I am building an extension and have created extra space in the loft in the process with all the new roof space being covered with breathable membrane/tiles but leaving a small section (approx 3m x 3m) of original roof with felt. I wish to insulate the new loft space (between and under rafter insulation as per the rest of the new habitable area) which should present no issues where there is already breathable membrane but I need to do something where I have the original roof felt. I don't want to remove the tiles and replace the felt from the outside so I am thinking that as it is such a small area then from the inside I could cut away the felt in the "empty" area between the rafters and replace this with strips of breathable membrane. If I am careful I should be able to just squeeze the edge of the membrane between the rafters and what is left of the original felt (on top of the rafter). I could then silicone the edges to provide a seal. Alternatively, I could cut away the old felt and simply fit the membrane as a continuous sheet underneath i.e. it would touch the underside of the tiles and then run underneath the rafters. Does anyone have any views or suggestions on my thoughts? I have actually bought the insulation (Celotex) already.