Thermafleece Cosywool which I'm hoping will hold in place using a breather membrane trailed between the joists.
Just wedging insulation between the joists sounds a bit risky to me - couldn't it fall out if the floor flexes due to temperature/humidity changes? Also, your joists will be cold spots and depending on whether the insulation is foil-faced you may end up with either issues with condensation or breathability.
Condensation and breathability are my biggest worries with this - I'm going to be putting a wireless hygrometer beneath the floor so that I can see if it looks bad. If it does and it looks like the moisture's coming from the house, I guess I'll have to relift all the boards (they're going to be screwed down so shouldn't be too bad) and fit a vapour barrier.
Day 2 and all the boards are up. One hearth reduced to rubble. Tomorrow is going to be preparing for the insulation:
- rubble clearance
- additional joist pieces
- securing loose joists (some have nails which barely make contact)
- bricking up the fireplaces where the concrete once was
- fixing timber battens around the room so there's something to tape the breather membrane to
- Checking airbricks are clear
- Checking cable routing under the floor
Then during the week I'm hoping we can actually lay the insulation in the evenings. The only noise should be the staple gun, which is a manual model so not too loud and lots of swearing. Stainless steel staples, so they won't rust (god they were a bitch to find).