Screed thickness with UFH, over PIR. How thick?

[saveasteading]

Opinions please  Cemfloor 50mm or 75mm?

 

We have designed the floor as 75mm Cemfloor over 125mm PIR.

Two contractors have suggested reducing to 50mm to save cost, implying that this is the normal thickness.

We can do this simply by increasing the insulation thickness.

 

The 75mm was not chosen randomly but from research on BH and elsewhere.

 

Very roughly the extra cost of insulation would be £10/m2 in PIR (or £5/m2 in EPS), and the saving on screed about £5/m2, so the premise of saving construction cost is wrong.

However, the question is interesting...A thicker more robust screed with greater heat absorption property, but slower to react?

Or more insulation?

 

[Conor]

More insualtion. 

[crispy_wafer]

When we finally get the last window in, and the UFH down, we'll be pumping in about 60-65mm - bit of a halfway house, as I'd read others doing 50mm and others going thicker, then I managed to get 150mm boards as good seconds so that made my mind up. 

 

Have you already laid the insulation?  If so I'm not totally sure of the minimum thickness boards you can get hold of at reasonable money to build up the level, I've seen 20mm, not sure if you can get any thinner.

[saveasteading]

No insulation ordered yet. 125 theoretical thickness which may turn out to be 120. Also wouldn't want to find a wobble in the existing concrete base that made the screed less than 50.

My main argument for thicker screed is that  I  think i recall someone on BH recommending a thicker screed to make better use of offpeak electricity.

But it would then take longer to react in the case of a cold turn, and waste heat if the temperature improved suddenly.

 

With the diminishing returns  proportional to depth, it could be reasonable to lay eps 25mm ish on the bottom, if that makes the thicknesses add up.

[saveasteading]

Have had quotes from local specialists.

 

The surprising thing to me is the sheer cost of the Cemfloor material, at about 3 x the cost of concrete.

 

That helps in the sums

 

Nobody has come back on saying 75 screed has advantages.

 

So we will now get PIR prices and see what thickness is likeliest so that we have 140/150 of PIR then 60/50 of Cemfloor.

 

We haven't  even prepared the ground on half the building, so could go a little deeper, but I'm happy with that PIR thickness.

[crispy_wafer]

I'd heard that a slight thicker slab will act more like a slow release heatstore but never experienced in practice so could not comment further.   Here's some prelim prices I had for screed if it helps for comparison, I think these are a bit on the high side though

 

[saveasteading]

Useful info thanks. I won't publish our quotes as it is a live discussion still. In principle though we have a bigger area and higher price. If we could do it in one pour then getting closer.