I want to insulate the ground floor of the house. It is a suspended timber floor and the boards are all up anyway, for other reasons. The joists, however, are only 100 deep. What I want to do is to fix 75mm of PUR insulation to the bottom of the joists (to increase depth of insulation and to reduce thermal bridging along the joists) and then fill the space between with glass wool insulation. I know it is going to be a bit fiddly to get the insulation under the joists, but it will be possible, though I will have to cut the boards. What fixings should I use to secure the board insulation under the joists?