Forgot to mortar down insulation board to floor.... - Tile Advice

[RenovateYourRoost]

Hi,
I bought some sheets of 6mm insulation board to install on my bathroom floor.
(It has blue foam in the middle of it.)
My subfloor is 21mm treated wooden floorboards. I've freshly installed them and they're nice and level and smooth. (I even planed some of the joists to make sure I'd have a good subfloor.) The joists are enormous.
I've fixed these insulation boards down with screws, taped the joints and spread a flexible tile adhesive over the taped joints and screw heads.
I've now just realised that it looks like I should have mortared down the boards using thin set as well..?
It's going to be pretty disruptive to pull the insulation boards up now... Do you think it'd be sufficient to just secure the insulation boards with lots more additional screws now? Has anyone else tried this?
I'm planning on laying underfloor heating mat on top of this and then hexagonal tiles (so will be a bit of a nightmare if I have to pull it all up at a later date!)
In an ideal world, I would have not been an idiot and I'd have mortared down the insulation boards. But I didn't. And I am.
I figure as long as they're well screwed down (and I fill all the additional screw holes,) the insulation boards aren't gonna move and I should get away with it..?
Or is that wishful thinking?

 

[Dave]

Did you use the washers for the screws as well ?

 

[RenovateYourRoost]

No, I didn’t use washers. If I was to use a load more screws to secure it down with, I’d use washers with them though (knowing what I now know!)

 

[Bond]

Think l would remove and bed with adhesive and screw, because if there is air trapped between boards and flooring, the action of moving on the floor will create a bellows type of effect the trapped air has nowhere to go,thus stressing the system, if you get my drift.

 

[RenovateYourRoost]

Think l would remove and bed with adhesive and screw, because if there is air trapped between boards and flooring, the action of moving on the floor will create a bellows type of effect the trapped air has nowhere to go,thus stressing the system, if you get my drift.

Thanks for your feedback, Bond. I ended up calling Prowarm to ask their advice (they make the insulation boards I bought.) They recommended using additional screws with washers (at 150mm intervals) so I did this, as it seemed like the simpler option.
I laid the heating mat and then used a flexible thin set mortar to smooth across the heating mat, filling in all the gaps between the heating cable (the heat cable is approx 6mm diameter.) I left it for set for a week, as have been away visiting family.
I just walked across it and can feel it cracking underfoot. Althouhg it seems to be cracking less the more I walk on it...
I don't know if it's just settling or something or if this is a cause for concern...
I'm considering maybe using thin set to mortar down either 6mm or 12mm cement boards on top of it all. I figured that'd spread the weight across it all and might help make a better surface onto which to tile..?
I think I can live with the height rise and it seems like a simpler and cheaper option than ripping it all up. I'd feel anxious to just tile it as is...
Any thoughts?
I might give Prowarm a call on Monday, but wondered if anyone's had experience of this before...?
I've not used this foam insulation board before. It felt worryingly spongey underfoot when I laid it initially. I'm kind of not surprised that 6mm of thin set on top of something spongey would crack when you walk on it. I guess the tiles will spread the weight of foot traffic a bit, but seems worrying to me....

 

[Dave]

Imho , its a disaster waiting to happen, been busy snd lost track of your post, in theory using washers should’ve worked but this depends on how close they were used. As too far apart will still create voids. As for spreading a thin coat of adhesive, i would’ve used a leveller with a min 5mm coverage over cables orcas advised by manufacturers.

 

[RenovateYourRoost]

Imho , its a disaster waiting to happen, been busy snd lost track of your post, in theory using washers should’ve worked but this depends on how close they were used. As too far apart will still create voids. As for spreading a thin coat of adhesive, i would’ve used a leveller with a min 5mm coverage over cables orcas advised by manufacturers.

The heat mat manufacturer recommended (in the manual) applying thin set to the height of the top of the cables, which is what I did. Prowarm reckoned additional screws with washers, spaced at 150mm intervals would suffice, so I did that. I’ve ended up using thin set to mortar down 6mm cement boards over the top of it. Having dried, it felt really solid. No cracking noises or movement. It felt like walking on a concrete floor this time, so really hoping that’s solved it. I’ve since laid the tiles, so fingers crossed. Will try using self levelling to 5mm above the cable height next time, as you suggest. Seems like a much better solution. Thanks for your feedback

 

[Perfect Tiling]

You seem to be asking us tilers questions that you then give your own answers to. When we tell you what WE would do .....you come back with what YOU have done!!!! If you want answers to problems you need to listen.....go ahead and continue with the rest of the floor......when you come back in a couple of months asking why there's cracks in it you should already have your own solution....

 

[Time's Ran Out]

He’s just gathering information for the next time. That’s what the forum is for information. Only a couple of replies in 3 days and he had to get on so he took the advice of the suppliers.
Not everyone is Perfect!