How to calculate the dew point / how VCL works

[wonderwire]

I'm insulating a lath and plaster ceiling, which means I can't place a vapour control layer beneath the joists as is recommended.

I am using a mixture of 170mm mineral wool and 50mm celotex insulating board. Electrical wires run over the insulation (not under it!), with 38mm battons spacing the chipboard above.

Assuming I don't insulate the rafters, is it worth running a vapour barrier up and down the joists, so it's under the insulation?

Otherwise I assume this would mean the due point would probably be on top of the insulating board (I assume the insulation stays warm)?

Thanks

 

[^woody^]

You dont need a VCL.

And there is no advantage in trying to install a VCL in that way as drawn - which in relality would not be acheivable and would leave lots of creases and voids which themselves become condensation traps.

You should not leave any air gap between the wool and the board either.