Loft insulation queries

[John74813]

Hello


My house, constructed in 2006 and extended in 2017 to include a bedroom with a vaulted ceiling, has only 100mm of insulation in the loft area. I'm considering adding more insulation (aware that I would unfortunately have to remove the existing boarding) and would greatly appreciate advice on the following:

1. Is there even any point insulating given I have several dormer windows which presumably I cannot retrofit additional insulation into?
2. Is there adequate ventilation in the loft for additional insulation? From what I can tell, there is no eaves ventilation, and only one roof tile vent. Unsure if the felt membrane is breathable, (suspect not though else a tile vent wouldn't have been needed?)
3. Any insight into why the main loft space is plasterboarded on the lower part of the roof truss?

Attached are pictures of the main loft area, the section leading to the vaulted ceiling, and a gap between the plasterboard and the roof line in the truss, showing the membrane (for assessment of breathability).

Appreciate any guidance on these queries.

Thank you.

 

[John74813]

Hi, aware I posted this on Boxing day when others were likely not on their computers! Any advice from members on the forum would be much appreciated. Thank you in advance