Floor screed choices

[crooksey]

I am currently in the process of a refurb of a 20 year old house, its off grid and we have just replaced the old LPG boiler/tank with a 16kw Midea ASHP. I am installing a 60mm PIR insulation board (possibly thicker, depending on floor screed choice).

 

We have striped the ground floor back to block & beam, new extension also has block & beam.

 

A lot of people have recommended a gypsum based anhydrite floor screed, I come across these alot (work in the flooring trade) and ideally would like to avoid these if possible. Main reason is their laitance, moisture retention when first laid etc. My thinking was I would buck the trend and go for a "traditional" sand and cement floor screed, creating a bigger thermal mass, maintaining a more even temperature and probably a more constant flow temperature from my ASHP.

 

The argument for anhydrite screeds is better thermal conductivity (e.g. quicker to heat up) but cement based screeds are known for retaining the heat for longer.

 

My thinking was a cement based screed at 60/70mm would be easier to maintain over winter when temperatures drop below zero (typically at night) and I could keep the ASHP running on a lower temperature to maintain a decent floor temperature. As opposed to the anhydrite screed which would more than likely reach its desired temp much quicker, cut the pump out then cut back in again as it needed to etc.

 

My thinking of course on the thermal performance is pure speculation, but would appreciate feedback from others and how happy you are with the floor screed you have? Is anyone running a 60/70mm sand/cement floor screed with a ASHP, how do you find it?

 

 

[Thorfun]

We had a 50mm cement based liquid screed (Cemfloor). I believe sand and cement needs to be 75mm min so you’d gain an extra 25 mm insulation and no laitance by using Cemfloor. 

[crooksey]

6 minutes ago, Thorfun said:

We had a 50mm cement based liquid screed (Cemfloor). I believe sand and cement needs to be 75mm min so you’d gain an extra 25 mm insulation and no laitance by using Cemfloor. 

How do you find the heat retention after the pump has turned off? I can go 60/70mm with something like Mapei TopCem which is something I was looking into. Cemfloor was something I had considered but wasn't sure if going thicker would give better performance.

[Conor]

We've 50mm cement based liquid screed. Solid and flat. Can't ask for much more than that. I think a lot of the thermal conductivity stuff is marketing hype from the sellers. Get the thinnest coverage product and maximise your insualtion. The more insualtion you have, the cheaper it will be to heat the house.

[Mr Punter]

51 minutes ago, crooksey said:

its off grid

I assume you just mean no mains gas?

52 minutes ago, crooksey said:

I am installing a 60mm PIR insulation board

 

It is not thick enough if you have UFH and don't want to heat up the ground underneath.

[PeterW]

So the idea that gypsum screeds hold less heat than cement based is incorrect - gypsum has a better heat capacity but it would be marginal unless you compare pure gypsum to pure OPC. 
 

60mm insulation is pointless with UFH - oversize your rads and put the most insulation you can under the floor as otherwise you’re just heating the floor void. Not sure how much you have removed or what BCO will say as for the extension it won’t meet regs either. 

[crooksey]

48 minutes ago, Mr Punter said:

I assume you just mean no mains gas?

 

It is not thick enough if you have UFH and don't want to heat up the ground underneath.

 

No mains gas or sewers, call it what you want, but not completely isolated, we have incoming mains electric and water.

 

60mm wasn't actually correct, I should have just said PIR, this hasn't really been decided and am waiting to choose a floor screed (as thin as possible with best performance) and then work out how much insulation I can fit below it.

[Thorfun]

1 hour ago, crooksey said:

How do you find the heat retention after the pump has turned off? I can go 60/70mm with something like Mapei TopCem which is something I was looking into. Cemfloor was something I had considered but wasn't sure if going thicker would give better performance.

I'll let you know once the house is finished and we've moved in.