Window jambs are often overlooked in flashing assemblies but are critical to achieving a continuous air and water barrier around fenestration openings. This article provides best practices for sealing and integrating jamb flashing into high-performance wall systems.
Jamb Flashing Fundamentals
Flashing the jambs ensures water directed from the head or sill does not bypass control layers. Jambs also help manage air leakage at rough openings. Integration with WRB, sealants, and insulation is essential for long-term durability and comfort.
Installation Steps
- Install flexible flashing or stretch tape from the sill upward to at least 6″ above the window head
- Embed jamb flashing behind head flashing and overlap it onto the WRB or air barrier
- Use sealant or fluid-applied membrane behind the flange or frame edge for secondary sealing
- Apply insulation behind the jamb frame cavity and protect from bulk water contact
Sequencing and Compatibility
Always follow the “shingle principle”: bottom to top, and inside to outside. Flashing tape must be compatible with WRB, sheathing, and sealants. In cold climates, select materials with freeze-flex characteristics; in hot/humid areas, prioritize UV- and mold-resistant barriers.
Field QA and Inspection
- Verify full adhesion of flashing at jamb corners and overlaps
- Check that flashing extends under head flashing and over sill flashing
- Look for compression gaps, tears, or reverse laps at tape edges
- Photograph finished rough opening before window installation
Mockup windows should include jamb detail reviews and be tested for air and water leakage under ASTM E783/E1105 protocols.
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