Undertile heating, removal of adhsve

[marc large]

Hi folks wondered if you can help..I'm laying an undertile heating system (loose wire) to an upstairs cloakroom toilet. I've taken the old tiles up and there is 22mm ply screwed down as my sub floor. Before laying the insulation board (4mm or 10mm?) should i need to try to take off all the old adhesive which is welded on?
I'm also doing exactly the same in the downstairs cloakroom loo which doesn't have the 22mm ply just screed floor.Hence the question about only using 4mm ins board upstairs

Many thanks in advance marc

[grumpygrouter]

It is always best ot have as flat a surface as possible to work with. If the adhesive doesn't come up easily is it possible and quicker to change the ply? if not then I would get it as flat as possible before laying the insulating board. How "lumpy" is the adhesive residue?

[marc large]

not too bad i've been at it most of the day today its only a small area & i'm a bit loathed to take anymore off for fear of digging into the ply i just need to know whether the heat will cause a prob with old adhsve

[grumpygrouter]

heat shouldn't be an issue. if you are laying your boards with a bed of adhesive, you may find that you can get the boards quite flat . The other alternative prior to laying the insulating boards is to level it with a levelling compound first. Mapei Fiberplan will be excellent for this. I would also suggest you use this to level over the top of your heating cable as well.

[marc large]

man thanks mate really helpful

[Dave]

Use the insulation boards on the concrete floor and just use the mapei fibreplan upstairs and no insulation boards...you won't need them on ply.....waste of time and money really...The use a cement based backer board for moisture control would be better..