I have newly moved to a property with a small cottage. It is located in a hot desert climate, close to the sea, with occasional rain during the winter.

The cottage is made from relatively thin planks and has rain damage inside. It's cheaply built, but I would love to preserve it.

The water damage is mainly on the back wall, which is on the rainy side and leans against a stone wall which is about three feet tall.

The walls look like this. (This is one of the side walls.)

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The damage from inside is visible in this terrible photograph:

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It appears the water seeps through the cracks, both from the rain and perhaps from what moisture the stone wall is preserving.

What are good ways to protect a wall like this from water creeping in?

Covering it in tarboard from top to bottom seems to be an option (and is available) but I'm not sure whether it is ideal.

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