ufh directly on to 18mm plywood

[dontknowitall]

working on a new bathroom with wet room area in one corner. I am using a maxxus former in the wet area and lastogum tanking so everything should be covered there. I have a perfectly level floor using 18mm WBP plywood with no step into the wet area. I plan to have ufh installed under the tiles in both areas.

reading the previous articles I understand that I need to seal the plywood with a quality PVA and use less than 150Watt matting to avoid substrate distortion. If I want better warm up time then I should also lay cement board. However, I'm reluctant to do this as I will have 6mm step into the wet area.

can anyone advise if my assumptions are correct and I can lay ufh electric matting (under 150Watt) directly onto WBP plywood, sealed with PVA without significant problems such as tile lifting. Or, are there alternative options available to me that will mean I don't raise my floor height ?

 

[Richard C]

You should be tanking the entire floor area if you want to avoid problems, not just the immediate shower area.

Is a replacement floor & not an overboard? If so, 18mm WBP ply is the minimum, the thickness you need will depend on size/pitch/span of your floor joists; did you seal the back & edges with acrylic primer before you laid it?

Don’t know which “previous articles” you’ve been reading but on no account use standard PVA to seal the WBP, its water soluble & if water gets to it your tiles will fall fail; also most cement based adhesives (which you must use) do not recommend any primer as this can affect tile adhesion. Use only quality trade tiling products, the DIY stuff is mostly crap.

UFH is very trendy a the moment but I’m personally unsure of the wisdom about installing it as a secondary heating system in any room. Unless you run it on the timer you will need at least a 30 minute warm up time, longer if you don’t install an under mat insulated backer board. Fine if you use the bathroom at exactly the same time every day but otherwise you’d do better to install an oversize towel rail or a conventional radiator. Can you explain what you mean by “under 150 Watt? Do you mean per sq/m? Ideally, you should also lay an SLC over the mat to avoid damage during tiling & if you want to stand any chance of repair if it goes wrong; bed the element in cement based adhesive & you will never be able to repair it. It can also be expensive to run.

If your new to this forum, I would advise you read the tiling sticky & forum archive posts, it may prevent you making disastrous & potentially expensive mistakes; come back if you have further questions.

 

[dontknowitall]

Hi Richard,

Thanks for your advice, very helpful.

The plan is to tank the entire area.

The room is small, 1700 x 3000, the 18mm plywood floor is directly on to the joists, the joist span is small under 300mm in all instances. The floor is not fixed yet so I will use an acrylic primer on all ends as suggested. Understood on the PVA!

The ufh it is a complimentary solution to take the chill off the tiles, there is already an adeuqate towel rail. Some of the ufh manufacturers I've looked at offer, 100,150 & 200 W/m2 and recommend 100,150 W/m2 for wood substrate floors.

I guess my main concern is heating elements on the plywood and causing some distortion of the substrate and eventual tile lifting. Anything I could do to prevent this that avoids raising the floor height significantly is the goal.

 

[dontknowitall]

http://www.thermilate.com/index.php?option=com_content&view=article&id=64&Itemid=70

anyone got an experience with this type of product?

 

[Richard C]

The room is small, 1700 x 3000, the 18mm plywood floor is directly on to the joists, the joist span is small under 300mm in all instances.

300mm can’t be the span, do you mean pitch between the joists? If so that’s unusually close! What is the cross sectional size of the timber joists & what is the span – the distance between the supported ends?

The floor is not fixed yet so I will use an acrylic primer on all ends as suggested. Understood on the PVA!

You should acrylic seal the edges AND underside, this is to prevent seasonal ambient moisture changes affecting the ply.

The ufh it is a complimentary solution to take the chill off the tiles, there is already an adeuqate towel rail. Some of the ufh manufacturers I've looked at offer, 100,150 & 200 W/m2 and recommend 100,150 W/m2 for wood substrate floors.

I guess my main concern is heating elements on the plywood and causing some distortion of the substrate and eventual tile lifting. Anything I could do to prevent this that avoids raising the floor height significantly is the goal.

Assuming this bathroom is on the second floor, you will get heat permeation through from below so the tiles will hardly be cold. I’ve had experience of UFH in cons, bathrooms & kitchens (you definitely don’t want one in there) although they sound brilliant, in reality they are impracticable as a secondary heat source. You would only need 200 W/sqm in a conservatory IMO & that’s what I have as the primary heat source in my own cons (a total of 4.4Kw) which works very well; with an insulated concrete slab & insulated backer boards directly under the element, response time is quick at around 30 mins but it costs bloody fortune to run in anger! I took the decision not to fit complimentary UFH in my personal bathroom & shower rooms & I don’t regret it. The tiles are not cold underfoot (especially if you stand on a bath mat ) & it would not have added much benefit IMO but the biggest factor is they just aren’t very practical in use as a secondary heat source. Unless you leave it permanently on during waking hours, you will invariably forget to switch it on, say, 45 mins before using the room & then what’s the point.

I think you’re right to be concerned about the effect of the heater mat on the tiles, hence the advice to use an insulated tile backer board directly under the heater element. Considering the cost & practicalities it’s not a risk worth taking IMO but your decision of course.

 

[Richard C]

Very sceptical ; there are a few on here who would argue if there is any benefit of adding a 6mm thermal backer board under the mat let alone a coat or two of “magic” paint.

 

[dontknowitall]

Hi Richard,

magic paint sounds a little too good to be true, I'll kick that one into touch!

It's a 50's built house with a very small 2 storey extension completed in the 80's.

300mm is the largest gap between each joists but not uniform distances apart, the joists are 47x175 supported at each end by the interior therma-block style wall on a bed of mortar for leveling. There is also an RSJ in the center of the floor running parallel with the other joists. I'm confident in the underlying structure as it's been there for 30 years but recognise that it's unlikely to adhere to today's building reg standards.

interesting advice on the ufh, i've had it in the last place and the wife loved it thus the requirement today. I will try and talk her round or maybe get her some slippers!

Thanks again.

 

[Richard C]

300mm is the largest gap between each joists but not uniform distances apart, the joists are 47x175 supported at each end by the interior therma-block style wall on a bed of mortar for leveling.

Even if the span is @ 3m you should be OK with that, especially with the RSJ as well.

Again, be carefull with your selection of tiling materials; use only quality trade products of the correct type for your tiles.

 

[tpt]

UFH is very trendy at the moment and i have to say (and i shouldnt as i sell it) that i think too much is made of how 'cold' tiles are.

They are only as cold as the substrate and the ambient temperature. if you are laying on a wooden floor in a heated house they wont be particularly cold at all. mid winter they will take in the temperature overnight and be a bit chilly in the morning, but thats all.

the 'cold tiles' thing has really originated from the days of laying directly onto old concrete floors that dont have the modern DPM etc to take some of the chill\damp out of it.

What several posters have stated about warmup time is true

 

[Richard C]

Dontknowitall; interesting that you now have similar views from the opposite side of the trade to myself in that TPT has a vested interest in selling this stuff whereas I just install it when I’m asked to.

I’ve formed my opinions based on installation, feedback from customers & friends & my own personal experience with my cons UFH system that dates from 2004. A primary, wet UFH system is probably the best you can get right now but I believe that electric UFH is grossly overrated as a practicable secondary heat source & can now be very expensive to run.

 

[bbtc]

If you use a digital thermostat that takes readings from both floor and air, you can leave this at a standard temperature to keep the chill off in winter, but will boost up at desired times, ie mornings and evenings, the running costs are not as bad as people expect, an average running cost averaged throughout the year is approx0.1kw/hr for 2 m2 at 150w/m2( average size floor) and aprrox 0.4kw/hr for a 10 m2 at 150w/m2
work out how much your energy provider is charging per kw/h and that should give some idea of running costs.
I would always use some kind of insulation, whether it is high density foam based or cementous based like hardy backer boards

hope this helps with your decision

 

[tictic]

if your going for elect ufh (in a wet room not needed imo,your only in the shower for...5-10 min...)

..then use a latex based self levelling compound over the ufh..