Ceiling Drips Six Months After Chilled Water Insulation

Start with the numbers. At 24 °C and 60 % RH the dew point is about 16 °C. Intact jacket at about 20 °C is above the dew point, so it stays dry - that tells you the 80 mm thickness is doing its job in these conditions and the space humidity alone is not causing the problem. Joints reading 9–11 °C are below the dew point by a wide margin, meaning the cold pipe surface is effectively exposed at those joints: the vapour retarder is broken and moist room air is reaching the cold surface, condensing, and wetting the insulation.

So this is a vapour retarder continuity failure, not a thickness failure and not a fibreglass failure. Contributing causes are visible in the evidence: cloth duct tape is not a vapour retarder - its adhesive dries out and lifts, and its permeance is far too high; reversed laps give water and air a path under the free edge; hangers crush the insulation and puncture the jacket, creating both a thermal bridge and a breach. The absence of joint-sealing photographs and mastic data suggests the joints were never properly sealed in the first place, which makes this a workmanship and inspection failure rather than a product failure.

Repair has to remove the cause, not just dry the ceiling.

1. Map it: open the ceiling, photograph, and mark every joint, hanger and fitting with a reading. Record surface temperatures and moisture readings before anything is touched.
2. Cut back to sound material: remove insulation until you reach material that is dry and whose jacket is intact and correctly sealed. Wet fibreglass has lost thermal performance and holds water against the pipe, so it is a corrosion and mould risk and cannot be left in place. Bag and dispose of it; do not try to dry it in situ behind a sealed jacket.
3. Replace with the same product and thickness, with a properly lapped ASJ: laps oriented to shed and with the free edge away from the likely air/water path, and every longitudinal and circumferential joint bedded in a vapour-retarder mastic or ASJ sealant rated for the purpose (look for a permeance of about 0.02 perm or better, not duct tape). Fittings get pre-formed covers with mastic-sealed overlaps, not a wrap of tape.
4. Hangers: replace the crushed sections with high-density inserts or saddles, jacket cut cleanly and sealed, so the vapour retarder is continuous over the support.
5. Verify: re-check surface temperatures at joints after a few days of operation and confirm they sit at or near the intact-jacket reading.

Document all of it with photographs at each stage; that record is the evidence that the defect was corrected.

The specification described an ASJ product but never defined the vapour retarder as a system, so the site filled the gap with duct tape. Fix that in writing.

• Name the sealant and the performance: vapour-retarder mastic or ASJ sealant with a stated permeance, applied to every joint and fitting, with tape permitted only as a temporary hold. List the acceptable products.
• Require the design check: confirm the jacket surface temperature against the design dew point for the space, and where the surface cannot stay above dew point, say explicitly that the vapour retarder must be continuous and that this is a hold point.
• Detail hangers and supports on the drawing, including high-density inserts at every support and a prohibition on crushing or puncturing the jacket.
• Inspection hold points: joints and fittings are photographed and signed off before jacketing is banded and before the ceiling closes; no ceiling closure without a signed joint-sealing record. Sample at least a defined percentage of joints per area and record the readings.
• Cure time and conditions: state the temperature and cleanliness requirements for the sealant, since mastic applied to a damp or dusty jacket will fail the same way.
• Handover pack: product data, batch numbers, photographs, and a moisture survey at practical completion, so the next failure is traceable to a person and a date rather than to 'the tape'.