Thickness of Screed for Electrical Underfloor Heating

Minimum distance admissible between cable and any metal object below or above is one inch (25mm). Normal practice is to directly bury the heating cable by pouring concrete and I would say that one inch cover of poured cement, concrete or aggregate would provide enough strength if you are choosing 1:2:4 mix.

Spacing between adjesent heating cables is also one inch when selection criteria is a quarter less than three watts per foot.

By the way, what's the 2 in 1:2:4? Is it the water?

In any case, I plan to avoid any excess water in the mixture.

The standard thickness of screen in the UK is 50mm with no reinforcement. Any thinner and it cracks. Much thicker and the insulation effect starts to overheat the insulation. The cable should be laid directly on the insulation board and clipped down with the clips provided in the kit (you are buying a kit arn't you?). Don't forget to bury the over temperature sensor tip at the same level as the cable.

One of the advantages of underfloor heating is the thermal inertia you have referred to so trying to minimise this would be counter productive. The other big issue is the thickness of the insulation boards fitted below the cable. In the UK the energy police have a calculation which usually means you have to lay between 150 & 200mm of Kingspan or similar unless you have high ceilings means things get a little tight.

The standard thickness of screen in the UK is 50mm with no reinforcement. Any thinner and it cracks.

Yes that's true. The question is whether I can bring it down to 3-4 cm by using reinforcement.

Moreover, as there is a concrete layer already, I bet there shouldn't be any problem even with a 1cm screed (I guess!), only that I have to take into account that the screed will not get in contact with the concrete layer directly, but with the insulators that will be in between. I plane to use extruded polystyrene with 30mm thickness. So the whole issue is whether the insulators will cause the screed to deform and consequently form cracks if it is not of adequate thickness.

A direct answer to your last question is NO. For reinforcement to be of any use it must be at least 50mm below the surface of a concrete mass with the same depth below. Anything thinner than 50 with or without reinforcement will crack.

Your proposal for 30mm of expanded polystyrene is way short of that required to be reasonably energy efficient. Do the arithmatic based on an insulation temperature of say 45°C and 30mm Polystyrene below and 50mm screed above compared with 200mm Kingspan below and 50mm screed above. You will better than 1/2 the loss to the floor.

The compressive strength of Kingspan phenolic foam is also much greater than that of polystyrene and would even at that would allow a 30mm screed to flex enough to crack.

Just a question about your roof insulation. What is it and how thick is it? With the current cost of fuel increasing your loft insulation from 100mm to 200mm typically pays for itself in 18months if installed yourself. Adding insulation under an existing under floor heating installation is a real ouch in the back pocket. If you are not stuggling with room height install as much Kingspan as you can afford with a 50mm screed. It will pay for itself in very little time at all.

Also check out the web sites for people who do this for a living.

Thanx for the info

You seem to refer to Kingspan, but I'm afraid I'm doing the implementation in Greece. From a quick browse, I saw that Kingspan also uses extruded polystyrene. I will have a closer search laster at the ease of my home and explore the possibility to import insulators if they are so much better.

In any case, 200mm of insulation is too much, as it would raise the floor beyond limits (otherwise, I would have to cut the door, add an extra step etc.) so that's why I try to investigate the case of putting something like 30mm of insulation and, say, 4cm of screed. These were claimed to be viable by so-called experts in the field. Moreover, although the house will be in a coldish area indeed, it won't be used as a main house: So the super duper insulation is not so vital. I have installed roof insulation though, to keep the house from radiating heat to the outer space, if this is of any interest to you.

By the way, I wouldn't insist to get as many opinions as possible, if I hadn't got so much conflicting advice from here and there...

Is the electrical undefloor system an off the shelf item of something you are cobbling together? Most of the "mat" style systems on sale in the UK would typically be installed in the grout layer directly under your tile, affixed to the insulation in say 10 mm of grout.

I installed such a system in a bathroom under slate time some years ago and it worked perfectly and led to an extremely cozy bathroom floor.

Is the electrical undefloor system an off the shelf item of something you are cobbling together?

I got the cables from Warmup (www.warmup.co.uk) They told me that there will not be a problem with as much as 30mm screed. Maybe I will make it 40mm after all. Nevertheless, I get advice (here in CR4 included) that this is too few, so I'm afraid I'm still confused...

It is important to put a good insulation barrier between the reinforced layer & the top coats otherwise you'll be heating the ground,I would suggest a 75mm foil faced expanded polyurethane sheets,you can then lay your heating coils over this then lay a Strong mix of finish sharp sand cement 75mm over the lot ,don't put reinforced steel in it as the heat will expand the steel more than the cement and crack the floor,also when you lay the top layer put some sheet polystyrene around the perimeter only 15mm, to allow for expansion,when it is dry you can put some expanding foam in the gap & tile over the foam, don't forget to leave a small gap at the edge of the tiles for expansion, about 5mm should suffice.

I agree in everything except from that 75mm you mention! That's too much, both because it will raise the floor beyond acceptable limits, and because it will act as a heat reservoir, which I don't want. That's why I mentioned something like 3-4cm (with 4cm being more probable.)

What you mention about reinforcement is something that didn't cross my mind. You seem to be right, but strangely enough, some implementors (who use Nexans cables) use it in order to strengthen the screed and also be able to tie the electric cables on.

Bloody hell, my confusion builds up!...

No... You want to put heating elements under the floor of a house? You will need Asbestos shoes. Bu,t it is a funny idea.

Hi,

my bathroom, shower room and cloakroom all have electric underfloor heating.

Don't forget the timer! And don't forget the insulation underneath if it is a ground floor insulation.

Tiles do slow heat transfer but mine seem to have worked out OK; I was a bit dissapointed with the amount of insulation in the downstairs room - but it seems to be working, probably just a Delta on the running cost.

I did not worry too much about the mechanical strength issues that you have gone into as 1 I have tiles over a grout layer in which the elements are buried and 2 I had the work carried out professonally to avoid such worries, I negotitaed a 10 yr guarantee which should be OK, no movement to date after 4 years.

Good Luck with your work.

Sleepy

try :ask this old house .com

I installed "off the shelf" electric heating during a kitchen and laundry room remodel several years back and it worked perfectly. The heating "element" was a serpentine wire which was attached directly to the sub-floor (backer board) and then buried in a mortar screed that was just the thickness of the element. The manufacturer recommended to do it this way and when the skim layer had set then a second layer was put down using the notched side of the trowel and the tiles were set in place.

The cable you are using is more than likely designed for direct burial in the mortar layer and is fully insulated which will not require any "excessive" layer (30 to 70mm as some have recommended) to protect from stray current. If you were installing bare copper wire in the floor then you may have to use a quite thick concrete layer for electrical insulation but the plastic covering on the heating element will provide the electrical insulation you require.

There was no timer, but the system had a thermocouple imbedded in the floor which ran to a thermostatic controller.

As indicated above, it made for a nice cozy floor in the winter!

Good luck with your project.