wet garden shed

[saanen]

I bought a garden shed (new) a year and a half ago. It is six feet by eight, standard make with wooden tongue-in-groove boards with a pitched felt-covered roof. It was treated with wood preservative before we bought it.
In the last few weeks I have noticed that some of the boards on two of the inside walls of the shed are soaking wet, with water dripping from them. The boards above and below the wet ones are quite dry. There is no obvious gap in the tongue-in-groove, and there are no knot-holes, so it seems as if the water is seeping through the wood. Is there something I can paint on the outside to prevent this happening? Does anyone have any ideas about what I could use to waterproof the shed, and why is it affecting some of the boards, but not others? Any advice would be appreciated.

[curlyfilm]

i use johnsons waterseal, on mine, its for brickwork but is an excellent water proofer on wood, just wait for it to dry thouroughly before application