Tri Iso Multipfoil in loft between rafters (T intersection)

[catgordon1]

I have a 1900 house with clay tiles and deteriorating roof felt.

The loft was converted to a bedroom and now only a small loft cupboad remains at the front of the house. Despite building regs certificates being provided when we bought the ouse the builders did a shody job and the top floor is freezing.

My roofer, who recently fixed the valleys after heavy rainfall, mentioned that I could consider retrofitting using a multifoil such as Tri Iso Super 10.

I have purchased some Tri Iso and am intending to install using staple gun to rafters then aluminum foil tape to seal overlaps. I might xonder further battens and plasterboard depening on how the first stage works out but my 2 questions are:

1. I have a T intersection roof with a triangular brick section at front of loft space. Should I be using multifoil to cover this vertical brickwork, or only use it between rafters on the sloping parts of the roof?

2. The loft space is a fiddly cobination of lots of triangular sections linked by valleys - should I be cutting my multifoil into the minimum number of pieces to avoid air leakage, OR can I maximise the expensive 10m2 rolls I have by using lots of smaller triangular and square pieces and relying on the overlap and tape to make it all airtight?

Do I cut the pieces then fit or apply the roll and cut in situ?

Any advice gratefully received!