John wrote:
Prior to putting the shiplap on a shed wall I though it would make it more
watertight and windtight to put a dampproof membrane. Had a look in wickes
and couldn't find a suitable product. Is this what Tyvek is for? What's a
suitable cheap option (and will I find it in a warehouse (open Sunday))?

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I wouldn't. You really need air flow both sides of the wood to avoid rot