Insulating under floor with spray foam.

[Grahamandcharlie]

Hi
I took my carpet up and I am in the process of sending my floor boards but the room is noticeably colder. There is a draft coming up through the floor boards and I have a decent sized crawl space to put some kind of insulation under there.
I was thinking easiest would de just get under there and spray the underside of the floor boards with spray foam insulation.
Would this work ?
Seems obvious but nobody has mentioned it on here.

 

[Deleted member 174758]

Don't do it! Spray insulation traps moisture and prevents the wood from breathing which can accellerate rot. By all means lift some boards and install tight-fitting PUR insulation (e,g, Cellotex, Kingspan, etc), but that may be difficult to do

You could use a more traditional approach: empty the room and lift the carpets then lay hardboard onto the floor, mesh side upwards. tacked down every 150mm with flooring tacks or diamond-head pins. That will cure most of the draughts. Ideally you should use the thicker 6mm stuff rather than the 3mm stuff. Before installing it you should go over the floor with a hammer and nail set (nail punch) and sink any protriuding heads beneath the surface. the mesh (rough) side of the gardboard should be dampened and left to dry out overnight (or a bit longer). This will cause the boards to expand a little, but when they dry out, they will pull tight and lay flat. If you don’t do this, and the room gets a bit damp, the hardboard will swell and can lift in areas. The hardboard should be installed at right angles to the original floorboards and you should avoid joints coinciding if at all possible.

Once the carpet is back down check for further draughts using something like a joss stick. You might still have gaps between the bottom of the skirting and the floor, or at the top of the skirting between the skirting and the wall. These can be filled with closed-cell foam piping such as Fossa Caulk Saver (available from decorator's stores) pressed-in with a blunt spatula or the like. You can use foam instead, but it tends to glue everything together - forever

 

[Grahamandcharlie]

Thanks for that advice.
I weren't going to take up the existing floor tho or relay a new one on top of the existing floor boards which I'm going to sand and varnish but I get the gist, Don't use spray foam on the underside.
Would stapling SuperQuilt Multi-Layer Reflective Foil Insulation to the underside of the joists work or would this need to lay just under the floor boards themselves?