Insulating suspended floor with multifoil?

[greensleeves08]

Hi, I have a 1970's house with a suspended floor. During the winter the floor gets very cold and I want to fix this.
I have access to under the floor through a small hatch.
I can not lift the floorboards up to insulate from above, so am trying to do it from below.
The floor currently has approx 4 inches of normal insulation (the itchy stuff) between the joists.
I was thinking about topping this insulation up with multifoil, my question is, would it be OK to staple this into the underside of the suspended joists, or does it have to go over the top of each joist? And would the screwfix superquilt stuff be OK to use?

Hope to hear from someone soon.

Many thanks

 

[mointainwalker]

Highly contentious product. Many people do not believe it works very well at all and what you are proposing to do does not meet fitting requirements, which are 25 mm still air-gap on both sides of the foil.