Timber subfloor options for tiling with heating

[mactabby]

Hi - I've read lots of advice here and elsewhere on tiling onto timber floors, including gcol's detailed FAQ here, but haven't seen this question answered before.

I have a new kitchen coming and I want to tile the floor. It used to be a lounge and the plumbing work has pretty much wrecked the original chipboard floor (as expected). I therefore have free reign as to what I put down under the heating and tiles.

Reading around, it looks like its best to use an insulating layer under the UFH, e.g. 10mm extruded polystyrene (Marmox or similar), and given this, what's the best option for going beneath it? 18mm chipboard on its own (cheap in both senses), 18mm ply with noggins (expensive and lots of work - it's a big room and my joinery skills are poor), or a combination of both (e.g. 18mm chipboard, 9mm ply, 10mm marmox).

The joists are 2x4s supported by bricks at about 1.5m intervals across a 4.5m span.

What would you do?

Thanks in advance.

 

[Jasonb]

Forget the chipboard, fit at least 18mm ply preferably 22 or 25 with noggins under joints.

As the floor is up you will get better insulation by fitting kingspan or celotex between the joists (also keeps height of floor down)

If you do go for an insulation board stick it to the ply with rapidset flexi, fix matt to insulation then tile with rapidset flexi and flexi grout.

Jason

 

[mactabby]

Thanks Jason, although I'm really trying to find a way to avoid noggins - if that's the only way then I'll get someone in. Would fitting 2 layers of 12mm ply at 90 degrees achieve a similar effect?

On the insulation, it's not so much heat loss I'm worried about, it's warm-up time - the advantage of overboarding the ply with insulating board (which has a much higher U value) is that more of the heat is conducted into the tile, which warms up quicker as a result. Fitting insulation below the ply reduces heat loss but slows down warm-up as the ply is warmed as well.

I thought insulation board provided extra rigidity in the floor as well... It's certainly sold on that basis.

 

[Jasonb]

If you can get hold of 22mm flooring ply, this comes in 8x2 sheets with T&G edges like chipboard you can probably do away with the noggins.

Two layers of 12mm will cost more, take longer to lay and need bonding together.

I agree that insulation under the matt will speed the initial warm-up but what happens once its warm - all the heat travels to the coldest side which could well be downwards.

The insulation boards with cement/mesh faces give a little extra stiffening but the plain polysomething ones do not. For example I've just fitted this ribbon heating that a client supplied and the insulation can be bent at rightangles, don't see that stiffening the floor, so I overlaid with 12mm ply. BTW I wouldn't reccomend this type of heating, takes a lot longer to lay than other makes and the earthing arrangement is a pore afterthought.

Jason

 

[mactabby]

Thanks again - as it happens that was the stuff I was going to buy, so I appreciate the advice.

Part of my problem is that it's not me doing the tiling (too big a room and large tiles - I've only done bathrooms before) and I rang the tiler today and he's happy with 18mm chipboard and 12 mm ply overboarded, without the insulation. I don't have the time to get someone else in - besides he's the mate of the kitchen fitter.