Don't waste your time and money - the quilted foil products are a rip-off.
Despite manufacturer's claims, they are nowhere near as good as they lead people to believe. A recent court case determined that their testing methods, and subsequent advertising claims, were flawed.
The quilts work almost solely by reflecting radiant heat. The layers of wadding between the foils have practically no effect on the total insulation value of the product, because they are so thin.
To have any effect at all, both sides of the foil have to be facing an air space, so fixing board directly underneath the foil defeats the object. In that case, the insulation value is only the reflected heat from one exposed surface, which will be quite small.
You would get the same insulation value, at a fraction of the cost, by applying a layer of foil-backed plasterboard to the joists first (foil facing away from the room), and then your second layer of plasterboard under that.