I've just had an extension built and I'm going to be installing wet UFH. The room will be an open plan kitchen / sitting room with a floor size of 35 square meters.

The UFH system is a polypipe system using trays to lay the pipes onto. We are looking a tiling on top of the UFH and though the doors into the existing house.

There is a concrete slab already laid (no insulation in as yet only DPM) leaving a total of 150mm to play with to floor level.

My current thinking was along the lines of:-

>Lay 75mm kingspan or similar insulation on top of slab

>Lay UFH trays

>Put pipes onto trays, 15mm pipes

>Lay 60mm screed

This will take me to the 150mm to get the floor levels.

Questions are as follows:-

What screed would you use, flow screed or sand and cement?

What kind of costs should I be budgeting for the screed?

What depth screed would you recommend? I'm guessing any space saved by less than 60mm screed could be made up with thicker insulation boards? I also guess it's better to have minimum screed depth to aid heat transfer?

Sorry for all the questions but I think getting the prep and spec right now will save me lots of time and hair in the future!

Cheers,

Ant

Has to be flowscreed for me very time with underfloor heating. Have you already bought he trays cos you could save quite a bit of cash and improve the thermal performance of your floor if you want to.

cheers for the quick reply!

Not committed to buying the trays as yet, wanted to get everything sorted first, although i was planning on ordering it later today.

would direct clipping give more thermal performance? Also what brand / type of flowscreed would you go for?

I could do with understanding what thickness kingspan or similar I should order, I'm paranoid about putting it in then not being able to get the right floor level with sufficient pipe coverage etc.

Not really appropriate to be selling over the forums but if yo want to ring me on 07545 932723 I can talk you through features benefits, upsides and downsides, and specs if you like. Where in the country are you.

Firstly I'm a DIY'er if that makes any difference but I'm also an industrial chemist and this bonding to calcium sulphate screed is really interesting....I'll get my coat ;0)

After having a chat with Ajax I've plumped for the Gyvlon screed, 35 square meters laid by these guys... TW Pump Hire - Liquid / Flowing Screed Application (ps thats not an advert as I'm not connected at all but merely recommending them from experience)

It's been in a while now and we've had the wet UFH on so it should be quite dry. Now as a DIY'er trying to find somewhere that did Gypsum based adhesive was neigh on impossible however by luck creative impressions is located about 20 mins from where I live. I took a drive down there yesterday and picked up some bags of their GBTA along with some primer.

I decided for the sake of the short while to prime the surface after ensuring all the laitence had been removed. The primer looks very much like PVA although it did smell like it had some type of ammonia based compound mixed with it. Two coats of primer went on easily and was no issue at all.

I'll report back as to the use of the GBTA and my experiences once completed.

Two tilters type questions first if that OK?

I'm putting Tavertine 60x40 stone down, what bed size would you plump for? I was going to work on 3mm however I'm not sure if thats sufficient?

Also the room is open plan so I was going tile half of the room, move the furniture, kitchen units etc to the other half once tiled and allowed to dry and do the other half... will this be OK?

Cheers for now..

Ant

Hi,
My experience with large tiles is that a resultant 5mm bed (meaning what you should end up with when the tile is bedded) is the shallowest I would use to ensure full contact.

I do not envisage any problem with your method of construction, GBTA has very low shrinkage therefore differential movement due to the different age will be minimal. may I suggest that you prime the second area when you are about to screed it. good luck

Have to show my hands here, I am the Technical Manager at Creative Impressions.

Sabo

Thanks for pointing that out Sabo...


Is there also a set period that GBTA should be left before commissioning the adhesive( GBTA ) to heat?

Hi Dave,

Depends on bed depth. At 5mm resultant depth, I would start to commission after 72 hours. Deeper beds may need longer - the idea is to create as little vapour pressure as possible, so allowing time for more of the mixing water to disappear should make sense. My rule of thumb for our GBTA is to allow a day for every 2mm resultant bed depth.

The process (for the sake of those not familiar) is to build the temperature 5 degrees per day until the maximum working temperature is reached. Some manufacturers say 3 degrees, personally I do not think it makes any difference which you choose although 3 degrees obviously allows more drying time for the floor.

Regards

Sabo

cheers.. i take it room temp does not dictate drying on the gypsum based adhesive then... cement adhesive will set above 5 dgs no probs but not up on gypsum adhesive drying times..?

Now i do know that gypsum screeds dry slower in low temps.. just if you could clarify for our members about GBTA..

Sorry to crash in on Anthony's thread but are all liquid pumped screeds gypsum based? I'll soon be laying the UFH piping for my self build ready for screeding and wondered if there's a recipe I should ask for, for floor tiling. I've been on different sites when different companies have poured screeds and some seem to set far quicker than others.

Thanks

Pete

Hi Pete, No!.. you can get poured cement based as well..

Hi Dave,

Yes, room temperature does dictate drying of Gypsum.

At the risk of getting technical, both systems use water to form new compounds. Anhydrite and hemihydrite are simply a form of de-watered gypsum. add water to these compounds and they become gypsum. In simple terms, think about them as equivalent to say, sun-dried tomatoes, prunes etc. You rehydrate thes by putting them in a bowl of water -excessive amount of water. Fully hydrated, you are still left with water to throw away. In floors, this excess water has to evaporate, evaporation requires heat and low humidity. Low temperature and high humidity means slower evaporation. This is the reason the gypsum screed manufacturers advise ventilation (so that any evapoated water gets taken out of the room, since there is a danger that moisture can be re-absorbed by the screed - a reason gypsum screeds appear to dry at slower rate).

Cement also uses water but different compounds are formed and more heat is generated internally due to these chemical changes. As we all remember from school chemistry, some chemical reactions, including the setting and hardening of cement are accelerated by heat. The hydration process in cement-based systems though takes more time in reacting the compounds - thus the need to 'cure' cement i.e. not allow it to dry too quickly.

GBTA is formulated to have less evaporable water and set at cement type temperatures.

Long-winded but hope goes some way to answering your question.

Sabo

Sure does , perfect reply.

Originally Posted by Ettr7920

Hi Dave,

Yes, room temperature does dictate drying of Gypsum.

At the risk of getting technical, both systems use water to form new compounds. Anhydrite and hemihydrite are simply a form of de-watered gypsum. add water to these compounds and they become gypsum. In simple terms, think about them as equivalent to say, sun-dried tomatoes, prunes etc. You rehydrate thes by putting them in a bowl of water -excessive amount of water. Fully hydrated, you are still left with water to throw away. In floors, this excess water has to evaporate, evaporation requires heat and low humidity. Low temperature and high humidity means slower evaporation. This is the reason the gypsum screed manufacturers advise ventilation (so that any evapoated water gets taken out of the room, since there is a danger that moisture can be re-absorbed by the screed - a reason gypsum screeds appear to dry at slower rate).

Cement also uses water but different compounds are formed and more heat is generated internally due to these chemical changes. As we all remember from school chemistry, some chemical reactions, including the setting and hardening of cement are accelerated by heat. The hydration process in cement-based systems though takes more time in reacting the compounds - thus the need to 'cure' cement i.e. not allow it to dry too quickly.

GBTA is formulated to have less evaporable water and set at cement type temperatures.

Long-winded but hope goes some way to answering your question.

Sabo

we don't mind technical, GBTA, is that Gypsum Based Tile Adhesive?

Spot on Doug