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"Ghosting" on hardi-type siding


Jim Baird
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It's a full open eastern exposure. Other side is shaded by adjacent house for part of the day. I too think it is temp diff between area over framing and area over substrate, which is likely half inch isocyanurate and likely foil covered where structural sheathing is not applied. Also is likely the only coat of paint it ever had.

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Probably the same phenomenon as ghosting on the interior with candles, etc. Dust particulates depositing unevenly due to thermal differences showing thermal bridging at studs, etc.

I doubt there is foam board in those areas, but then 1/2 inch is pretty skimpy for much thermal benefit.

I thought the same thing, but it won't scrub off. The vinyl itself actually changes color.

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The photograph gives me the impression that there is also a dark horizontal strip approximately where the 2nd floor rim joist would lie. It appears to match the color of the suspected stud markings. If that coincides with the actual appearance, the framing is certainly affecting the issue if not directly causing it.

Another thought is that periodic, subsurface moisture might have "washed" some of the zinc from the fasteners slowly over time. Precise nailing on the studs would keep the drainage rather vertical. Weep holes would drain the wash close to the nails.

Are there any evidence of irrigation over-spray or misting?

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