I would advise against a vapour barrier in the way you propose Dave. You could take up the floorboards and put down a continuous sheet across the joists. A breather membrane would probably be a better choice. Or, if you want to just stop the draughts and don't need the floorboards exposed then pin down hardboard sheets on to the top of the joists before you lay your carpet or floorcovering.
 
I don;t envy you having to crawl about under there. Is there a concrete over-site below the void or just soil?

Thanks for the advice Keith,
I will be leaving the floorboards exposed (and won't be taking them up) so do need to draft-proof / insulate from below.
 
There's soil / rubble beneath the floor but I've been down there before and kind of cleared it up a bit so it's not too bad.
Dave

I agree with GBP Keith
 
I am a manager for an insulation company. We would normally use 200mm thick ordinary glassfibre held in position with plastic mesh stapled to the underside of the joists. you can purchase the rolls ready split to suit your joist widths. Eurethane boards and lambs wool would do a similar job but are much more expensive.
Do not include a vapour barrier underneath as this may trap water vapour.
You will need protective overalls, mask and goggles.
Good luck

I would advise a vapour barrier otherwise moist air from the room can condense on a regular basis on or near the joists.
 
It is common practice to have a vapour barrier on the warm side of the insulation, why not on a floor? It will if cleverly detailed also act as an air tightness barrier too.

You are right Tony but Dave is proposing to just do it between the joists which will in-fact put extra risk on the joists.
 
If the floor is ventilated as I suspect then a breather membrane would work between the joists or, if he was prepared to take up the floorboards then a comprehensive vapour barrier could well suffice.

"I don't envy you having to crawl about under there"
 
I've done it a few times , I quite enjoyed it , its a bit like pot holeing

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When you insulate your floor the floor cavity will become a lot colder in the winter months as will the structural elements e.g. timber floor joists and my primary concern would be the formation of interstitial condensation within the timber members during winter months rather than anything to do with surface condensation.
 
The problem is that the vapour control layer would need to be laid over the top of the floor joists as you will be trying to stop the warm air (which holds more water vapour than cold air)coming into contact with a material at a colder temperature, where the warm air meets a cold porous structural element the air may be cooled to such a point (especially as the floor cavity will be a lot colder than present) that the air becomes saturated (technically speaking it reaches its 'dewpoint', further cooling of the warm air then results in condensation forming, this can be in the form of surface and/or interstitial condensation - interstitial meaning within the structural element be it timber, brick or any other porous material.
 
There would be no benefit in placing the vapour barrier to the underside of the floor joists as it will be the drop in temperature that will trigger the release of the water vapour from the warm side to the cold side of the structural element.
 
To answer your question, introducing a vapour barrier to the underside of the floor joists would be a complete waste of time that is other than to assist in the air-tightness of the building (which is another bone of contention)whereas introducing a vapour barrier over the top of the floor joists may well provide some long term benefits by reducing the amount of water vapour introduced to the timber.
 
The options now open to you are: 1. do I go with the risk and avoid all the work necessary to introduce the vapour barrier over the top of the joists; or 2. do I bite the bullet and do an all singing all dancing routine, and perhaps sleep well knowing that a good job has been done? A third option is a no brainer which would be 'do nothing'!
 
Regards

you could lay the vapour barrier over the floor boards.

Using staples to fix a membrane in a damp place (here: the suspended ground floor)is not sustainable. In a few years these will have rusted away and the proposed insulation will come down.