10 Essential Steps For A Proper Window Installation

Thinking about replacing a window yourself? Here’s how to install replacement windows the right way, whether you’re swapping out one window or working your way through a whole house full of them.

We’ve installed windows and doors across Chicago since 2010, and the steps themselves aren’t the hard part. Measure, remove the old window, prep the opening, set and shim the new one, then fasten, insulate, and seal it. What actually trips people up is everything around those steps: whether your frame can handle an insert, whether Chicago requires a permit, and when it’s smarter to call in a pro.

This guide covers:

Grab your tape measure. Let’s get into it.

Full-Frame vs. Insert Replacement: How Are Windows Installed

An insert replacement window slides into your existing frame. You keep the old jambs and trim, and only replace the sash and glass unit. It’s faster and cheaper, the kind of project a handy homeowner can pull off in an afternoon.

A full-frame replacement means tearing out the entire window, frame and all, down to the rough opening. It’s the only real option if your frame is rotted, warped, or you’re changing the window’s size.

We cover this in more depth in our guide to types of window installation. Short version: solid frame, go with an insert. Compromised frame, don’t fight it.

Do You Need a Permit in Chicago for Home Window Installation?

For most straightforward swaps, no. Per the City of Chicago’s Department of Buildings, replacing a window in-kind (same size, same opening, no structural change) doesn’t require a permit in most residential buildings up to four stories.

You will need one if you’re changing the window’s size, adding a new opening, or working near safety-glazing locations. Egress windows in bedrooms and basements have their own code requirements, too. If you’re in one of the suburbs we serve, like Naperville, Evanston, or Schaumburg, rules can differ slightly, so a quick call to your local building department is worth it.

Tools and Materials You’ll Need for DIY Window Installation

Tape measure and a 4-foot level

Pry bar, flathead screwdriver, utility knife

Cordless drill or driver, hammer

Caulk gun and exterior-grade sealant

Low-expansion spray foam

Composite or plastic shims

Flashing tape

Safety glasses and gloves

Skip high-expansion foam. Too much pressure can bow a vinyl frame inward and throw off the seal.

Step 1: Assess Your Existing Window Frame

Press a flathead screwdriver into the wood around the frame, especially the sill and bottom corners. Soft or spongy wood means rot. Solid resistance means you’re in good shape.

• Good signs for an insert: no soft wood, the frame is still square, no visible drafts, and the windows are 20 to 25 years old or newer.

• Signs you need full-frame: rotted or water-stained wood, an opening that’s noticeably out of square (common in older Chicago two-flats and bungalows that have settled), or water staining on interior walls nearby.

If you’re in an older home in a neighborhood like Portage Park, Austin, or Pilsen, this step matters even more since original frames have often been painted over many times, which raises the next issue.

What About Lead Paint? (Homes Built Before 1978)

Homes built before 1978 are assumed to have lead-based paint. Under the EPA’s Renovation, Repair, and Painting Rule, window replacement specifically counts as an activity that disturbs paint. Any paid contractor working on a pre-1978 home must be lead-safe certified and follow containment procedures.

If you’re doing the work yourself on your own home, the rule doesn’t apply directly. But it does apply for rental properties, and it’s not optional there. A lot of Chicago’s bungalows and greystones fall into this pre-1978 category, so it’s worth knowing going in.

Step 2: Measure the Rough Opening

Measure width at the top, middle, and bottom, and use the smallest number. Do the same for height. Old openings are rarely perfectly square, especially in a house that’s settled over the decades.

Then measure jamb depth from interior trim to exterior stop. This is the number people forget, and it determines whether the new window sits flush. Take these measurements before you order.

Step 3: Remove the Old Window

Score the interior trim with a utility knife to break the paint seal, then pry it off gently. Save it if it’s in decent shape.

Old double-hung windows often hide sash cords and counterweights in the jamb. Cut the cords, remove the sashes, pull the weights.

Most insert replacements go in from the inside, since the new unit is easier to maneuver through the house. Full-frame jobs are usually set from the outside, especially on upper floors. Check your manufacturer’s instructions, since some specify one direction only.

Step 4: Prep the Opening

Clean out loose paint, old caulk, and debris. This is your last chance to catch any rot you missed earlier. Deal with it now. Sealing a new window over rotted wood just traps the problem.

Check plumb and level with your level, then measure the diagonals corner to corner. Equal diagonals mean the opening is square. If they’re more than a quarter inch off, you’ll need to compensate with shims.

Step 5: Set the New Window and Shim It

Run a bead of exterior-grade caulk along the sill and exterior stop first. Tip the window in from the inside, resting the bottom on the sill, then tilting the top against the exterior stop. Don’t force it.

Shim at both bottom corners, both top corners, and near the lock hardware. Recheck level and diagonals. Patience here pays off. A window that’s slightly out of square now will bind or leak later.

Step 6: Fasten, Insulate, and Flash

Drive screws through the side jambs into the shims and the original frame. Don’t overtighten, or you’ll bow the frame before you’ve finished.

Fill the gap with low-expansion foam in thin layers because it can expand more than expected. On full-frame jobs, apply flashing tape above the opening in a shingle pattern so water always drains outward.

Step 7: Seal It Right (This Matters More in Chicago)

Chicago’s freeze-thaw cycle is brutal on caulk. Every swing from a 40-degree afternoon to single digits overnight expands and contracts your seal. Cheap caulk or a rushed job cracks within a season, and you won’t notice until you feel a draft on your neck in January.

Apply exterior-grade sealant around the full perimeter where the frame meets siding or brick. Then seal the interior side too, where the frame meets trim or drywall. Skipping that interior bead is a common shortcut that costs you comfort later.

Step 8: Reinstall Trim and Finish

Put the interior stop molding back, or install new trim if the old pieces got damaged. Fill nail holes, sand, and paint to match. On the exterior, replace trim and caulk the seams. This step affects curb appeal more than any other; a mechanically perfect window still looks like a DIY job if it’s trimmed sloppily.

Energy Efficiency Numbers That Matter in Chicago

Two numbers matter most on a window’s spec sheet: U-factor and Solar Heat Gain Coefficient (SHGC). U-factor measures how well a window insulates; lower is better. SHGC measures how much solar heat it lets through, which matters less here than in warmer climates.

Chicago falls in the Northern zone for ENERGY STAR’s window program, which currently calls for a U-factor of 0.22 or lower to qualify. Look for the NFRC label, the independent rating ENERGY STAR certification is based on, for real numbers instead of marketing language.

A lot of our clients come to us after a winter utility bill makes them do a double take. Correctly rated, properly installed energy-efficient windows are usually where that number starts coming back down.

How long does it take to remove an old window and install a new one?

Plan on two to four hours for a single insert replacement. Full-frame jobs run four to six hours per window, more on upper floors. We cover typical project timelines in our guide to how long window installation takes.

DIY vs. Hiring a Pro for House Window Installation: The Honest Breakdown

• Easy to underestimate with DIY: specialty tools you don’t own, mistakes on your first window, a full weekend for just a few windows, and manufacturer warranties that sometimes require professional installation to stay valid.

• Where a pro earns the cost: full-frame jobs on older homes, multiple windows at once, lead paint disturbance on rental property (legally required), egress or safety-code replacements, and historic or architecturally specific windows.

Kevin, a client doing a full renovation in Wicker Park, is a good example. Handy enough to do most of his own work, but once he hit the custom bay window phase, he realized that particular install was outside of what he wanted to take on solo. Knowing your limits is part of doing the project right.

If professional installation makes more sense for you, we offer free, no-pressure quotes and back our window installation work with a lifetime warranty and price-match guarantee.

What are the most common window installation mistakes?

• Measuring once instead of at three points per dimension

• Skipping the back dam or getting the flashing sequence wrong

• Over-tightening fasteners and bowing the frame

• Using high-expansion foam

• Skipping the interior seal because it’s less visible

• Ignoring rot discovered during removal, instead of fixing it first

What Types of Windows Work with This Process?

This approach applies to most double-hung, single-hung, and sliding windows. Casement and awning windows follow a similar sequence with some hardware differences.

What is the average cost to install a replacement window?

It depends heavily on window type and whether it’s an insert or a full-frame. We break down real Chicago pricing in our guides to window replacement costs and Chicago-specific costs.

DIY or Done for You? Either Way, We’ve Got You Covered

Whether you tackle this yourself or bring in a pro, the goal is the same: a window that’s properly sealed, sits square, and holds up through a Chicago winter.

If you get partway through your assessment and realize the frame is worse than expected, or you’d rather just have it handled, we’re happy to take a look.

We serve Chicago and communities within about 60 miles, including Naperville, Evanston, Schaumburg, Aurora, and Elgin, and every quote is free with no pressure attached. Get in touch today!