Conservatory floor-18mm chipboard over 50mm insulation OK?

[elvira]

Urgent advice needed please.
The concrete slab was laid about 6 weeks ago and is close to dry now. I did not put insulation (50mm) under the concrete slab, as I wanted it to go on top of the slab. I decided to do this because I am having electric underfloor heating and I thought it would be more efficient this way, otherwise I would be heating up the slab.

After the concrete slab, a layer of self-levelling was applied to give a smooth finish to the floor level. This has been drying for about a month now with a dehumidifier.

I have bought 50mm floor insulation (polystyrene) and 18mm chipboard to lay on top. My questions are:

1) how should polystyrene be fixed to the concrete, & the chipboard to the polystyrene?

2) Can this floor be tiled with 60 x 60 cm porcelain tiles ?(I’ve read about risks with floating floors and tiling but presume the phrase “floating floor” applies when the chipboard is laid on joists, not when it is laid on polystyrene as in my case or am I wrong?)

3) Should I make my floor stronger by adding joists first with the polystyrene fitted between them. I really don’t want to do this as I prefer to avoid having any gaps in the insulation sheet.

4) Instead of joists can I make the floor strong just by adding a layer of tile backer board or plywood eg 3mm or 6mm?

5) I have had a demudifier since mid December to dry the concrete slab. The conservatory has been sealed with all windows closed. The floor does seem very dry now but I still get a bucket of water – could this be from the air. Whats the best way to know if it is now dry?

NOTE I can’t replace the 50mm insulation with screed because the insulation is required.

Many thanks for any advice.

 

[WoodYouLike]

Never opt for chipboard, this will restrict your option in floor covering tremendously.
It's the builder's friend, but the pest of everyone else!

Replace the chipboard with 18mm exterior grade plywood, much better, stable and suitable to adhere many different types of floor coverings to.

As for humidity. Open the windows every day for about 15 - 20minutes so any excess moist can escape naturally. have you got a humidity reader you can use to check the air humidity?

 

[elvira]

Thanks for your advice. I don't have a humidity reader but last night i switched the humidifier off to see if there was any condensation in the morning. There was only a little on the windows which would be normal.

I also put a small plastic sheet down and sealed the edge by placing a large heavy paint can on top and when i litfted it this morning there was a round damp patch where the paint can was placed. So i guess its not dry yet.

The humidifier is not a domestic one but a builders one borrowed from a friend and it has been on 24/7 for probably 5 weeks by now and I have also put on a fan heater from time to time. Would it normally take this long to dry out in your experience. The floor area is 26 sqm.

thanks.



__________________
Wood - what's not to like!
.

 

[elvira]

Woodyoulike - sorry one more question as I have already bought the chipboard , instead of replacing it altogether could I not put say, 6mm plywood on top or use a tilebacker board - they have it in Selco called "hardibacker tile board". I bought the chipboard a while back and don't know if i have the receipt for it to get a refund.
thanks again.

 

[WoodYouLike]

You can always ask if they take it back?
We're not tilers, so can't say if your alternatives will work, but adding plywood sounds like an idea if you can't bring the chipboard back

 

[mointainwalker]

It may not be your question, but I am puzzlede by the way you are laying the UFH. if I understand correctly there will ony be chip/ply and tiles. Right ?

I'm puzzled because although you had the opportunity to lay the UFH in a slab/screed you didn't.

The advantage of this would have been the mass of the slab/screed acting as an inertial mass to give a continual 'smooth" heat ( using off-peak electricity ).

You now have something more like a hidden electric-fire with additional insulation ( chip/ply) added on top of the elements to further impede the system efficiency and running on peak-rate power.

Did you do any in-depth research on this with the UFH suppliers or the Net ?

 

[Richard C]

I have bought 50mm floor insulation (polystyrene) and 18mm chipboard to lay on top.

Polystyrene (if that’s what you mean) is not ideal; there are far more efficient specialist insulation materials for use in slabs & for use with UFH.

1) how should polystyrene be fixed to the concrete, & the chipboard to the polystyrene?

I wouldn’t use polystyrene directly under the flooring material; the main insulator should have been laid in the screed with additional thermal insulation/tile backer boards laid directly under the heater element, self levelling screed over the heater mat & then tiled. As already advised forget the chip (crap) board, it’s the worst possible flooring material you can use & is not a suitable tile base.

2) Can this floor be tiled with 60 x 60 cm porcelain tiles ?(I’ve read about risks with floating floors and tiling but presume the phrase “floating floor” applies when the chipboard is laid on joists, not when it is laid on polystyrene as in my case or am I wrong?)

Chipboard (or any other sort of board) fixed to joists is a suspended floor not a floating floor. A floating floor is one where the floor is not attached to the subfloor or the walls, which you may well end up with if you do it the way you plan; floating floors are not good for tiling.

3) Should I make my floor stronger by adding joists first with the polystyrene fitted between them. I really don’t want to do this as I prefer to avoid having any gaps in the insulation sheet.

Forget it, your going the wrong way.

4) Instead of joists can I make the floor strong just by adding a layer of tile backer board or plywood eg 3mm or 6mm?

Your going in the right direction with waterproof backer boards, ply is not a good solution over a concrete screed for many reasons but you must keep the number of layers to a minimum or it’ll create problems.

5) I have had a demudifier since mid December to dry the concrete slab. The conservatory has been sealed with all windows closed. The floor does seem very dry now but I still get a bucket of water – could this be from the air. Whats the best way to know if it is now dry?

It may look dry but will still have a high residual moisture content, a sand/cement screed will typically dry at the rate of 1mm per day, half that if the screed thickness exceeds 40mm. Do not be tempted to finish off & tile before the screed is dry. I assume there is a damp proof membrane in the floor?

I can’t replace the 50mm insulation with screed because the insulation is required.

The primary floor insulation should have been laid within the screed. Too late now & 50mm is a lot to make up but you need to rethink what you’re doing & the way you’re doing it; do some more research on floor insulation alternatives & visit the Tiling/Building Forums!

 

[elvira]

mointainwalker : noted your comments, the only reason i did not go for screed was to save the drying time. the suppliers of the ufh suggested that instead of laying the insulation underneath the concrete slab i can lay it on top followed by chipboard. If i had the insulation underneath the concrete slab then my ufh would have to heat up 100mm of concrete slab plus the 50mm of screed on top of that which would take longer to heat and cost more, (although they said the advantage was that it would retain the heat ).

I have now decided that on top of the chipboard I am laying 10mm of heatstore tile backber board which retains heat and reflects back up to the tiles, so hopefully that along with the 50mm of insulation and 18mm of chipboard will make the ufh more efficient.


Richard C: thanks for comments. as mentioned I am now putting tile backer boards on top of the chipboard (ive been told the insulation can support 20 tonnes per sq meter and that the back boards 30 tonnes per sq meter so I am hoping that these 3 layers (insulation, chipboard and tile backer board) will create a hard enough floor surface that will last. i am sticking down the tile backer boards onto the chipboard with tile adhesive.

 

[Richard C]

I still dont advise you use chip, WBP every time. Seems silly to risk failure for a few quid extra when you consider the cost of the heater mat & tile job but, hey, it's your choice.

 

[Richard C]

Just re-reading this thread & I’m not clear how you intend fixing the chip to the polystyrene or, indeed, how effectively you can; polystyrene has no restraining properties or strength at all. You may be fixing the backer board to the chip but if the chip or ply is not securely fixed to the sub-floor, you will effectively have a floating floor which is not a good proposition for tiling. Any sort of wood will not be dimensionally stable & if it’s not securely fixed, it will able to move under the influence of both changes in ambient temperature & moisture content due to changes in seasonal humidity; these are the principle factors for tile failure.