I've built a summerhouse at the bottom of the garden, and when testing moisture levels (before laying laminate flooring) I noticed that the baseplate, of the front timber frame wall, is registering over 22% moisture - moisture levels at the back and sides are around 14%. Moisture levels outside - the cladding, subfloor and foundation are registering 12 to 14%.
Construction as follows: railway sleeper foundation, then a layer of DPC, timber frame/OSB sub-floor, another layer of DPC on the top of the OSB and then wrapped down the side of the subfloor, baseplate and timber frame above.
The timber frame is wrapped in breathable membrane, then a layer of featheredge cladding secured onto batons. The timber frame (unintentionally) overhangs the subfloor by 1cm and are secured using 25 cm timber screws, driven into through the subfloor and into the foundations. I've attached a diagram which hopefully helps.
Where is the moisture coming from? Should I be worried and what should I do about it?
Also, I was going to use foil insulation on the inside as a vapour barrier. Is this a good idea and which way should the foil face? Insulation is Wickes own brand.
I'm a complete novice and any help, suggestions will be gratefully appreciated.