rigid board floor insulation

[Philip Morant]

I am going to replace chuck out the floorboards in my living room and replace them with rigid board insulation sandwiched between osb or plywood or hardboard. I'll be using 30mm or maybe 40mm polyiso boards (Kingspan, Celotex, or one of a hundred other brands). Joist spacing is

47cms (18.5 inches).

Polyiso isn't strong enough by itself of course so I need wooden boards below and on top. My questions are:

OSB, plywood or hardboard ?

What thickness ?

I want to keep thicknesses to an absolute minimum so as not to raise the level of the floor any more than necessary. At present I'm thinking 6mm ply below and 9mm or 11mm on top.

[Grimly Curmudgeon]

If it were me, I'd experiment.

6mm under board, polyfoam glue 25mm Kingspan Therma to it, glue 10mm ply on that (all with appropriate setup time with weight on top) and see what kind of rigid sandwich has resulted. On a slightly larger scale, I glued 100mm aerobord (a type of eps) to 12mm ply and was astounded at how rigid the pair became.

[Phil L]

His joists are spaced at 18.5 inches apart. He knows this because his opening statement was that he was going to throw out all his floorboards

[John Rumm]

You will need them above, you could skip them below - its fairly strong in compression.

On top, OSB or Ply or Flooring Chipboard.

19mm

[geraldthehamster]

Agreed. That's the orthodox way to do it - insulation between joists, fix boards to joists. It's done that way because it's easier and better. Putting insulation on top of the joists would reduce the room height and create steps at thresholds; you might need to trim the bottom of doors and would have issues with your skirting. So I suggest you don't do what you're suggesting.

Cheers Richard

[Philip Morant]

Thanks, very useful to know that the bottom layer can be dispensed with.

Does anybody know if the extruded polystyrene products might be more practical than polyiso ? The marketing literature says they're the most robust type of rigid board insulation product. I think that they might not 'dust off' like the surface of polyiso seems to, so they might glue better to plywood.

[The Natural Philosopher]

Extruded poly is a bit less good at insulatin but way tougher than polyisocyanurate.

Its main drawback is fire hazard - very toxic fumes and it burns well.

You can in the limit glue them to ply with PVA - its not brilliant and takes ages to dry out, but it does work.

Most modellers use foaming PU glue instead. Or hot glue.