The jacket is the vapour barrier
Duct wrap insulation is faced with foil-scrim-kraft, a laminate of aluminium foil, glass scrim and kraft paper. That facing is the vapour barrier, so every seam, staple line and penetration has to be sealed or the insulation absorbs moisture. Foil-scrim-kraft tape is the matching closure.
FSK tape carries the same laminate as the jacket, so it moves, expands and reflects identically. Plain foil tape will also work on a clean seam, but it tears more easily and offers less tensile strength across a butt joint. For a jacket that will be inspected, matching the tape to the facing is the defensible choice.
Closing a jacket properly
Cut the jacket with a straight edge, butt the ends, and tape with a 50 mm overlap onto each side. Roll the full length so the adhesive wets out, then seal the staple line separately, because staples punch holes straight through the barrier. At hangers, tape a patch over the compressed area rather than wrapping tape around the duct.
- Use 63 mm or 75 mm tape so it spans the joint plus 25 mm each side
- Apply at room temperature; FSK adhesive struggles below 10 degrees Celsius
- Keep tape off the aluminium outer jacket of pre-insulated panels
- Photograph each sealed joint before the ceiling closes
Condensation is the failure that shows up a season later. A pinhole at a seam lets humid air reach the cold duct surface, and the resulting drip stains a ceiling far from the actual defect. That is why inspectors run a hand over every joint before signing off, and why a second pass with the roller pays off.
FSK tape is supplied in 50 m rolls at 63 mm, 75 mm or 100 mm, usually 16 to 24 rolls per carton. On a large project, order the same width as the jacket overlap so installers do not improvise and leave a gap.
