Underfloor heating over uneven subfloor??

delfagio

Hi all,

I am currently at the stage now on our self build Structural Insulated Panel timber frame house where I need to install the underfloor heating pipes and rigid floor insulation, and hopefully all going well get our liquid screed installed just before the Christmas break.

The concrete subfloor is not level as it was only given a tamped finish. This along with the fact that the timber frame house was raised up on shims, mean that the subfloor is off level anywhere between 5mm to 25/30mm in places. Before I install the floor insulation sheets I want to get a level surface to lay the floor insulation sheets onto.

So my question is, does anyone have an idea of how I can achieve this. I was thinking of using a sand and cement dry mix screed and using this to bring the subfloor to a common level and then lay the floor insulation sheets directly onto this sand and cement layer...... any ideas or thoughts

Many thanks

Steven

DOCARCH

delfagio wrote: »

I was thinking of using a sand and cement dry mix screed and using this to bring the subfloor to a common level and then lay the floor insulation sheets directly onto this sand and cement layer.

Assuming the engineer is happy with the sub-floor in the first instance, I'd agree with your suggestion above.

delfagio

Thanks DOCARCH,

Yea engineer has previously signed off on the subfloor. Thanks for the reply, Did you ever come across this issue before

Steven

DOCARCH

delfagio wrote: »

Thanks DOCARCH,

Yea engineer has previously signed off on the subfloor. Thanks for the reply, Did you ever come across this issue before

Steven

Not really, but sounds a sensible way to deal with the issue....before I got down to the end of your post I had said in my head 'sand and cement'. Keep it simple!

Important (for drying of the floor screed and for consistent heat) that the floor screed be of fairly equal depth throughout.

delfagio

I agree, the whole point of doing this is so I can have a consistent 50mm finished floor screed over my underfloor heating pipes giving an evenly uniform heat release into all rooms on the ground floor.

Another person did suggest just laying the rigid floor insulation directly onto the uneven subfloor and let the liquid screed remove any discrepancies. I was dead against that as it would mean I could have 50mm Floor screed in one corner and then 75mm screed in another area and 65mm is another spot......seemed maddness to do that.

Strolling Bones

Why was the subfloor not installed level and smooth - as it should have been

delfagio

It was installed fairly level, not exact, and as it was only a subfloor it only was tamp finished not power floated

GreeBo

its unclear (to me) if the floor is not level or not flat?
i.e its flat but off level or is the floor just uneven with peaks and troughs everywhere?


I think you are going to have cracking problems with a sand/cement screed of 25mm and less tbh, would you try a self levelling compound?

BryanF

What depth of insulation is allowed for & what depth of screed?

delfagio

Yea the subfloor is slightly off level but also has the up and down troughs or ripples from the tamp finish. I had initially thought about using a high flow conbextra grout and use this to even out and level the sub floor. Then let it harden before laying the insulation.

The insulation will be 150mm with underfloor heating pipes on top and then a liquid floor screed of 50mm ontop again for my finished floor

Strolling Bones

delfagio wrote: »

It was installed fairly level, not exact, and as it was only a subfloor it only was tamp finished not power floated

only only only.

The steep learning curve. Given the known layers of materials that were to be placed over whose decision was it that only would suffice?

If it was your own then welcome to building. But if you have va contractor you must insist he sorts it at his expense

delfagio

Yep, my first self build, so I'm the contractor, ha. I will know for the next time I build a house lol, it will be 100%perfect.

It's not a major issue in the grand scheme of things, just looking to see other people's ideas.

GreeBo

delfagio wrote: »

Yea the subfloor is slightly off level but also has the up and down troughs or ripples from the tamp finish. I had initially thought about using a high flow conbextra grout and use this to even out and level the sub floor. Then let it harden before laying the insulation.

The insulation will be 150mm with underfloor heating pipes on top and then a liquid floor screed of 50mm ontop again for my finished floor

Even with Conbextra you will need to build up a minimum of 10mm in all areas so its going to cost a bit, but I wouldnt try to avoid it, cracking wont be fun.

Strolling Bones

delfagio wrote: »

It's not a major issue in the grand scheme of things

Please accept my advice in the indended (helpful) spirit. One builds in layers. In this case the follow on layer is more difficult for you because what went before.

Lose the following phrases from now on

only
it will do
almost
sort of
failrly

Another issue for you here - the gap between your sole plate and floor slab has to be made air tight. Would have been alot easier to seal if the gap was level and smooth.

delfagio

Thanks for that Strolling Bones, advise taken on board..........

Drift

As a matter of interest OP .. where is your DPM and/or radon barrier going?

delfagio

Hi Drift, the Radon barrier is under the 6" concrete subfloor

Drift

So the only need you have for levelling it is to provide you with a flat base on which to lay your insulation?

If that's the case would minor cracking of the levelling material be the end of the world? I'm only throwing it out there as a thought - people with more experience than myself in screeds might be able to clarify what level of cracking to expect?

DOCARCH

Drift wrote: »

If that's the case would minor cracking of the levelling material be the end of the world? I'm only throwing it out there as a thought - people with more experience than myself in screeds might be able to clarify what level of cracking to expect?

I'd agree and see no issue with the sand/cement option. If it cracks a bit, over time, it's really not going to be an issue.

delfagio

Thanks for the reply Drift, to be honest, I don't see any issue with the levelling material for the subfloor cracking. As this would be under all the floor insulation so would never be seen.

It is however unclear to me if this would cause any cracking in the finished floor screed which is to be poured on top of the floor insulation as it seems to be suggested by other people. I can't really see how it would.

hexosan

What's about just using a dry sand and cement mix

delfagio

Hey Hexosan, yea a dry sand and cement mix is what I was originally planning on using just because of the fact that it would be dry. If I were to use say a wet levelling compound, or a wet grout it would mean I'd have to wait a while for it to dry out before laying down the floor insulation