Insulating roof space

[Beebobs]

Hi! New to forum, just wanted to ask a bit of advice...

We have a 1960s semi-detached house, with the loft converted into an 'occasional room'. Its not a full conversion as there aren't any dormers etc put in so its pretty restricted on height.

We want to insulate the eaves areas, which we were going to do with some rock wool and raise the joists to board over and provide a bit of storage space.

The bit I was uncertain about is the space in the sloping roof between the plasterboard and roofing felt. There's some polystyrene foam there but it's only 2cm thick and probably not doing much. I was considering replacing it with maybe rockwool, but was concerned about leaving space to prevent condensation building up. There's about 7-8cm depth of space.

What would you say the best way to insulate this is?

I've attached a photo in case it's helpful but it's difficult to get a clear view up there from the eaves

 

[wwwebber]

Use PIR board. If you have a non breathable membrane (likely) you must leave 50mm gap between the PIR and the membrane to prevent condensation issues. When insulating the eves do not block airflow to this 50mm gap.

 

[Leofric]

A ' warm roof ' ( insulation following slope of roof ) with non breathable membrane, like yours appears to be ,requires ventilation at the eaves and an air gap between roofing membrane and insulation as the previous post says. There should also be ventilation to the air space over the insulation at the ridge of the pitched roof.
If it happened to be breathable membrane the construction for a warm roof with insulation between rafters is slates or tiles roof finish on tantalised softwood battens on breathable membrane over rafters, ventilated air gap, insulation , vapour barrier under rafters, plasterboard and skim finish.

 

[wwwebber]

Deleted

 

[Beebobs]

Thanks for the answers guys. We're going to try some kingspan on a section and see how it goes. Will be aiming to have it up against the plasterboard and leave approx 5cm between the insulation and felt.

 

[Leofric]

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