The Devil is in the Details of Air Tightness

Critical Details for High-Performance Building

Air tightness is the foundation of high-performance building, and windows and doors represent the most vulnerable points in any building envelope. Properly installed air-tight window and door installations deliver superior energy efficiency, moisture management, and occupant comfort. However, achieving true air tightness requires attention to critical details that many builders overlook and not every product will help you actually achieve your goals. That’s why we focus maniacally on products we know move the needle toward air tightness!

The European Advantage: “Innies vs. Outies”

One of the most significant considerations in window installation is the mounting approach. European design philosophy favors “innie” installations, where windows are recessed into the wall assembly and integrated with continuous air and thermal barriers. Due to a couple of reasons, this approach affords better air tightness compared to traditional North American “outie” installations with nail fins where the windows are located to the very exterior of wall assembly.

We’re starting to see more “innie” approaches to windows with this European influence. However, it’s still a mix. Regardless of which window styles you work with, you want to make sure you’re working with high-quality windows and ones that are designed for high performance. That’s why we love Alpen and Advantage products, and also why you’ll see us reference their great educational resources on installs here!

Critical Flashing and Sealing Details

So much happens behind the scenes with windows and door framing! The products you use make a big difference. We’re fans of the SIGA products for air sealing, even though they’re working literally behind the scenes in a building. 

Proper window flashing requires a systematic approach that addresses water management and air sealing simultaneously. SIGA’s Wigluv tape system offers high-performance flashing that maintains adhesion in extreme temperatures while providing vapor permeability to allow diffusion when there is condensation.

The installation sequence is crucial: sill flashing must be installed first with proper drainage slope, followed by jamb flashing that overlaps the sill installation. Head flashing completes the water management system, with each layer building upon the previous to create a continuous barrier.

Corner details also present particular challenges. The modified eye-cut technique demonstrated in professional installations creates clean transitions while maintaining the integrity of both the water-resistive barrier and air barrier systems. These corners require careful attention to prevent air leakage.

Don’t Forget Interior Air Sealing

While exterior flashing receives significant attention, interior air sealing is equally critical for achieving true air tightness. SIGA’s Fentrim IS 20 provides interior air sealing that also functions as a back dam, preventing water infiltration while blocking air movement.

Regardless of the specific products used, incorporating a back dam or sloped sill is essential. These features provide secondary water management, help you work with gravity, and contribute significantly to overall air tightness performance.

Advanced Installation Techniques

High performance builders generally understand that air tightness extends beyond product selection to installation technique. Proper shimming locations prevent frame distortion that can compromise seal integrity. For operable windows, shims should be placed at the proper locations to prevent operational stress that can break air seals over time.

The integration of insulation and air barriers requires careful coordination. Spray foam or other insulation materials must be applied to limit thermal bridging while maintaining air barrier continuity.  Note that some believe spray foam is the air barrier and it might work for the short term but generally will have cracks and delamination over the long run. The sequence of operations matters: structural attachment, insulation installation, and air barrier integration must follow the specific sequence to achieve optimal performance.

Lessons Learned and Best Practices

Weather conditions during installation can significantly impact long-term performance. While many products don’t “stick” well during higher or lower temperatures, SIGA products perform reliably across a fairly wide temperature range.  Installers must understand how environmental conditions affect adhesion and curing.

Achieving consistent air tightness requires mastering numerous small details that collectively determine performance. Cross-blocking locations for tilt-turn windows, proper sealant application patterns, and integration with weather-resistive barriers all contribute to the final result.

The most successful installations follow manufacturer specifications precisely while adapting to project-specific conditions. This requires understanding both the science behind air-tight construction and the practical realities of field installation.

Ready to elevate your window and door air tightness? Contact AE to learn more about complete flashing and air-sealing systems as well as high-performance windows and doors.