I have insulated a 'sweaty' pipe to prevent condensation from forming and dripping on to ceiling tiles (hyperlink provides context). I have been advised that the insulation foam should have the slit at the 6PM position.

  • What is the reason to orient the slit at 6PM and not 12PM?


The PVC pipe in questions carries cold water from second-story (AC evaporator) to the outside. Because it runs above paneled ceiling on the first floor, the pipe 'sweats' and the condensation drips onto the ceiling tiles (resulting in stains and mold).

  • I would buy the insulation with the glue installed so no piping was exposed.
    – d.george
    Commented Jul 23, 2017 at 9:35

2 Answers 2


Because there will still be a tiny bit of air access and thus water condensation. You want the insulation acting like an inverted bucket, not a bucket that will catch the water and hold it against the pipe. Catching the water will cause metal corrosion, mold, and waterlog the insulation.

Making the insulation heavier will also tend to make it want to fall downward. If the top of the insulation is a crack, it will tend to want to yield and slip out. Whereas if the crack is at the bottom, the insulation is "just sitting" on the pipe.


Speculation would be that as heat rises more heat would be lost through the slit. The more expensive foam insulation has a self sealing adhesive film to seal the seam. At some measurable level the sealed seam would be less effective than continuous foam.

  • 1
    Heat doesn't rise. Parcels of fluids suspended in cooler fluid tend to rise due to lower density. Heat has no density in that respect. Unless there's a substantial air gap, this answer doesn't apply.
    – isherwood
    Commented Jul 23, 2017 at 12:12

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