Insulation under florboards.

[greensleeves]

I've sanded and varnished my lounge floor. I also removed one of the two radiators from it. I'm pretty pleased with the effect, but in very cold weather the heating is only just managing to cope. The floor has a 1 metre deep void underneath.
I can feel cold coming up from the void. Not huge draughts but certainly cold.
I've decided to insulate underneath the floorboards.
I bought a pack of Knauf loft insulation space boards today as an experiment. These are easy to cut to the width of the joist gap and wedge between the joists and push up so that they are touching the floorboards and no nails etc. are necessary to hold them in place. As far as I can tell, it has improved the situation. There is no question of rot or anything from this, is there?

I noticed one panel was flush with the floorboard at one joist but had a gap(very small) at the other, maybe because of some floorboard irregularity. I was thinking of putting a very thin layer of loft insulation or some other compressible material between the space board and the floorboard so that everything would be flush.
Any comments on these ideas?

 

[Deluks]

Shouldn't be a problem with rot unless the timbers are already damp. There should also be no probs with mixing insulation types, just don't insulate below the bottom of the joist, as you'll be encroaching on the necessary ventilation in the void.
I would run some lengths of thin batten or even staple some string to the bottom of the joists just in case the stuff falls out. To save you having to go back under and refix it in place.

 

[greensleeves]

Thanks deluks, I'll bear those points in mind. I managed to get some wadding, which I'm putting on top of the polyfoam battens before pushing them up against the bottom of the floorboards. The ones I have done so far are a very tight fit, but if any future ones are looser I'll use your string idea.