Loft Insulation - Joist widths - Wickes Space blanket

[hemelhandyman]

I want to use Wickes Space Blanket but it only comes in 400mm width. My joists are 300mm.
Can I use it (ie compress it to 300mm) or would it likely pop out. Also, would the insulation properties be adversely affected by the compressed fibres?

 

[big-all]

still wrapped cut 2 rolls with a wood saw 300/100 and one roll 200/200

some rolls come ready part cut to allow different widths