Our contractor installed an AC compressor and did a poor job of it.
He ran the AC coolant lines through the middle of the wall and then just squirted some expanding foam into the hole, like so:

(Click images for larger views.)
Bogus flashing 1 Bogus flashing 2

What is the correct way to flash and/or seal this opening?

Around here, the way its usually done on hardboard-sided houses is to pass the lines underneath, in the crawl space and through a vent, like so:

Typical line routing

But I guess that's not an option in this case?

  • 1
    The installers on the other homes must have intentionally place the outdoor unit near the vent, for simplicity of install. I have only see retro hack installs done this way. The installer bends the screen out of the way and violates its purpose for vermin proofing and runs the line. All new installs has the siding cut around the line and looks just like your install does. You can or could use one of the flashings in another post, but some types do no more than what the pipe does already, with the exception of protecting the black foam pipe wrap from deterioration. Which is pretty good.
    – Jack
    Jan 6, 2016 at 16:50
  • @Jack, Didn't say I liked that vent pass-thru, but it is much more weatherproof. The photo came from one of about 80 houses in a newish subdivision. They were all built like that. Jan 6, 2016 at 20:30
  • If the same person did the disconnect installation I would also be concerned about how watertight the cable penetration through the wall is there. Not to mention air leakage if this is a tightly sealed house. Sep 18, 2018 at 21:31
  • @DaveInCaz, and you would be right to be concerned. Disconnect penetration was fixed by me before this question even posted. Couldn't think of a professional/durable/cosmetic way to do the pipes though. Sep 18, 2018 at 23:21
  • We normally enter the home through the wall and calk the opening. Going through the vent in my area is not allowed, you can bore a hole through the foundation, but most lines run in the wall / ceilings to the air handler so this method is industry standard in my opinion.
    – Ed Beal
    Sep 19, 2018 at 13:46

3 Answers 3


There are all kinds of options, but the cleanest are probably purpose made flashings specifically for AC lines, like these ones:

enter image description hereenter image description here

Note that it is very important to tie a flashing like this into the overall water-resistive barrier to the house, similar to a flanged window.

The "vent" entrance method would only work on a raised foundation structure, the home in your picture appears to be built on a concrete slab.

  • 1
    Those things look interesting, but how do they keep out the critters?
    – Tester101
    Jan 6, 2016 at 12:18
  • 1
    It has a special rubber seal up inside where refrigerant or other lines penetrate. Jan 6, 2016 at 12:24
  • Tough to use on T-111 in OPs situation, although he could surface mount it, seal with caulk, and paint it maybe. AC tech would have to disconnect those lines though. Jan 6, 2016 at 12:28
  • Yeah, I was looking for something like this. (My Google-fu had failed me.) Jan 6, 2016 at 20:15
  • 2
    @Damon, the OP simply asked what was the best way to do the job. In my opinion a finished flashing is better than spray foam. OP did not ask for retro-fit solution. If you think a manufactured flashing made specifically for the purpose is not the best way, then down-vote if you want and/or propose something better so we can vote on your solution. PS- I can think of retro-fit ways to utilize that flashing that would not "completely negate it's purpose", it would work way better then spray foam and goop, and look professional as well. Jan 7, 2016 at 1:05

Depends on where the indoor unit is that he had to connect to, in comparison of where the other were in your neighbors house. The important thing is, the tech did slope the pipe down and out of the wall at a heck of a good angle. The only problem I really see is the type of sealant he used as a finish, unless the plan was to come back, carve off the excess foam and use that as a backer for a good brand od polyurethane caulk to finish sealing the hole where the pipe goes into the wall.

The foam does a great job of insulating around the pipe in the wall, in an otherwise difficult area to seal.

If the tech never comes back, all that is needed now is as mentioned before, is to clean the foam back enough to leave a small recess, say 1/4", not a "dished out" 1/4" but a clean, flat recess that has the edge of the cut exposed, not covered by a skin of foam, so the caulk bonds to the siding, not foam that will degrade over time by exposure. If a little more is removed than the 1/4" I spec'd, no biggy, it is just more caulk, and it will be a little less flexible.

Use tape to cover areas you do not want caulked, poly caulk is contrary, but it cleans up with paint thinner... Still messy when not prepared for it.

  • 1
    It's not pretty but there's no question this is the "right" answer. However, after years the slightly-exposed edges of the wood can rot slowly. If possible, liberally prime & paint the wood after trimming back the foam. That way moisture will stay out of the wood.
    – pbristow
    Jan 6, 2016 at 14:01
  • Perhaps I need to detail the answer better, where I wrote "but a clean, flat recess that has the edge of the cut exposed, not covered by a skin of foam, so the caulk bonds to the siding," The caulk is to seal the siding from water infiltration. The tape mentioned will be keeping the amount of caulk that is needed to seal to the siding to a required minimum, while keeping it neat.
    – Jack
    Jan 6, 2016 at 16:42
  • Oh sorry, I missed the comment about the caulk. Yeah caulk or paint... something to protect the wood. Good call.
    – pbristow
    Jan 6, 2016 at 18:20
  • How is this a good solution? It will still look ugly as sin and the caulk will not last in the sun and vibration. We'd have to reseal it every year or two, which is unacceptable. Jan 6, 2016 at 20:12
  • 2
    The poly caulk I refer to, is VERY flexible, and please research this, it has a very long life span, MUCH longer than 2 or 3 years, it should be 15 or 20, long enough to be checked or redone if needed when the house is repainted. Of course you could use a flashing that will have to get sealed with caulk in the same fashion and have to look at that too. The vent the other guys used is violating the purpose of what it was intended, and you are ok with that? What you have and what I propose is by far more than what is usually done in this type of situation. Or, you could use the flashing.
    – Jack
    Jan 6, 2016 at 20:58

Duct Seal is a common product used in the industry to seal around wall penetrations. It stays soft and flexible, so it handles expansion/contraction well. It's fairly easy to remove, and reapply or rework.

enter image description here

  • Yeah I like this stuff. Have you seen it used in a vibration and temperature-extreme scenario? Also, this would still look rather ugly. ;) Jan 6, 2016 at 20:20
  • @BrockAdams It's paintable, if that makes it any more appealing.
    – Tester101
    Jan 6, 2016 at 20:21
  • 2
    I have seen the aftermath of using it, I am sorry to say it is a standard way of sealing just this type of opening. It fills the hole but does not stick.
    – Jack
    Jan 6, 2016 at 20:52

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