wet ufh between joists and timber overboard advice

[kilty55]

hi all, i have a job to look at for a customer who is getting conflicting advice from the ufh installers and the joiners

the ufh installers are putting in an inbetween joist system of some sort and are advising screwing 22mm ply to the joists as an overlay and tile direct

the joiners are advisign her to lay chipboard over it

none have given her any advice on comissioning the ufh...now does this type of system with no screed need to be commisioned in the same way with the ply overlay or would it differ?

also i was going to spec ditra if the ply overlay was agreed...btw alsoa natural stone finish yet to be chosen

any advice would be appreciated

 

[AliGage]

Never done a wet UFH. What system is this? I wouldn't of though warming any timber substrate that's to be tiled would be a good idea.

Having said that, if it was a choice between the two, it'd be ply with a de-coup.

 

[Ajax123]

22mm ply screwed to the joists. As with all systems it should be commissioned and run prior to tiling. They are horrible systems in terms of efficiency. Personally I would get shot of the diffusion plates and run a flowing screed in between the joists onto a deck of sterling board supported on battens nailed to the joists and insulated from underneath.

 

[Stef]

Not sure Jamie, never heard it like that.
What would give more expansion/contraction?
I think the ply would.

 

[kilty55]

thanks guys..job is upper floor level i beleive so may be why they have gone for the pipes run inside the joists with just an overboard....to me it doesnt sound very efficient either ajax have you come across many clients who have had these put in and what was there opinion?

 

[Ajax123]

thanks guys..job is upper floor level i beleive so may be why they have gone for the pipes run inside the joists with just an overboard....to me it doesnt sound very efficient either ajax have you come across many clients who have had these put in and what was there opinion?

i know of lots of these installations. Trouble with diffuser plates (thin ally plates) they have no mass so cannot develop thermal inertia so they heat up and cool down really quickly. Trouble is when you then put an insulator on top (floor boards) you deaden the response so that the heating becomes very inefficient.

Ive actually got wet heating between the joists in my lounge, bathroom and attic room but I overlaid them with a screed to give thermal mass.

 

[kilty55]

this sounds much more sensible way to do a job liek this imo ajax it doesnt make sense to me doing the pipe system and laying just ply over it

 

[jonnyc]

I have laids dozens of stone floors over water fed systems between joists.
sometimes they have had screed between joists sometimes insulation and plates.
until about a year ago i would always say minimum 18mm plywood glued and screwed.
i would then ditra mat on top.
never had any problem but schluter do not recommend ditra now over this type of system.
last few jobs i have done we have stuck down hardibacker 6mm and screwed or the trimline cement baoad that is similar.
this does not stop heat coming through but important to remember the that the subfloor must be rigid and loadbearing before cement board is fixed.

 

[Qwerty]

schluter do not recommend ditra now over this type of system.

I was unaware of this. Interesting to see who does

 

[DHTiling]

Like Jonny i have previously used Ditra over the ones i have done and they were ply over lays... i did hear about Schluter advising against this now... So do schluter also advise against using with wet UFH in a screed as well. ?

 

[kilty55]

urgh.....thanks for that up to date info johnny...customer has just told me they are screeding in the ufh also my plan was ply and then ditra.... might have to re think

 

[jonnyc]

urgh.....thanks for that up to date info johnny...customer has just told me they are screeding in the ufh also my plan was ply and then ditra.... might have to re think

best thing would be call schluter diect.
no problem using ditra over a normal screed with under floor heating its just between joists that is an issue.

 

[kilty55]

update direct from the plumber....

they are battoning joists and sheeting then fitting kingspan and a screed to joist height

wonder why they used to endorse ditra but not anymore?