Wet ufh screed

[Rossymcg]

A question for anyone in the know, I'm retro fitting wet ufh down stairs in my house, I'm taking up the timber floor boards acro proping the timber joists to help carry the addition load,fitting Kingspan in between the joists, then the pipe work,
The screed to use is my question, does anyone know if I could use a standard concrete mix if not why not? I've been involve many times in new build wet ufh, where we've used sharp sand and cement damp mix and the pumped flow screed, but I'd only be doing a room at a time and would be better if I could mix the concrete and lay myself

 

[Ajax123]

I would and have used anhydrite to do this. It gives better thermal performance and response than concrete and is stronger at thinner depths.

 

[Rossymcg]

I would and have used anhydrite to do this. It gives better thermal performance and response than concrete and is stronger at thinner depths.

How much of a better thermal performance? I'll be screeding to a depth of 100mm, I'm gona need 2m3 for 1st room, how would costs stack up having your stuff pumped in against concrete?

 

[Chalker]

How much of a better thermal performance? I'll be screeding to a depth of 100mm, I'm gona need 2m3 for 1st room, how would costs stack up having your stuff pumped in against concrete?

100mm ! That's a lot of weight on top of joists.

 

[Rossymcg]

100mm ! That's a lot of weight on top of joists.

That's why I'm acrow propping the joists from below,

 

[Chalker]

Normally when I'm fitting ufh between joists, I fit kingspan on bearers between the joists 50mm below the top. Clip the pipe to the kingspan and fill the gap with a weak sand and cement. Then self level the whole floor. Install edge insulation around the perimeter for expansion, Then lay fermacell over the top. It's a quick heat up and you can tile or carpet straight on to.
p

 

[Rossymcg]

Normally when I'm fitting ufh between joists, I fit kingspan on bearers between the joists 50mm below the top. Clip the pipe to the kingspan and fill the gap with a weak sand and cement. Then self level the whole floor. Install edge insulation around the perimeter for expansion, Then lay fermacell over the top. It's a quick heat up and you can tile or carpet straight on to.
p

it an old house and the joists are only 100mm deep, I was considering fixing the Kingspan to the underside of the joists, minimum dept for sand and cement screed with ufh is 75mm which would have left me with 100mm to fill with concrete, but the 100mm of concrete is quite a bit of mass to heat up,
i like your style with this way, how come only a weak compo mix?

 

[Chalker]

it an old house and the joists are only 100mm deep, I was considering fixing the Kingspan to the underside of the joists, minimum dept for sand and cement screed with ufh is 75mm which would have left me with 100mm to fill with concrete, but the 100mm of concrete is quite a bit of mass to heat up,
i like your style with this way, how come only a weak compo mix?

You will get 75 mm with the system I proposed. 50mm of screed and 25 mm of solid fermacell. If you are tiling, that is also part of the mass. The screed is a weak mix as its not structural, its just there as mass.

 

[Rossymcg]

Where abouts are you getting the fermacell birds from?
I've googled them but can't seem to find a stockist

 

[Chalker]

Where abouts are you getting the fermacell birds from?
I've googled them but can't seem to find a stockist

I get mine through MKM builders merchant.

 

[Ajax123]

If your joists are 100mm you can put 70mm of king span supported on bearers then on top of that 30mm of Gypsol TimBRE screed on top. That will bring you to the tops of the joists and you can put your floor boards down. The performance of anhydrite vs concrete is about two
Twice as responsive. And will give you better performance at lower flow temperatures.

 

[Rossymcg]

If your joists are 100mm you can put 70mm of king span supported on bearers then on top of that 30mm of Gypsol TimBRE screed on top. That will bring you to the tops of the joists and you can put your floor boards down. The performance of anhydrite vs concrete is about two
Twice as responsive. And will give you better performance at lower flow temperatures.

Can I buy the gypsol timBRE in bags and mix myself? Or does it come ready mixed on the back of a wagon?
what makes it twice as responsive?

 

[Ajax123]

Can I buy the gypsol timBRE in bags and mix myself? Or does it come ready mixed on the back of a wagon?
what makes it twice as responsive?

you could buy sureflo in bags from CCF which is the same thing to all intents and purposes. Or you could have it mixed on the back of a wagon. When I did my lounge I needed just under a cubic meter which I mixed by hand. It took a couple of hours. What makes it more responsive is to do with compaction, mass and the fact it is gypsum which is inherently more conductive than cement based screed. Thermal conductivity of sand cement screed is approx 1.1W/m2k whereas Gypsol sureflo is around 2.2. It is self compacting which means no air entrapment. Air is a good insulator so you don't want it there. Because Gypsol is a thinner screed option it has less physical mass to heat up so is more responsive.

 

[Ajax123]

http://www.gypsol.co.uk/Technical/technical.html

More info here

 

[Rossymcg]

you could buy sureflo in bags from CCF which is the same thing to all intents and purposes. Or you could have it mixed on the back of a wagon. When I did my lounge I needed just under a cubic meter which I mixed by hand. It took a couple of hours. What makes it more responsive is to do with compaction, mass and the fact it is gypsum which is inherently more conductive than cement based screed. Thermal conductivity of sand cement screed is approx 1.1W/m2k whereas Gypsol sureflo is around 2.2. It is self compacting which means no air entrapment. Air is a good insulator so you don't want it there. Because Gypsol is a thinner screed option it has less physical mass to heat up so is more responsive.

Sureflo sounds the one, I don't have meters and meters to justify ready mix,, I know ccf well and will talk to them on Monday,
cheers Ajax