Retrofit window installations must bridge existing construction with new fenestration while protecting against water, air, and thermal infiltration. This article explains how to detail retrofit windows in both face-sealed and drainage-based wall assemblies.
Retrofit Conditions and Challenges
Common retrofit scenarios include insert windows into existing frames or full tear-out replacements. Each condition presents challenges for aligning new flashing with aged WRBs, dealing with unknown framing conditions, and ensuring consistent air barrier continuity across dissimilar materials.
Best Practices for Retrofit Integration
- Use low-profile head and sill pans that integrate with existing WRB or cladding
- Seal behind retrofit flanges or casing trims with flexible flashing and compatible sealant
- Apply liquid-applied flashing to the interior/exterior joint to bridge substrate gaps
- Backer rod and sealant must be installed in a toolable, compressible configuration
Thermal and Air Sealing Considerations
Install low-expansion foam or backer rod in rough opening cavities to improve air tightness. Include insulated casing or trim systems where feasible. Apply peel-and-stick air barrier membrane to connect old sheathing to new window flanges.
Mockup and QA Protocol
- Create at least one full window retrofit mockup for each wall type or condition
- Review flashing tape adhesion to existing substrate and age-weathered sheathing
- Inspect back dams, head flashing, and drainage slopes prior to cladding reinstallation
- Verify airtightness with blower door depressurization and IR imaging if available
Document each retrofit installation zone with photos and approved sealant/membrane products for warranty submission.
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