Insulate first floor screed. Or not?

[Mr Blobby]

Accepted practice is to install perimeter insulation around screed, which makes a lot of sense in a traditional build on the ground floor where the UFH would otherwise be heating cold walls.  But does the screed need to be insulated in a well insulated floor?

 

Our 100mm screed will be going over first floor hollow core slabs.  Block walls have 250 cavity and no thermal bridges anywhere as far as I know.  Foundation is Kore slab.  We will have some UFH loops in the FF screed, but this is hopefully not required for heating, but is there just in case. We may need it for cooling.  Do we really need to insulate the screen from a very will insulated perimiter?  Is the insulation still required to prevent cracking in the screed? 

 

Our builder also wants to insulate below FF screed by first pouring 50mm insulated screed and then 50mm flow screed on top.  I would be quite happy for the UFH heat/cool to pass to the hollow core slabs rather than have quick response times.  Is that mad?

 

UFH flow temps will of course be much closer to ambient temps than a "normal" house.

 

In a well insulated passive house then, is there any need to insulate the screed...

1. Do I need 50mm insulated screed below 50mm flow screed to insulate the FF Screed from the hollow core slabs below?

2. Is there any point in installing perimeter insulation when the walls will hopefully not be "cold"?

 

 

[Conor]

We did it exactly as your building is proposing. If you don't have insulation, fitting the UFH is harder work as you have to attach to concrete rather than soft insulation.

 

yes, perimeter insulation is needed as acts as an expansion gap.

[Mike]

On 22/10/2025 at 10:40, Mr Blobby said:

Accepted practice is to install perimeter insulation around screed, which makes a lot of sense in a traditional build on the ground floor where the UFH would otherwise be heating cold walls.  But does the screed need to be insulated in a well insulated floor?

You still need an expansion strip around the walls + movement joints in doorways.

 

On 22/10/2025 at 10:40, Mr Blobby said:

I would be quite happy for the UFH heat/cool to pass to the hollow core slabs rather than have quick response times.  Is that mad?

I'd probably do the same, using clip rails to fix to pipes.