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.

[Kelvin]

I’ve got a decision to make on this coming up. We’re 200mm insulation. The build up is 20mm allowance for final floor material over 75mm screed (traditional sand cement) However reading the Cemfloor website it says that using it allows for thinner screed thickness with maximum depth of 60mm. Optimal depth between 40mm-50mm. If I go Cemfloor I’ll need to add more insulation and go to 225mm (150mm + 75mm), not a bad thing of course with 50mm Cemfloor Therm screed. 
 

Just interested why people have suggested going more than Cemfloor recommend? 
 

[Dave Jones]

What is your wall construction, brick and block ?

[Kelvin]

Timber kit (302mm CPS panel) on block foundation. Top block is a 300mm insulated trench block. 

[Dave Jones]

As long as your inside FFL to outside is still ok i'd go for 75mm screed.

 

 

[JohnMo]

Our floor is 200mm PIR, 100mm concrete screed.  UFH pipes at 300mm centres.  Running low flow temp (about 32 at -3). Slow reaction time to heat inputs, long release of heat after heating turned off. You have to be careful with flow temps, to little heat up time takes forever, to much heat you overheat the house.

 

House has a nice feel as no real heat occurs in the house, think that comes from the low flow / floor temp.

 

Either batch heat or low and slow with thick screed.  Thinner the screed the more like a radiator it becomes from I can gather.

 

The thinner the insulation the higher the flow temp required.  So if you are going heat pump now or in the future, add more insulation at that depth use 150mm PIR.

[Kelvin]

28 minutes ago, Dave Jones said:

As long as your inside FFL to outside is still ok i'd go for 75mm screed.

 

 

Sure but why do Cemfloor state that 60mm should never be exceeded? That’s pretty emphatic. I’m going to call them anyway. 

[JohnMo]

1 minute ago, Kelvin said:

Sure but why do Cemfloor state that 60mm should never be exceeded?

Possibly due to shrinkage cracks? If they say don't exceed and you do and something is not right after, they will just say you exceeded the max depth, the problem is yours to sort out.

[Dave Jones]

1 minute ago, JohnMo said:

Possibly due to shrinkage cracks? If they say don't exceed and you do and something is not right after, they will just say you exceeded the max depth, the problem is yours to sort out.

 

never use liquid screed, always dry.

[JohnMo]

1 minute ago, Dave Jones said:

never use liquid screed, always dry.

Confused, what are you saying? 

 

I used fibre reinforced concrete, mine was installed before the walls went up.

[Kelvin]

Just spoke to Cemfloor. They said the information on their website is incorrect and you can install thicker depths than 60mm with no problems. They recommend you don’t just for performance reasons. They are going to update their website with the correct information. 
 

 

[saveasteading]

5 hours ago, Dave Jones said:

 

never use liquid screed, always dry.

I was really surprised how liquid the screed was. I knew it had to be, to come through a 50mm pipe and to self level, but I was still surprised. 

This is not through excess water though, but the use of fine aggregates and plasticiser additive.

The finished floor had a few cracks but mostly where 2 pours met and were reworked together. 

 

The thickness was about 60mm but varied thinner where there were errors in the base layer.

300m2 in 6 hours. Impressive how easy they made it look.

[Kelvin]

Was that Cemfloor? 

[Happy Valley]

Our Cemfloor was 60-70cm over 200mm insulation with UFH pipes run approx every 150mm

[joe90]

3 minutes ago, Happy Valley said:

Our Cemfloor was 60-70cm over 200mm insulation with UFH pipes run approx every 150mm

Ditto and no cracking 👍

[saveasteading]

1 hour ago, Kelvin said:

Was that Cemfloor?

Yes. 

[Kelvin]

6 hours ago, Kelvin said:

Just spoke to Cemfloor. They said the information on their website is incorrect and you can install thicker depths than 60mm with no problems. They recommend you don’t just for performance reasons. They are going to update their website with the correct information. 
 

 

 

Cemfloor have updated their website already and removed the incorrect information. 

[crispy_wafer]

I'm another with cemfloor, I'll have to check through my previous posts, but 65-80mm thickness over 150mm pir, no cracks and solid.  Overall Im pleased with it

[saveasteading]

I don't think cracking is proportional to thickness in this situation.

Our specialist was thorough in putting crack inducing joints across doors and large expanses..just vertical strips of hardboard,  not even straight.