Draft from under the first floor level

[waherne]

I have a serious draft from under my first floor bedroom, the ground floor room below the bedroom is also approx 8 degC colder than the rest of the house. When I lift a small section of floor boards at first floor level, its similar to opening a window! The bedroom is located at the back of the house, where there is also a tiled roof on the ground floor extension to the back of the house.

1. Would sealing with foam or Rockwool in-between the floor joists along the external wall at the perimeter solve this problem? or alternatively completely insulating between the first joists running parallel to the back wall?