How to Properly Install a Window (And Why It Matters)

When we bought our 1965 home, we knew we’d be tackling some upgrades. One of the more recent projects was replacing the original French doors in our den with a new window.

We decided to hire a local contractor for the job—what should’ve been a straightforward install. But watching them work, I was shocked to realize they weren’t following the most basic principles of proper window installation. It reminded me of something a foreman told me on one of my first jobs in construction:
“Think like a raindrop.”
It’s advice that’s stuck with me for over two decades and applies perfectly to sealing windows.

What a Proper Installation Looks Like

Let’s walk through the correct way to install a window—especially when replacing a door, as we did.

1. Framing:
   The rough opening should be properly framed and square. This creates the base structure for everything else.
2. Sheathing:
   Usually plywood, the sheathing gets attached over the framing. This acts as the structural and moisture-resistant surface for the wall.
3. Water-Resistive Barrier (WRB):
   Over the sheathing goes a membrane—either house wrap like Tyvek or tar paper. This layer is your first line of defense against water intrusion.
4. Window Installation with Sealant:
   The window should be installed directly onto the membrane, with high-quality sealant applied behind the nailing flanges. This ensures a tight bond and stops wind-driven rain from sneaking in behind the window.
5. Flashing Tape (In the Right Order):
   Here’s where “thinking like a raindrop” really matters:
   • Bottom first: So water runs off and doesn’t collect behind the tape.
   • Then sides: Lapped over the bottom, not tucked behind.
   • Then top: Lapped over the side flashing to shed water like shingles.
6. Insulation or Foam Board:
   Insulation fills the gaps around the window—either spray foam or backer rod with caulk, and sometimes rigid foam board for energy efficiency.
7. Siding:
   Finally, the siding goes back on, overlapping the layers below and maintaining a continuous water-shedding surface.

What the Code Says: Air Sealing Requirements

If you think air leaks around a window are just a minor inconvenience, consider this: both the building code and energy code require windows to be sealed properly for air leakage. And it’s not just best practice—it’s code-mandated.

2020 New York State Residential Code

• Section R702.7 – Vapor Retarders (and related wall envelope requirements):
  Not all regions require a vapor retarder, but where applicable, improper window sealing can break the envelope and lead to condensation or mold.

Section R703.4 – Flashing:

"Flashing shall be installed in such a manner so as to prevent entry of water into the wall cavity or penetration of water to the building structural framing components."
This reinforces the importance of flashing tape and sealant in the correct order—bottom, sides, then top.

2020 Energy Conservation Code of New York State (ECCCNYS)

• Perimeters (jambs, sills, and headers)
• Intersections with walls
• Interface with the WRB (weather-resistant barrier)
• Blower Door Test (R402.4.1.2):
  New homes must test at ≤ 3 ACH50 (air changes per hour at 50 Pa). Poorly sealed windows can be a major contributor to test failure, which is why proper air sealing around windows is no longer optional in modern construction.

Table R402.4.1.1 – Air Barrier and Insulation Inspection:
Requires verification that:

"Window and door frames are sealed"
"Foam sealant, backer rod, or equivalent materials are used between the framing and window/door unit"

Section R402.4.1 – Building Thermal Envelope:

"The building thermal envelope shall be constructed to limit air leakage..."
Specifically, doors and windows must be sealed at:

Practical Translation:

In plain terms, the window needs to:

• Be flashed and integrated with the WRB to manage water
• Be sealed with foam or caulk between the window frame and rough opening to block air
• Be inspected as part of your blower door test (if required in your jurisdiction)

If it isn't sealed, it isn’t code-compliant.

Why It’s So Important

You only get one chance to get it right.

A window that isn’t sealed and flashed properly will leak. Maybe not today. Maybe not even next year. But eventually, water will find its way into the framing—rotting studs, swelling sheathing, and creating a breeding ground for mold.

It’s not just water either. Poor sealing can lead to drafts, energy loss, and even pest entry. And once damage starts behind the wall, it’s out of sight and out of mind—until the repair bill is thousands of dollars.

That’s why it surprised me so much to see a contractor overlook these basic principles. These aren't "extras"—they're part of any good install.

How to Inspect Existing Windows for Signs of Failure

If you're not opening up your walls, there are still ways to catch failures early:

• Interior Water Stains: Look for brown or yellow stains around the inside window trim or drywall. These are often signs of past or active leaks.
• Soft or Warped Trim: Gently press on the interior and exterior trim. If it feels spongy, it’s likely rotted behind the surface.
• Drafts: On a windy day, run your hand around the edges of the window frame. A cold draft is often the result of poor air sealing.
• Foggy Glass (in double or triple panes): If you see condensation or fog between the panes of glass, the window’s seal has failed and needs replacing.
• Exterior Caulking Gaps: Check around the perimeter of the window for missing or cracked caulking. Gaps can let in water and air.
• Peeling Paint or Bubbling Siding: On the exterior, these could be signs that water is getting trapped behind the siding due to flashing failures.

If you notice any of these issues, it may be worth having the window re-flashed or replaced entirely, depending on the extent of the damage.

Final Thoughts

If you’re replacing a window or door, don’t assume your installer is doing it right. Ask questions. Look for flashing tape. Watch how they layer materials. And most importantly, remember:
Always think like a raindrop.