Backer rod external and internal..?

[carax]

Installing windows for the new construction. There is horizontal paneling on the outside of a wooden frame.

Doing a classic installation with backer rod and caulking. However, the carpenter suggests also placing a backer rod on the outside to prevent it from driving into the insulation.

Is it possible to have backer rods both outside and inside the insulation?

 

[carax]

I have read that it is important for the inside to be vapor-tight and the outside to be vapor-open. The question is whether the backer rod allows moisture to pass through? In other words, is it vapor-open? If it is, then there shouldn't be any problem in placing one on the outside as well??

You seal on the inside, which I assume creates the vapor barrier?

Help me...

 

[Ewald]

Old thread, I know, but here is a good manual on the subject:

https://www.sp.se/sv/index/services/buildingsystem/Documents/montagestudie.pdf