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I understand that closed-cell spray foam can be waterproof, but is it an appropriate material to use for filling around the external wall penetrations where AC lines pass through?

I don't know if it is simply not advisable to ever use spray foam to create a watertight seal or if it is an installation glitch, but we are having leaks from water dripping along the AC lines into our house. It seems no spray foam was used in between the drain line and the refrigeration line, which is understandable as the foam would expand in between the two lines. However, that does leave a small gap.

If spray foam is not a good choice for this application, what's a good remedy? Sealing on top with a silicone caulk?

  1. The lines are insulated and covered by a line hide so the foam may survive UV longer.

  2. We would see a leak after raining while the AC was not running, so it was not due to the condensation.

I've added the following as a comment, not sure why it is considered duplicate:

"Compared to the various linked questions on what can be used to seal AC lines, my question is not so much about what I should use if I were to do this again from scratch myself -- I would probably use duct seal or some outdoor-rated silicone -- but now that spray foam is already used, whether it is fundamentally problematic and would always be leaky. If so, what's the best way to remedy (e.g. scrape or caulk on top)."

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    Hi, but the first question you ask is whether the foam is appropriate. If its too late and it's already used, then please remove that question. In this form, I cannot vote to open. And your title is the same. Try changing them to say something like how do I fix the leak Commented Sep 29, 2023 at 22:13
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    @RohitGupta Thanks for the clarification. I'll open a new question about fixing the leak, as they are still leaking even after applying another layer of silicone on top. However, regarding this one, if you check the linked question to which mine is considered a duplicate, the answer there suggested duct seal but does not answer my question: whether spray foam is appropriate or fundamentally problematic and would always be leaky.
    – P. B.
    Commented Sep 29, 2023 at 23:30

2 Answers 2

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I'd say that the spray foam is "appropriate, but suboptimal." If there's a gaping oversized hole, then spray foam is the best product--just be sure to buy spray foam with UV resistance (Loctite makes such a product, but I don't know if UV resistance is common among other foams). If the hole size match is pretty good, then I would use Henry 212 ("Crystal Clear" sealant which is typically located around roofing stuff at the hardware store). If the hole size match is pretty good and you have silicone caulk leftover from another project, then I would use that.

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  • I don't think it's at all appropriate. It'll degrade in sunlight and soaks up water. It must be removed or covered, in my opinion. Gaping holes should be eliminated, not filled. Fix the wall.
    – isherwood
    Commented Oct 3, 2023 at 19:39
  • @isherwood, you can even flush cut it and then paint it.
    – popham
    Commented Oct 3, 2023 at 19:42
  • Paint on foam is never going to cut the mustard, in my book. I can hardly tolerate stucco on foam, as is the modern trend. It's fragile and usually ugly.
    – isherwood
    Commented Oct 3, 2023 at 19:43
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The spray foam can be used to fill a larger void near the exterior.

However any foam exposed to the sun must be covered to prevent degradation.

This can be simply painting or covering with exterior grade caulk.

Before using the foam, you should be certain that the water is coming from outside and not condensation on the AC lines. Sealing a hole may just seal condensation in the wall.

If the problem is condensation, the pipes should be covered with a foam insulation. Then if needed the foam can be used to seal the opening at the exterior.

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  • To clarify: Caulking on top of the foam is acceptable for further waterproofing the penetrations? That would make things a lot easier without having to remove the foam. I've also edited my post to add: 1) The lines are insulated and covered by a line hide so the foam may survive UV longer. 2) We would see a leak after raining while the AC was not running, so it was not due to the condensation.
    – P. B.
    Commented Sep 28, 2023 at 17:15
  • The Loctite foam has UV resistance. I don't know about other products, though.
    – popham
    Commented Sep 28, 2023 at 17:21
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    @P.B. , The caulking on top of foam would provide both secondary sealing and protection to the foam, if needed.
    – RMDman
    Commented Sep 28, 2023 at 17:36

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