Replace a basement window by measuring the rough opening, removing the old unit, leveling the new frame, and sealing it with low-expansion foam.
The process is straightforward for the prepared homeowner, but a few basement-specific pitfalls trip up plenty of first-timers.
Before you order anything, check one critical thing: if that basement room has a bed in it, the window likely needs to meet local egress code. That single requirement shapes which window you can buy.
Measure the Rough Opening Before You Order
Careful measuring prevents a wasted purchase. The rough opening is the framed or masonry hole the window sits in, not the window itself.
- Measure the width at the top, middle, and bottom. Use the smallest measurement as your width.
- Measure the height the same way—left, center, right—and use the smallest value.
- Measure the wall thickness and the depth of the opening. Basement walls vary widely, and custom sash sizes are common.
Masonry openings are often uneven, so shimming room matters.
Check Egress Requirements for Bedrooms
If the basement bedroom is used as a sleeping room, the window must comply with International Residential Code egress minimums.
Your local building department may amend these values, so confirm the current rule before ordering. The Appleton, Wisconsin building department’s egress handout details how these limits apply to basement windows.
Window type also matters here. A casement window often achieves egress compliance more easily because the entire sash swings open. Sliding and double-hung windows leave part of the sash closed, so they need a much larger rough opening to hit the required clear dimensions. Fixed glass block windows are non-operable—they cannot serve as egress at all.
Remove the Old Window and Prep the Opening
Start by cutting through the old caulk and paint around the exterior trim with a utility knife. Pry the frame loose with a flat bar, working carefully so you don’t damage the surrounding wall. Pull the old unit out and clean the opening thoroughly.
Inspect the exposed framing or concrete for rot, mold, or masonry damage. Fix any structural problems now; a new window mounted to soft wood or crumbling concrete will fail fast. If the sill is too high for the egress limit, you may need a permanent step or platform inside to bring the floor up to code.
Set the New Window, Shim, and Seal
Dry-fit the new window before fastening anything. Check that it sits level, centered, and square in the opening.
Fasten the window per the manufacturer’s instructions. For wood framing, use the supplied screws through the pre-drilled holes. For concrete or masonry openings, switch to masonry screws or anchors. A word of caution: don’t overtighten. Warping the frame now means a sticky window and air leaks later.
Seal the exterior with a waterproof sealant rated for the frame material, then fill the gap around the interior with low-expansion spray foam. High-expansion foam is a common mistake—it can bow the frame and cause the sash to stick or leak.
Finish the interior trim once the foam cures. The job is done when the window opens freely, the seal is watertight, and no daylight shows around the frame.
| Window Type | Egress Fit | Best Use |
|---|---|---|
| Casement | Easiest to meet code—full sash opens | Basement bedrooms needing egress |
| Sliding / Double-hung | Needs larger rough opening | Non-sleeping rooms with space |
| Fixed glass block | Non-operable—fails egress | Light and privacy only |
If you’re weighing which type fits your basement and your code situation, our breakdown of top-rated basement replacement windows compares the practical options side by side.
FAQs
Can I replace a basement window myself?
Yes, a DIY replacement is realistic for a handy homeowner, especially with a prefabricated window in good condition. Budget a full weekend per window. The tricky parts are accurate measuring, leveling the frame, and sealing it properly. If the opening has rot or structural damage, call a pro first.
How much does a basement window replacement cost?
Your cost rises with window size, type, and installation difficulty.
What is the egress window size for a basement bedroom?
U.S. egress code generally requires a net clear opening of at least 5.7 square feet, with a minimum width of 20 inches and a minimum height of 24 inches. The sill must sit no more than 44 inches above the finished floor. Your local building authority may amend these numbers, so verify before you order.
References & Sources
- City of Appleton, WI Building Inspection. “Basement Egress Requirements Handout.” Details the 5.7-sq-ft, 20-inch, 24-inch, and 44-inch egress limits.
- Home Depot. “Egress Windows Buying Guide.” Explains window types, egress trade-offs, and cost ranges.
