Why so thick screed layer? Am I being an idiot

[jimmyhorns]

i am having a bit of a nightmare working out what is going to be the best way to install underfloor heating in one section of a room that is going to be a kitchen / diner / living room.

 

The bulk of the room (27m2) where underfloor heating is going to be laid is new - and so has an insulated floor, and there is plenty of room above (75mm) to screed over the UFH pipes in the traditional way. I think the builders will be using the traditional method for screeding.

 

The kitchen area is a similar size, but only 17m2 of this will have underfloor heating due to cabinets etc. This floor is existing, and appears to be traditional screed (60mm) on concrete. It was made in the early 80s and I doubt is insulated.

 

Taking up the screed is a piece of cake with a breaker. The concrete not so much. I could have the screed out of the areas required in half a day, but this will only give me 60mm depth to play with.

 

The UFH heating pipe I have is suitable for being buried under screed. It is also suitable for being retrofitted in insulation boards directly below the finished floor surface - I.e. it is strong/durable enough for this. These boards are covered with a very thin layer of flexible self levelling compound.

 

I have read that a lot of the thermal loss from floors is laterally - and it will be easy to fit insulation laterally around edges of the removed screed to reduce this. To fit the 16mm pipe I would be looking at 40mm PIR insulation with 20mm screed on top. I would fit 100mm wide strips at the edges to prevent lateral spread. The area will eventually be tiled.

 

So a couple of questions 

 

1) Why does screed need to be so thick if you are laying it as liquid screed/self levelling compound?

 

2) How crazy is my above idea

 

3) Are there any other 'reasonable' alternatives (there is no space for radiators)

 

Thanks

 

James

[nod]

You are fine to allow 50 for liquid 

But you need 75-100 for sand & cement Anything less than 60 will come up  

[jimmyhorns]

35 minutes ago, nod said:

You are fine to allow 50 for liquid 

But you need 75-100 for sand & cement Anything less than 60 will come up  

 

But why can't self levelling (liquid screed) be a lot thinner than 50? It is specified for use at 6mm for levelling (some brands you need to add sharp sand if you want to lay thicker than 6mm).

 

Its why it needs to be so thick that confuses me. No pun intended.

 

[Mr Punter]

On top of insulation it needs to be 50mm liquid, 75mm sand cement.  The thin layers of self levelling would just be for evening out existing concrete floors.

[PeterW]

45 minutes ago, jimmyhorns said:

But why can't self levelling (liquid screed) be a lot thinner than 50? It is specified for use at 6mm for levelling

Because it’s not a structural product. They are different to liquid screeds and have very different properties. 

[jimmyhorns]

17 minutes ago, Mr Punter said:

On top of insulation it needs to be 50mm liquid, 75mm sand cement.  The thin layers of self levelling would just be for evening out existing concrete floors.

 

but the big question for me is why? is it because of strength and the flexibility of what is underneath?

 

 

[Gone West]

14 hours ago, jimmyhorns said:

Are there any other 'reasonable' alternatives (there is no space for radiators)

Have you looked at skirting heating systems.

[Mr Punter]

Thin screed on insulation will crack up.  If you want a really thin layer above underfloor heating pipes you can use some of the interlocking cement board products like ScreedBoard20.