Superfoil underfloor insulation

[Mr Blobby]

In the next few weeks the screed will be going over the UFH pipes on our first floor hollow core slabs.  The original plan was to have 50mm insulated screed, attach UFH pipes to that and then 50mm flow screed on top.  The hollow core slabs are however higher on one side of the build  meaning that the screed depth in this area will be too shallow for two screeds, and so now we plan to install just flow screed without any insulated screed.  Which I'm ok with.

 

Just for context, we have a passive standard block cavity build.  The first floor UFH may be used more for cooling in the summer than heating in the winter and so I'm not too bothered about insulating between the hollow core slabs and the UFH pipes.

 

The screed installer has suggested foil insulation under the UFH.  I'm not convinced it will be at all useful because, firstly, I think there is no requirement to insulate the slabs from the UFH pipes, and secondly, I think superfoil is a cr4p insulator anyway.  But of course the screed company would normally install this because normally there would be outragous heat loss through the floor slabs.

 

As you can imagine, my suggestion to omit insulation is met with shock and disbelief.  Am I crazy to go naked, with nothing between screed and hollow core slabs?

 

(which leads me onto a second question.  I have been told that if there is no foil insulation, then there is nothing between screed and slabs.  No PVC sheet laid out under the screed, just perimeter expansion, which seems odd, as the screed is going to pour down the sides of the slabs (there are some decent gaps) all over my lovely plastered walls below.  That can't be right, surely there needs to be someting to hold the screed?)

 

Edited 9 hours ago by Mr Blobby

[-rick-]

Have the precast slabs already been grouted?

 

I thought the screed layer was often used as grout to tie everything together on precast? (Depends on how it's specified).

[Mr Blobby]

23 minutes ago, -rick- said:

Have the precast slabs already been grouted?

 

 

Yes, if by grouted you mean filling the gaps between slabs with dry-mix concrete.  That was done a long time ago, and before blocks were laid above FF. 

Sorry, I should have been more specific, the gaps of concern are around the perimiter, where the slabs run in the same direction as the walls. There is maybe a cm or two gap in places.  To my lovely plastered walls below.  

 

Not having done this before, I imagined the sreed to be liquid enough to run through the gaps, maybe it doesnt?

 

 

Edited 9 hours ago by Mr Blobby

[-rick-]

5 minutes ago, Mr Blobby said:

Sorry, I should have been more specific, the gaps of concren are around the perimiter, where the slabs run in the same direction as the walls. There is maybe an inch or two gap in places.  To my lovely plastered walls below.  

 

No practical experience so hopefully someone with some will be along but fill the gaps with foam? I guess you'd want something with a little give in it to allow for movement.

[JohnMo]

49 minutes ago, Mr Blobby said:

suggested foil insulation under the UFH.

Once compacted it's just very thin insulation, with same thermal properties as thin insulation - it has zero reflective properties without an air space to reflect into. Would use any other insulation than this. If it's first floor just 25mm EPS or XPS.

[Mr Blobby]

1 minute ago, JohnMo said:

it has zero reflective properties without an air space to reflect into

 

Good point, thank you.

[Mr Blobby]

Just been told that this is the actual product...

 

 

fqml1.5m10m6mm---ybs---product-guide-1671023508.pdf

 

Apart from the acousitcs I remain unconvinced of any benefit. (except maybe for stopping the screed from falling down any perimeter gaps)

 

Am I crazy to think no insulation is actually better anyway?

[saveasteading]

You say first floor so I hope this is a UK definition, ie the the ground floor has been fully insulated.

 

BUT just in case this is US definition I'll write as if that is the case because it would need immediate attention.

 

 

Superfoil is utterly useless in this context.  as others say above. Which also demonstrates that your builder is not knowledgeable on such matters.

fyi, it is ok for improving the wall or roof of an attic or outbuilding. It is not used ever by commercial designers or contractors (with knowledge).

 

 

2 hours ago, Mr Blobby said:

hollow core slabs are however higher on one side of the build 

 

Most also curve up to the centre. Be sure to check levels  everywhere.

 

2 hours ago, Mr Blobby said:

we have a passive standard block cavity build.

I don't see why that is relevant.

 

 

With more detail I can be more precise and relevant. 

[Nickfromwales]

@Mr Blobby 

 

You need to get the slab fully grouted and fully filled, and get this so there is nowhere for the screed to disappear down or in to.

 

You mention your lively plastered walls, but seriously? Nobody’s going to expect you to pour a liquid upstairs that ends up downstairs. This worries the deck out of me to hear you say that.

 

Time to stop the ambiguity mate, and grab this by the balls.

 

If you’ve builders around, or not, all that needs doing is to make up a semi-dry mix of 6:1 sand/cement, get a gauger trowel and a pointing trowel, and get filling in every single nook and cranny.

 

A wet sponge can then be used to work the last of the mix into a filling paste, that you then ram into any remaining cracks / voids etc where wall meets slab and so on. 
 

Get this right, and you’ll be fine and ready for the pour.

 

Use foam perimeter skirting (insulation if you will) to run around the entirety of walls to give some expansion relief. Choose the one with the polythene skirt, and the weight of the screed pouring out over this actually creates the seal.

 

As for anyone’s thoughts on which BS insulation layer to waste money on……”none”. 
 

Just get the slab mopped and near saturated with 50/50 SBR/water with a janitors mop immediately before the pour and crack on; this removes or locks in any contamination or debris and allows the slab to be primed and cleaned so it adheres well.

 

Give me a buzz if you want to chat