Insulating Gaps between joists & wall

[nheyward]

Hi,

First-time poster here looking for some advice before I embark on upgrading the insulation in the loft space of our single-storey extension!
Current insulation in the loft is a single layer between the ceiling joists, and I'm looking to upgrade by putting a second perpendicular layer on top.

A question I have is about a gap of around 2 inches between the first joist and the original brick wall of the house (where the extension starts). Should I insulate this gap or will it create a bridge between the wall and joist for moisture? It's not an external wall as it's between the house and the extension, but it is a solid brick wall.
If I should insulate, can I use cuts of the fibre-roll or is expanding spray best?

Thanks in advance!
N

[welsh brickie]

you can use either, but the fibreglass will be cheaper