Under floorboard Insulating - okay for gap between floorboard and insulation?

ChuckleVision_2

Details:
1930's house, ground floor floor boards (with 2 foot cavity below joists).

Due to ventilation below joists I want to insulate under the floorboards. To save lifting all floorboards I've decided to tac battening to the joists then run Jablite insulation board along the battens. I've just lifted boards from either side of the room to attach battens and slide under the insulation board.

I understand that there must be ventilation in the 2 foot cavity, so will make sure that the air bricks are not blocked.

My Question:
I was about to fit the battens to the joists and noticed that I can easily just rest the battening on the three beams that are holding up the joists and in-turn place, once I've cut to size, the insulation board on the battens.

This will make the job easier and give more surface area for the insulation to rest on (as the battens will be flat rather than on their side. The only issue is that there will be a gap of about 10-20 cm between the floorboards and the insulation boards.

Can anyone see any issue with doing this?

Many thanks for reading!

27col

A gap is exactly what you do not want. The whole principle of insulation is to provide a layer of still air. If there is a gap then air can flow along it and completely negate the concept of still air. You need to ensure that the insulation is tight up against the underside of the boards for it to have any effect.

ChuckleVision_2

As the air flow (from air vent bricks) will be flowing in the other direction to the joists I thought it might be okay. Thanks for letting me know anyway.

One other questions, whilst down there I've cleared all the crap from the air vent and found a load of white power in one. I can push my hand up in the wall cavity and there seems to be quite a bit of it in there.

It moves and crumbles to a fine dust.

I read somewhere on my google searching that it might be damp in the walls/lime residue?

Any ideas? No evidence of rotting or dampness there though. Possibly evidence of an old leak in the attic? (that was repaired before I moved in).

Thanks

ChuckleVision_2

What a job this is turning out to be!!!

I'll screw the battening to the joists so there is no gap between the floorboards and the insulation.

Need to somehow stop the garden plants growing though the air vents!

Not sure about the white power though.