Raft foundation + UFH

[Water]

Hi all,

 

I've wanted ufh in my new build from the start and we are getting into the nitty gritty details now. I've spoken to the builder who says the ufh will be separate to the raft foundation, I was surprised by this as all the ones I've seen its incorporated into the foundations. I guess this might be the difference between insulated and not? 

 

It would be so great if someone could help fill in the blanks and maybe give the pros and cons to the different ways.

 

Thanks again for all your help and input!

[Russell griffiths]

You can only install it into a fully insulated raft, otherwise you will just lose all the heat into the ground. 

Does your design not show this ?

it needs to be insulated underneath it as well as a large insulated upstand to stop the heat traveling out of the sides. 

[Water]

That is what I thought but the builder seems to think otherwise... I will have to ask him what he is thinking.

 

I don't suppose anyone knows how much insulation is normally used under the raft? 

[Russell griffiths]

200-300mm what has it got to do with the builder, it’s down to the structural engineer to design a raft foundation, and then you to say what you would like to achieve, then you jiggle the ideas around and come up with a plan, most builders haven’t a clue about new technologies, he would be the last person to talk to unless he is used to doing insulated raft foundations. 

[Water]

i say builder they are the main contractor who are dealing with everything including the structural engineer etc

[wozza]

Can you post up a picture of the drawings you have?

 

@Russell griffiths is correct about builders not having a clue about new stuff, a lot of the contractors we had on site were very stuck in their ways and wanted to do things their way. 

Edited July 28, 2020 by wozza

[Temp]

Sorry if this is obvious but.. Do you actually mean a raft foundation or are you referring to a floor slab?

[Conor]

8 hours ago, Water said:

That is what I thought but the builder seems to think otherwise... I will have to ask him what he is thinking.

 

I don't suppose anyone knows how much insulation is normally used under the raft? 

 

This should all be detailed in your architect's drawings, engineering drawings etc. U values would have been needed for your SAP report and building control would also have required this. What overall I values are you aimer for in the build? What's your targeted SAP rating?

[oldkettle]

From conversations with several builders in our area I noticed that the idea of an insulated raft or insulated slab was completely foreign to them. They really lost interest when I asked about this option. 

[Water]

Thanks for the input.

 

They said the ufh was separate from the raft foundations, I'll ask for the drawings but I agree maybe they don't fully understand? They said they had done it before..

 

I don't really have a SAP rating in mind, maybe I should? I was just confused as I've only seen the ufh pipes go into the foundations on the videos I've seen so wanted to question it.

 

5 hours ago, Temp said:

Sorry if this is obvious but.. Do you actually mean a raft foundation or are you referring to a floor slab?

 

Definitely raft foundations.

[Declan52]

3 minutes ago, Water said:

Thanks for the input.

 

They said the ufh was separate from the raft foundations, I'll ask for the drawings but I agree maybe they don't fully understand? They said they had done it before..

 

I don't really have a SAP rating in mind, maybe I should? I was just confused as I've only seen the ufh pipes go into the foundations on the videos I've seen so wanted to question it.

 

 

Definitely raft foundations.

But there is a massive difference in a raft Foundation and an insulated raft Foundation.

He could be building block and then pouring a concrete raft which ontop he will put your insulation and then another thin screed which holds your ufh pipes.

[Bitpipe]

If you're doing a new build then you will need to have done a SAP calculation to satisfy building control that you'll meet the minimum standard. You can do this once you have decided the fabric and design of your floor, walls, roof and windows, plus your heating system. This is where you can see how improvements to the fabric impact on heating requirements  etc. and figure out whether its a good investment or not.

 

As Declan says, a standard raft is reinforced concrete, with additional strengthening where there are load bearing points, usually cast onto a compacted layer of type 1 with a membrane on top. Normal shuttering methods used to cast it. 

 

It is a cold slab, directly connected to the ground below so any heating upon it (UFH) needs to be thermally disconnected  from it by means of  insulation, usually 100-150 XPS. The UFH pipes are clipped to this insulation and a screed is poured on- top of that.

 

When the UFH comes on, only the screed layer heats up which means response times are quicker but you tend to need to run the system hotter and the insulation under the screed is isolated from that in the walls, so less thermally efficient all round. However you can still get a decent standard.

 

An insulated raft is built out of 200-300mm thick EPS formwork which acts as both insulation and shuttering. the UFH pipes are connected to the slab reinforcement steel (whether bar or mesh) and the concrete is taken up to near the finished floor level - sometimes power floated to give it a finished surface. 

 

This is a warm slab, completely insulated from the ground and with good design, the perimeter insulation can be tied into the wall cavity insulation to give a highly insulated envelope for the whole house which will greatly reduce energy requirements. The UFH is usually run at a low temp (usually 35^o) as one big zone and the slab has a slow response time, but if very little heat is required then this is not an issue. if you're using an ASHP vs gas, this can nicely drive the slab heat requirement and some people here run it in a cooling mode in summer, keeping the slab just above dew point. Even when heating is not required, running the UFH pump can distribute heat away from areas that get direct sun and even out the temp.

 

Downside of an insulated slab is the increased cost of the formwork and labour to place it properly, plus additional excavation. You save on avoiding the traditional screed sandwich though so needs thinking through.