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So when you install wood siding over a tyvek house wrap, you take care to flash and seal all penetrations from plumbing, HVAC and electrical connections to make sure no water can get in... then you riddle it with nail holes when you nail the siding in place!

Why isn't there any concern about water penetration into the house through the nail holes? Does Tyvek seal itself around the nail holes? Or does nobody worry about it?

  • Are you going to paint or caulk over the nail holes in the siding? That's going to seal it, right? – JPhi1618 Oct 31 '17 at 16:37
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While products like Tyvek do act as a drain plane to some extent, they're not truly waterproof. They're designed to allow vapor to easily pass, and therefore they're inherently an imperfect waterproofer. They should be considered a second line of defense against flawed siding installation and occasional wind-blown water intrusion.

The flashing around windows and other penetrations has as much to do with stopping airflow as it does with stopping water. Housewrap is mostly concerned with air leakage, which can dramatically affect the energy efficiency of a building's envelope.

The bottom line is that nails in the field are less likely to be subject to water on a regular basis, and they do somewhat self-seal. Window flashing is much better at self-sealing and is used in high-risk areas. Housewrap should not be considered a substitute for proper siding drain planning. Steel and vinyl siding channels, for example, must lap properly in order to direct the flow of water outside the siding.

  • Allowing vapor to pass does not in any way affect the material's "perfection" as a waterproofer. Vapor is not liquid water, and several products stop liquid water completely but pass vapor just fine. That's what differentiates them from vapor barriers. – Ecnerwal Oct 31 '17 at 16:39
  • I agree, but I didn't want to get to pedantic. We're talking about Tyvek and similar products, which are not perfect waterproofers, probably due to manufacturing compromises. – isherwood Oct 31 '17 at 16:48
  • You'll notice that DuPont's literature language is careful to use the term "bulk water". It's fairly obvious why to me. :) – isherwood Oct 31 '17 at 16:51

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