UFH Pipes over an insulated concrete slab

2 Sep 2015
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United Kingdom
Newly constructed basement I have a concrete slab with 100mm of insulation beneath it.

What would be the best method to use when fixing the UFH pipes? If insulation was above the slab I know it can be easily fixed to that.... architect specd for insulation to be put under slab!

Membrane > UFH pipes clipped directly into the slab > Screed

Would this not result in heat going in both directions? As in heating slab down and also the floor up.
Is there some kind of foil i can place down before fixing the UFH pipes?

I could think of adding 25mm celotex and fix pipes to that, however I do not want to reduce height of room.
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Clip rail system would work
It will take longer to heat up than a insulation above slab system but as long as you operate heating with correct back set temperature should not be a problem.
What's screed thickness?
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I am sure i have seen some kind of foil sheet to lay on top of the slab to reflect the heat back up?

Planned screed thickness will be 65/70mm.
Unless you have already done it get a specification/layout from one of underfloor suppliers.
You will then have some come back
We are polypipe registered installer's normally get specification back same day as we email plans.
Lots of others out there.
ive reached out to a few and one has suggested to use the crates.

Is there anywhere i can get a pipe layout design? atm all i am getting is the breakdown of components required, or is the design only provided once you complete the purchase.
TBH that should all have been considered as part of the slab design and the required ceiling height. Unfortunately you are now falling foul of that. Apart from using the egg boxes there aren't many other ways to hold the runs properly until the screed is poured and sets.

You could use a mix of SS screws and clips and drill but that's a bit of a farf but it would be cheaper, the other option would be to channel the slab but that's more major work. The are other overlay system that minimise the increase is height as you wouldn't need a screed then but there is a added cost to that as well.

Any slab below UFH embedded in screed on top will always absorb some heat but that's per design and the whole thermal mass will then heat the space above and wouldn't be lost downwards given the insulation below. You just need to work with it properly as suggested with correct set back etc.
Having had a few UFH installers in, 2 have suggested to use aluminium layer on the current concrete slab, to reflect the heat back up.

2 products mentioned are:


Vast difference in price! Understand that the amazon product is just a layer to reflect heat back up, where as the superfoil is also an insulator (not show how good though!)

Would it be beneficial to install one of the items above?
So build up will be as previously mentioned>

Insulation > Concrete Slab > Waterproofed Slurry Tanked Floor (Basement) > DPM > Aluminium Reflective Layer > UFH pipes clipped > Screed

Would I need the DPM if the floor has already been tanked using a waterproof slurry?
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Is this suggestion to maximise headroom? Could you not stick 100mm celotex down?

Personally I can't super foil being as efficient as PIR.
Having already put 100mm of rigid insulation below the slab, I cannot have that much over the slab now due to keeping an eye on head height.

The aluminium foil will be just to reflect heat back up (as much as it can). I understand that the method in which the floor has been constructed will result in the slab being heated until it hits the insulation below.
Sorry, didn't read properly, If the insulation is under the slab, then aren't you good to go.

It just means you'll be heating up a large slab, instead of just a screed layer, this will take longer to heat initially, but will stay warmer longer.

If you use superfoil malarkey, are you intending to screw clips/strips into the slab? They will have to be well screwed in, as the pipe has a fair amount of spring in it especially when dealing with curves.

If the trays are too expensive... (not sure on your meterage to cover) then wondering if you could use reinforcement mesh and tie pipe to it? and screed the lot.

What about spreader plates? again cost??
I think i will tank the basement floor (i have already done the walls) and add a DPM over the top just for extra protection. I am planning to use clips to hold the pipes down which will be via using a nail gun - clipping the pips directly through the DPM into the concrete slab.

As you have mentioned, heating will take longer initially however will stay warmer longer - i think i can live with this.

The foil idea was just a thought to get more heat reflected back up, rather than going down through the slab.
The slab (and the U/F insulation) should have a DPC under it extending up the walls.

With the slab sitting on top of the Insulation it is intended that the Slab is heated up with the U/F heating system. Once warm it will stay warm longer so (hopefully) uses less energy
use aluminium layer on the current concrete slab, to reflect the heat back up.
Hah. No, alas. If that worked then you wouldn't burn yourself picking up a stainless steel pan full of boiling water by grasping the sides because the mirror surface would be reflecting the heat back into the pan

Shiny surfaces reflect radiant heat, not conducted heat. Your screed slab with UFH pipes in will conduct heat to the aluminium it is in direct contact with; no heat "reflection back up" will occur. It's also likely that the aluminium will corrode and possible that it will subsequently damage the screed chemically. An aluminium layer on top of a slab with screed poured on top of the Alu would do absolutely nothing to prevent conducted heat reaching the slab. A floor-on-floor of a network of battens with aluminium spreader trays between and a chipboard floorboarding over would do next to nothing to stop heat reaching the slab, the only insulation in that direction being offered by the air gap under the spreader plates

Your UFH pipes should perhaps (depending on the use pattern of the room, see below) have been clipped to the insulation below the slab and the slab poured over them. Now they should be clipped to the slab or to something clipped to the slab (clips, clip rails, egg trays) or heavy enough to stay put (reinforcement mesh, but take care not to stand on the pipes tied to the mesh) or to another layer of insulation barriered with a plastic sheet so screed can't get underneath it, to stop them floating when the screed is poured. Even if they were full of water (and it's recommended to pressurise them before pouring but doesn't have to be with water), without clipping they would float in the screed..

So long as your room is often used, It doesn't matter about heating the slab because on/off/on/off is how radiators are run, not UFH. You use UFH to heat a well insulated slab to some low temperature like 25 degrees (more if you have an insulative floor covering like carpet) inside a well insulated room. The room then maintains a temperature like 22, and you pay to constantly heat the slab a small amount as it loses a small amount of heat to warming the room. The insulation below the slab, which is hopefully quite a thick layer like 100mm+ (of PIR or equiv) inhibits heat loss into the ground so the heat loss from the slab (that you pay to replace) goes to heating the room. Insulate your room walls well, and ensure that poured screed is insulated from the walls (run the wall insulation right down to the under slab insulation) and it won't matter that the slab takes 5h to get up to temperature because once there it stays there with minimal ongoing input

If the room is used only occasionally, UFH may not be the best choice but you can make a faster responding UFH that you can blast on and off like a radiator with eg a grooved cement boards layer on top of a thick layer of insulation

UFH needs designing properly for the use pattern of the room, ideally before slabs are poured.
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