Why Window Sealing Matters in Thermal Envelope Testing

We recently inspected a new residential build in Clark County that didn’t pass its initial thermal envelope test. The issue? Inadequate air sealing around the windows. It’s a simple fix, but it’s one that can cause big problems if missed—especially when you’re up against tight schedules and inspection deadlines.

In the case of this project, the window units were installed correctly, but the air barrier around the window frames wasn’t continuous. There were gaps between the window frame and the rough opening—areas that were either under-sealed or completely unsealed. That allowed outside air to bypass the insulation and enter the building, causing the house to exceed the allowable air leakage rate.

During a blower door test, we’re measuring how much air leaks into and out of the building through the thermal envelope. Windows are a major part of that envelope, and if they’re not properly sealed, air can easily move around the framing and into wall cavities.

The Result: A Failed Test and a Return Trip

Because the air leakage rate was too high, the project didn’t pass the initial inspection. The crew had to come back, identify each leak, reseal the windows, and schedule a re-test. That meant delays, added labor, and another day lost waiting for inspection.

How to Seal Windows the Right Way

To avoid this issue, make sure all gaps between the window frame and rough opening are sealed with appropriate materials. Low-expansion spray foam is a good option, as it fills irregular gaps without warping the frame. Backer rod and caulk can also be used in smaller joints.

Don’t rely solely on exterior flashing or trim to block air movement. While those elements are important for water management, they’re not a substitute for proper air sealing inside the wall cavity.

Lessons for General Contractors in Las Vegas

In our work as third-party energy inspectors across Clark County, this is one of the most common issues we come across—especially in residential construction. It’s easy to assume the windows are "done" once they’re in and trimmed out, but if the air sealing isn’t tight, it’ll show up in the blower door results.

A quick check during installation and a little extra sealant can be the difference between passing on the first try or calling everyone back for rework.