Insulation under floorboards

I'm a bit stuck! I bought 200mm insulation to go under the floor boards in a ground floor room I'm decorating. 1930's house.

After taking up the floor, I see that the joists are 4x2" rather than the 6x2" I was expecting.
I will use chicken wire to hold the insulation in place between the joists, but should I :

1/ Take it back and get 100mm or 150mm instead.

2/ squash the 200mm down to 100mm (level with bottom of joists)

3/ lay it at 200mm so it will be 100mm below the joists.


I will ensure that the underside of the joists are not covered to allow air to them.

There is approx. 12" (300mm) of crawl space (right term?) from the bottom of the joists to the floor.

Is this enough for air circulation if i add the insulation?

We live in a high radon area so air circulation under the floor is essential.
The room is always cold partly due to the solid wall construction and partly due to cold air under the floor coming up through the carpet. The floor is always freezing on bare feet. I am hoping this will warm the room a bit. Or maybe the room is haunted lol.

I would go for option 1 and use 100mm. Option 2, compressing the insulation, will just reduce its efficiency and therefore be less cost effective (assuming the 200mm is a lot more costly than 100mm) and option 3 could end up reducing the underfloor circulation if you're not careful.

I am a little surprised that the floor is so cold as you have carpet down already. Have you eliminated draughts from the equation before going to all this trouble for what could be very limited benefit?

The room is being replastered so taking up the floor is only an extra days work as all the skirting is off already etc.

The 100mm was £3 cheaper a roll so not much in it. I'll go back and get 100mm.

Thanks for the help