Pitched Roof Insulation Where Pitch Continues Down into First Floor Rooms

[Pharding]

We have moved to a detached house with a pitched roof where one pitch continues inside and forms approx 800mm of the ceiling creating a partially sloping ceiling along one side of the first floor. The gap in the roof space between the ceiling and the roof felt is the depth of the roof beams; 130mm.
I notice that this section is not insulated, maybe to enable roof space ventilation.
My question is if I go ahead with the easiest solution of inserting mineral wool of glass fibre blankets into the space will it completely block any air flow or will sufficient air find its way through the blanket itself?
I could insert polystyrene slabs which could be sized to allow some air space but i'm concerned there is a higher fire risk.

[CraigShipton]

Hi Pharding

If you have a 130mm gap I personally would not fill it beyond say 75mm as you will need to keep the ventilation gap to allow air to flow out of your vented soffit.

The Glasswool material is very open but will not allow enough airflow to stop the build up of mold IMO..

The mineral wool or a polystyrene sheet would both siffice in this application (mineral wool my option) just make sure its tied into place somewhat as you dont want it sagging or slipping down and covering the soffit vents

Hope you manage to get this sorted out

Regards
Craig Shipton

[Pharding]

Thanks Craig, I get your point that ventilation is the main criteria.