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We're going to use metal siding on our house. We're using a corrugated profile. Where the siding meets walls/corners/windows, it will run into a strip of C-channel (or J-channel).

Visually, that looks great, but doesn't seem to be much of an actual weatherproof seal. I've searched quite a bit and even asked our siding provider if that is sufficient but there doesn't seem to be a whole lot of info out there on installing metal siding horizontally like this--at least not at the residential level.

Is it OK just to have the siding run into the J-channel? Or should there be more of a seal between the two?

The only possible viable option I've found is expandable foam tape. The idea would be we'd run that along the inside of the C-channel, and then when the siding is inserted that'd create a bit more of a seal.

Any other thoughts/suggestions?

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Here's a rendering of the issue. The orange(inside)/blue(outside) is the J channel along the side of the window. The corrugated siding then is inserted into the channel.

  • How about a picture of what you're concerned about. – JACK Oct 22 '19 at 20:05
  • @JACK good suggestion. Picture added! – DA01 Oct 22 '19 at 20:26
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I'm not a professional but I've replaced a few siding panels. In all the houses I've lived in (US), there wasn't any insulation or seal in the J or C channel. The one thing I wonder about is the colors. All of them I've seen the color matched the siding making gaps even less noticeable, I don't know if your channel actually has an orange interior. If there were a seal to be concerned about, it would be between the channel and the wall.

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  • Not worried about aesthetics (the J-channel ends up covering the end of the siding). But wondering about weather. There's really nothing stopping driven rain from being blown into the J-channel and then behind the siding. Which maybe isn't a big deal. I don't know (we do have a proper housewrap behind it) – DA01 Oct 22 '19 at 21:18
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The weather proofing and sealing should be done prior to installing the siding. The "J" channel will be installed against he window trim which should be sealed. I've never seen any sealing inside the channel. The siding will expand and contract so any sealing, like caulk for foam tape, wouldn't be effective for long. Good luck

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  • The windows are sealed. NO worried there. Just trying to avoid excessive water getting in behind the siding. – DA01 Oct 22 '19 at 22:05
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    Any water that gets in there will just run down the channel and out. – JACK Oct 22 '19 at 22:09
  • I don't think that's entirely the case. While most water will run out, there is no seal between the panel and the J-channel. It just sits in there. So any wind-driven rain can easily wick in behind the panel. But maybe that's not as big of a deal as I think it may be. – DA01 Oct 22 '19 at 22:17
  • I think you're over thinking this. Sliding glass doors, in house or showers, have J channel that gets water thrown into them, aluminum windows have tracks and channels that get water into them. the water runs out and if a small amount remains, it evaporates. – JACK Oct 23 '19 at 13:18
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It is a fair question. My experience is the panels just rest inside the J trim. If water does wick into the back side of the J trim then it will just be on top of your WRB and assuming you have rainscreen/air gap behind your siding then that water will either evaporate or drain out to your bottom flashing or a mixture of the two. It shouldn't be that much water. Really a building is pretty tight with just WRB if installed properly.

What is your siding contractor doing about the j-trim to window frame interface. Do you caulk this everywhere except the bottom or do nothing about it?

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