Putting insulation on top of wooden boards on solid concrete floor

[benjiman]

Hi,

I have a solid concrete floor, with joists sunk in the concrete, on top of which 22mm pine boards are screwed to, pre 1930's property.

I'm doing underfloor heating, 75mm celotex, then 75mm screed on top.

Can I put the celotex directly on top of the wooden boards with a dpm underneath, or do i have to remove all the pine boards and have the insulation directly on the concrete?

I have enough head height so not concerned losing 22mm if I can avoid the hassle of removing and disposing of all the timber.

Thanks,

 

[AronSearle]

Don't see any reason to remove them as long as they provide a reasonably flat surface.

 

[benjiman]

Thanks AronSearle,

Do you think there will be any issues with the board rotting/condensation due to lack of air?

 

[^woody^]

If there is no DPM below the concrete, and you seal the top with a DPM or insulation, then you will potentially trap moisture within the floor which will rot the timber boards and that in the concrete.

 

[AronSearle]

Technicall yes, unlikely to be an actual risk IMO.

 

[^woody^]

Technicall yes, unlikely to be an actual risk IMO.

Well it is a risk or it is not.

But it's not just a technical probability, it's an actual one. If that timber gets wet and can't dry, then it will rot.

 

[benjiman]

Thanks chaps.

Theres no DPM beneath the existing slab. I don't think I'll take the risk! The wet underfloor heating and screeding is a costly job so not going to risk there being problems in the future and having to rip the whole thing up.

Cheers.

 

[^woody^]

Are these actual deep joists set into the concrete, or just shallow timber sections just for fixing the floorboards? And are the timbers flush with the concrete floor surface?

 

[benjiman]

They are about 3 inches deep and have been set into the concrete.

And yes the timber boards on top are flush with the concrete. They have been nailed/screwed through to the sunk in wooden joists.

 

[^woody^]

OK, they are not joists then just battens to fix the floorboards to.

Take the floorboards up, lay the DPM and continue as you proposed. De-nail and if there is any great difference in surface levels, lay self leveller.

If the timbers then do rot, it's not a big deal as the concrete is providing the support to the insulation and it won't drop.

 

[benjiman]

Thanks Woody, will do as you propose.

Cheers.