Cold roof ventilation

[Morninglory]

Hi,

Do my rafters need more ventilation? I'm redoing the existing loft conversion in my house where the original rafters sit on wall plates. I'm planning on doubling up the rafters for depth, and insulating between the new rafters, then insulation backed plasterboard on top. Will they get enough ventilation through the bottom if I put in insulation as below? Will I need a vapour control barrier or membrane as well as the vapour control from the insulation backed plasterboard?

Thanks in advance

 

[blup]

Dropping the rafter cuts off the path for the cold air to exit at the soffits, or is there no insulation between the new rafters?

Blup

 

[Morninglory]

Hi. Thanks for replying.

There is no soffit for ventilation. The original rafters have sat on the wall plates since it was built in the 1890s. The new rafters I'm attaching to the originals are just for the depth of insulation required. I'm just curious about how to manage ventilation with this detail, or if the trickle ventilation from general gaps will be adequate because of the vapour control that will get put in.

 

[blup]

Maybe the answer is to leave gaps at the base of the rafters.

Blup