Chilled-Water Condensation and Vapor-Barrier Repair
A 6 in chilled-water line at 40°F runs 120 ft indoors through a pump room held at 80°F and 80% relative humidity in summer. The pipe is currently insulated with 1 in flexible elastomeric foam; seams are wrapped with ordinary duct tape, and the jacket is open at three valves and at the wall penetration. Condensation is dripping from the valve boxes and forming puddles; maintenance has added rags and a bucket. The specification requires a continuous vapor barrier and no condensation on the insulation surface. The building dew point is about 73°F. The contractor proposes removing the tape and adding a second layer of 1 in foam with butt seams staggered, then painting the outside with vapor-barrier mastic. The line runs above a finished ceiling and any leak will stain the tiles.
1. Evaluate whether the proposed 2 in of foam will prevent surface condensation; state the controlling principle and one check you would require.
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The controlling principle is that the outer surface temperature must stay above the 73°F dew point at the design room condition, not just above the pipe temperature. Two inches of closed-cell elastomeric may be enough, but it must be checked with the manufacturer's thickness table or a calculation using the 80°F and 80% RH condition and the 40°F pipe. The existing 1 in is clearly inadequate. The proposal also fails because ordinary duct tape is not a vapor barrier and painting mastic over a wet system traps moisture. Require a continuous closed-cell vapor barrier; if the calculation shows 2 in cannot keep the surface above 73°F, increase thickness or reduce room humidity.
2. Detail the vapor-barrier repairs at valves, the wall penetration, and hangers.
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Remove all tape and wet foam. At valves, use removable insulated boxes with closed-cell insulation and gasketed covers, not open jackets. At the wall penetration, seal the annular space with a vapor-barrier mastic and a sleeve; the vapor barrier must be continuous through the wall. At hangers, use high-density inserts and seal the jacket around the insert. Glue all butt seams and longitudinal seams with the manufacturer's adhesive; do not rely on tape. Terminate the vapor barrier at each end with mastic.
3. What verification would you perform before accepting the repair?
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Verify after the system is at operating temperature and the room is at design humidity. Use a contact thermometer or IR camera to check surface temperatures at seams, valves, hangers, and the wall penetration; all should be above the 73°F dew point with margin. Perform a visual check for condensation after 24-48 hours, and use a moisture meter on the foam. Pull a sample seam to confirm adhesion. Document the readings and compare them with the design calculation.