Stop shed wall cladding leaking/water ingress

Evening all,

Long story short. I have a shed (20x12) with loglap cladding treated with two coats of a decent solvent based wood preservative. I've had it a year but haven't been happy with it as have had a few issues.

I need to insulate it as it is to be used for a model railway.
The problem is that there are a lot of knots in the wood and when it rains heavily the water seems to soak through the knots. I've been covering the knots in mastic as and when I find a leaking one but it takes a lot of time (I've been tipping water down walls to fault find). A friend has suggested painting the entire inside with a product called 'Black Jack' which is a bitumin liquid dpm. What are people's thoughts on this please?

I have a roll of tyvek membrane which I planned to retrofit (staple to cladding) in between joists and then place 50mm celotex insulation in and overboard with osb3. I plan to leave a 10mm gap between the tyvek and the foil faced celotex for air movement to try and combat damp buildup.

I guess what I'm asking is would you recommend the 'black jack' treatment as well or just go for the tyvek approach knowing that in very heavy rain there is some water ingress through the cladding?

Any thoughts/advice appreciated.

Thanks