Insulating between rafters - purlin detail

[Frankofthenorth]

I'm in the process of insulating between and under the rafters in an Victorian attic bedroom - decided to go with PIR due to the lack of rafters depth, and have finished the between rafters layer, so too late to switch tactic now to a breathable system.

There is an exposed purlin, which supports the rafters at mid span and my question is how to I achieve a VCL around the purlin. I presumed the VCL should go behind the purlin however where the rafters touch the purlin this isn't possible so I would end up with holes at these points which could lead to damp timbers. I wasn't planning to add insulation around the whole purlin as this would be on the warm internal side of the insulation. Would Installing the VCL on the internal face of the purlin be a bad idea which could trap moisture in the purlin and stop it drying out?

 

[blup]

Can you not insulate over the purlins to get a continuous line

 

[Frankofthenorth]

Can you not insulate over the purlins to get a continuous line

It's an option however I'm not sure if restricting the purlins ability to breathe and trapping it on the cold side of the insulation is the best idea?

 

[blup]

It's an option however I'm not sure if restricting the purlins ability to breathe and trapping it on the cold side of the insulation is the best idea?

If you are not using the loft as living space, it doesn't need insulating - properly insulated the loft floor will protect the house

 

[Frankofthenorth]

If you are not using the loft as living space, it doesn't need insulating - properly insulated the loft floor will protect the house

It is an existing room so hence wanting to insulate at rafter level

 

[blup]

It is an existing room so hence wanting to insulate at rafter level

You probably have to lose some height to maintain a continuous air gap from ridge to eaves