There's no such thing as waterproof siding. Over time, moisture will work its way behind even properly installed siding. Housewrap provides good protection against the moisture that does get in, but it must be detailed properly at windows and other penetrations to work.
In this article, I'll explain the housewrap flashing details I use when installing flanged window units. These details will prevent any water that gets past the siding from damaging the windows or rotting the framing.
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