'd lose by squaring it off.¹

How do you keep moisture and cold out at the concrete?

Basement Remodeling Co. treats a basement window opening as a moisture detail first and a trim detail second. The concrete around the opening is the coldest, dampest surface in the wall. Keep a treated or foam break between wood and concrete, and seal the room air away from the concrete with foam and caulk.

Warm, moist room air that reaches cold concrete condenses on it. Behind drywall, that water feeds mold you won't see until it smells. The foam layer keeps the inner surface warm, and the sealed edges stop air from getting behind it.

Water from outside is a separate problem, and framing can't fix it. Under the 2021 International Residential Code, section R310.4.3, window wells for egress openings must drain to the foundation drainage system. There's an exception for well-drained soils.³ HomeGuide puts a drainage system at $1,000 to $4,000.⁴ Extra waterproofing around the window and foundation cut may be needed where rain is heavy or the water table is high.⁴

If you find mold on old framing or on the concrete, have it assessed by a qualified pro before the wall gets closed. A matched contractor checks moisture at the free on-site estimate, so leaks and damp spots get settled before anything is framed.

What is the code for a basement window?

Under the 2021 International Residential Code, section R310, a basement needs at least one operable emergency escape and rescue opening. Every basement bedroom needs its own. Section R303.1 also requires glazing of at least 8 percent of a habitable room's floor area.³ Local adoption and amendments vary, so confirm the rules with your building department.

These are the 2021 IRC numbers that shape how you frame and trim the opening:

A few of these change the framing work directly:

• Net clear opening comes from normal operation. The code measures the opening as the result of normal operation of the window.³ A deep stool, a casing or blind hardware that stops a casement from swinging fully can shrink the opening you're relying on.
• Covers must release from inside. Bars, grilles or covers over an egress window or its well must be removable from inside without a key, tool or extra force.³
• Light has exceptions. The glazing rule allows exceptions where the room has mechanical ventilation and enough artificial light.³

Finishing an existing basement doesn't always mean adding egress. Section R310.7 says new sleeping rooms in an existing basement need an escape opening, and other alterations or repairs don't trigger one.³ Replacement windows get their own path. Under R310.5, a replacement can skip the size rules if it's the manufacturer's largest standard size that fits the existing opening, it opens the same way or wider, and it's not part of a change of occupancy.³

If you're adding a bedroom or a rental unit, the rules tighten. Our guide to basement egress windows covers sizing, wells and ladders, and the page on basement apartments and in-law suites covers a separate living space.

Should you replace the old window before you frame?

Replace an old basement window before you frame if it's a single-pane metal hopper, the frame leaks or the sash sticks. Then the buck, foam and trim go around the new window once. A sound, modern window can stay, and you frame around it.

HomeGuide lists these signs a window is due: condensation between the panes, drafts, a sash that's hard to open or close, and windows more than 20 years old.⁵ On one forum thread about framing around a basement hopper, a reply put it bluntly: if you're finishing the basement, change the window.²

What it costs, from HomeGuide's national figures:

• Standard replacement: $450 to $1,500 per window on average, for single-hung windows in basic aluminum or vinyl frames.⁵
• Egress window: $6,000 to $8,000 on average, including the window, well, excavation and labor.⁵ Complex projects can exceed $10,000.⁴
• Permit to change the window size: $50 to $200 for reframing the opening.⁵
• Egress permit: typically $200 to $500+, depending on local rules.⁴

Those are national averages from one cost guide. Your own number is priced at your estimate. Replacing several windows at once usually lowers the cost per window, because units cost less when bought in bulk.⁵

If you're measuring for a replacement, measure the width between the jambs at the bottom, middle and top. Measure the height at the left, center and right. Use the shortest of each.⁵

If the basement is already finished and you're reworking it, window swaps fit naturally into a basement remodeling project, since the trim is coming off anyway.

What are common window framing and installation mistakes?

Common basement window framing and installation mistakes are untreated wood against concrete, trim that blocks the sash, squaring off a sloped opening, unsealed foam edges and finishing around a window that already leaks. Most show up months later as a stuck sash, damp drywall or a cold draft along the wall.

• Untreated wood touching concrete. A plain lumber buck or rough sill sitting on damp concrete can rot and grow mold. Use pressure-treated lumber and a foam or gasket break.
• Framing or trim that cuts into the egress opening. The net clear opening is measured with the window in normal operation.³ Test the full swing before the casing goes on.
• Squaring off a sloped opening. Framing to the bottom of the window on a sloped block sill loses daylight.¹
• Foam with open edges. Air that gets behind the foam finds the cold concrete and condenses. Caulk or tape every edge.²
• Finishing around a leak. Poor drainage around window wells can lead to water leaks.⁴ Fix the well drain first.
• Measuring a replacement once. Openings in old concrete are rarely square. Take three measurements each way and use the shortest.⁵
• Overfilling with foam. High-expansion foam in the gap can bow a vinyl frame so the sash binds. Use low-expansion window foam.
• Covers that lock. A well cover that needs a key or tool fails the egress rule.³

What should you ask your contractor about the window openings?

Ask your contractor which return and sill option they'll use on each basement window, what goes between wood and concrete, and whether each window meets egress. Then check that every answer shows up as a line in the written, itemized quote. The quote is where you catch a skipped detail before it's built.

Good questions to bring to the estimate:

• Which return will you build on each window, and what's the sill made of?
• Is the buck pressure-treated, and what's behind it?
• How will you seal the foam edges around the opening?
• Does any window need to meet egress, and will you pull the permit?
• Should any window be replaced before framing, and is that priced separately?
• How will you check the window well drainage?

With Basement Remodeling Co., your request goes to one vetted, licensed basement contractor for your metro. You get a free on-site estimate and a written, itemized quote, and nothing starts until you sign. Your contractor pulls the permit and books the inspections. When you're ready, get a written quote for finishing your basement, or use the basement contractor ZIP finder to check your area.

Permit offices and building departments differ by city. If you're in Colorado or Georgia, start with the local pages for finishing a basement in Denver or finishing a basement in Atlanta.