Would you insulate this roof space?

[Scarletpimpofnel]

My (old) house has some internal sloping ceilings similar to that shown below.

Essentially the ceiling plasterboard is nailed to one side of a rafter that has the battens supporting the roof tiles nailed to the other side. That leaves a gap of about 3 inches. This gap is currently uninsulated.

My mother always says to not block the eaves to allow air to flow through this gap. On the other hand its a cold space, should I pour micafil or similar down the gap? It is otherwise inaccesible for anything other than the likes of micafil.

Should I or shouldn't I?

(I have insulation where the red arrow is, the area I am talking about is where the blue arrow is) -

[8-P]

I think the risk is of moisture transfer from roof to plasterboard.

[Inbox]

No, you will block off the ventilation into the loft which is bad news.

[essayer]

Our house has solid insulation in the sloping part with an air gap to the roof. Might be an option but maybe a pain to retrofit

You can see in the thermal camera where the cross beam prevents a consistent join between the mineral wool and the solid insulation - prone to condensation here

Edited by essayer on Tuesday 25th November 19:50

[Aluminati]

If you maintain the gap in the rafters as your diagram, you’ll be fine. If you have a 75 mm rafter ( Unlikely) maintain 50 mm airflow. 25mm of pir insulation doesn’t sound much, but better than nothing.

Check if you have any over fascia or soffit ventilation. Do not push the insulation past the wall plate either.