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I got new siding installed last year (Hardie board). They replaced the sheathing as well (it was that crumbly tar covered stuff) with OSB. They also installed house wrap (as I previously did not have such a luxury).

While they were at it, they replaced the insulation (from the outside) as well - it was decades old and had been thoroughly chewed through by mice.

But now I've got this odd condensation pattern outside. The solid line across the top is the attic floor, and the wet parts down the wall seem to basically match up with the studs of the inside wall.

Have you seen this before, or have any theories on what could be causing this?

enter image description here

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  • Was a vapor barrier installed? Where in the world do you live?
    – FreeMan
    Jan 8 at 18:19
  • Nothing was installed inside anyway - the drywall remained intact. The contract says they used faced fiberglass, but.. obviously it couldn't be stapled to the inside like you'd normally do.
    – negacao
    Jan 8 at 18:36
  • Which way does the facing face and what part of the world do you live in? The vapor barrier changes sides depending on heating/cooling requirements and may have been installed backwards.
    – FreeMan
    Jan 8 at 18:39
  • Ah sorry, in southwestern Pennsylvania. As far as I can see, the insulation paper is facing inside (poking around some outlets where I can).
    – negacao
    Jan 8 at 18:43
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    Is it like that all day or just 1st thing in the morning? Is the attic vented? What was the old siding? How does the temperature change night to day when this happens? Jan 8 at 19:29

1 Answer 1

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Assuming that it was cold outside when this picture was taken and that the stud pattern is dry (i.e. condensation forms over insulated cavities), I think this is a byproduct of thermal bridging. The strange pattern is nothing to worry about. It shows that your insulation is working.

Wood conducts heat better than fiberglass insulation. Warmth inside your house is traveling outside via the wood framing; condensation forms on the colder parts of the siding but not the warmer. It probably didn't happen before renovation because your insulation was terrible and the entire wall was warmer. I don't think the sheathing or water-resistant barrier are factors.

The same effect is common in cold climates: You'll see the framing visible in frost patterns unless the builder put rigid foam insulation outside the framing.

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