Please please do not do that as it's a complete waste of time and money! It will not insulate your loft at all and you are only paper overing the cracks so to speak because it's leaking it will just carry on leaking causing more damage to your timbers which then become a major problem. The money you spend on that will probably come 70% towards having a brand new roof tiles/felt and battens. So either have it repaired properly or consider a full replacement.
Hope this helps.