insulation between floor joists

[mazears]

My kitchen floor joists are 100mmx75mm.I'm in process of putting 25mm ply down as it will be tiled over...Is it worth me putting insulation between joists as now would be easy to do? I was thinking rigid foil backed same depth size (100mm) with nails tapped below sideways into joist to keep them from dropping. Also. does it matter how tight fitting i fit the ply along the wall edge - is it best to leave a minimum gap?

 

[mointainwalker]

I find your question "is it worth me putting insulation between joists ...?
rather odd.

Can you think of any reason not to ?

 

[mazears]

Do i fill between the floor joists with rigid insulation jablite which is the same depth as the joist (100mm) making it flush with underside of ply floor, or do i need to leave some sort of an air gap?

 

[mointainwalker]

You can use whichever type of insulation you wish, however they all have the same requirement: there has to be something to hold it in place.

I favour fibre-wool* because it is cheap and you can cut it slightly oversize so that it completely fills the gap between the joists. Fixing is by nailing/stapling some kind of mesh across the underside of joists. If you use this*, paper-face upwards

If you choose some kind of board, you can cut it to be a friction fit between the joists but I don't know if that would hold long term: probably better to bang in a few nails to hold it up.

No air-gap, flush with top of joists.

 

[ajrobb]

I find your question "is it worth me putting insulation between joists ...?
rather odd.

Can you think of any reason not to ?

Inadequate air bricks.

 

[ajrobb]

Also. does it matter how tight fitting i fit the ply along the wall edge - is it best to leave a minimum gap?

I suspect you should leave a 10mm expansion gap for plywood. To prevent drafts, you could fill the gap with something like an acoustic sealant:

http://www.customaudiodesigns.co.uk/soundproofing/sealant.htm