Is the "In Screed" method the most efficient Underfloor Heating system?

[swimmydeepo]

As I've understood the In Screed method (using 7mm cable) is the preferred UFH system for new buildings - so we can conclude this is the most efficient/economical of the lot? I'm dealing with a small bathroom - the screed covers only half of it (basically the bath areas was laid on top of the concrete slab. The screed on the remaining other half is only 2-3cm thick and not in a great state (a few cracks). I'm tempted to dig out the rest of the screed (easy job) and take out a bit of the concrete slab too (this is going to be a bit harder) - then I can lay some insulation for the UFH (7mm in screed cable) and finally a new screed on top. The only limitation is going to be the existing thickness of the concrete slab if the screed was only 2-3cm what thickness is expected for the concrete slab? Failing to find the right thickness (to be checked in a few days) I'll settle for the 3mm cable on top of 10mm boards and under the new screed - less work to do in the end...

[The Natural Philosopher]

Not especially. But few houses are made with suspended ground floors these days so it lends itself to modern construction.

I'm dealing with a small

Is this an upstairs location?

If its against the ground you need 60-75mm of insulation to avoid wasting heat downwards

Upstairs if its JUST a bathroomn I'd just lay microbore under the floor and fit a TRV. and shove some lagging underneath it - rockwool maybe. It works pretty well.

[swimmydeepo]

The bathroom is from an extension on the ground floor... I was expecting that sort of thickness for insulation (60-75mm) - as I've = mentioned, the only issue is going to be the total thickness of the concret= e slab - if I need to fit 60mm of insulation I would need to dig out the sa= me of concrete... What about these 10mm UFH insulating boards? Are these only for upstairs or= suspended floors? Failing to find enough thickness for proper insulation I= might have to use them...(better than nothing - I suppose).

[The Natural Philosopher]

You need about 60-70mm of concrete, DPM , 60-70mm of insulation and

60-70mm of screed for a wet system. I am not sure what to do with an electric system

But 10mm of insulation is pointless. . Best to jackhammer up the whole slab and start again, or build a 3/4" plywood floor on top of the insulation and pop the cables under that. Plywood tiles nicely. Make sure its rigid though

[harry]

d UFH system for new buildings - so we can conclude this is the most effici= ent/economical of the lot?

sically the bath areas was laid on top of the concrete slab. The screed on = the remaining other half is only 2-3cm thick and not in a great state (a fe= w cracks).

it of the concrete slab too (this is going to be a bit harder) - then I can= lay some insulation for the UFH (7mm in screed cable) and finally a new sc= reed on top.

slab if the screed was only 2-3cm what thickness is expected for the concr= ete slab?

ttle for the 3mm cable on top of 10mm boards and under the new screed - les= s work to do in the end...

The reason underfloor heating is popular is that it enables the water temperature in wet heating systems to be kept low so making condensing gas boilers more efficient. Also, no radiators cluttering the place up.

The snag is that it doesn't lend itself to turning the heating off/up down on a daily basis or to meet outside air temperature. as it takes a while t warm up and cool down. Several hours minimum. So, in poorly insulated houses it can waste energy. All depends on your lifestyle. ie are you at home all the time or out at work through the day.

[Roger Mills]

mentioned, the only issue is going to be the total thickness of the concrete slab - if I need to fit 60mm of insulation I would need to dig out the same of concrete...

You can't realistically shave some off the top of the concrete slab. You need to break the whole lot up and start again - in which case you can excavate to a suitable depth to allow for the new slab, plus the insulation, plus the screed.

What are the chances of making it a wet system, with pipes - rather than cables - installed within the screed, and fed (via a suitable mixer valve) from a gas-fired CH system. It would cost a hell of a lot less to run than an on-peak electrical system.

[swimmydeepo]

You are talking of wet systems...at this stage I'm only considering the electric UFH - simpler to install and is only for 1 small bathroom. In fact I'm tempted to use UFH for the whole house and maybe leaving a Combi Condensing boiler for hot water only...

[swimmydeepo]

I thought of using an angle grinder - with due care it should work and keep the DPM intact. The wet systems looks more complicated (surely more can get wrong) - and at this stage the focus is for a small bathroom only - the elctric system compares well with traditional radiators - can't be that bad.

[Steve Firth]

You can. It's not cheap to do though.

[Roger Mills]

this stage the focus is for a small bathroom only - the elctric system compares well with traditional radiators - can't be that bad.

"Compares" in what sense?

[swimmydeepo]

I've never considered using plywood in the bathroom but that would be very useful in this instance:

1) no need to bother with the screed layer (I'm not that keen on working with cement). 2) there's going to be more room for the insulation as the plywood is only 18mm

- the screed should be thicker than that...

Yet, I'll have to abandon the idea of using 7mm electric cables as these are designed to be embedded into the screed (surely can't be embedded in plywood)... Most importantly, there is something I haven't grasped - the cables should be installed over the insulation and under the plywood? That seems a bit too dangerous to me... On the other hand; if the electric cables are to be installed on top of the plywood (under the tile's adhesive), then I can only use 3mm electic cables or mats.

[The Natural Philosopher]

plywood (under the tile's adhesive), then I can only use 3mm electic cables or mats.

Actually that is probably the 3way to go. I have no exp. of leccy UFH, only wet.

[swimmydeepo]

Considering the area of the bathroom (2.7 square meters) it should be easy.= ..anyhow some form of leveling is necessary as the floor right now is only = screed-ed on half of the bathroom (there is a step of about 30mm).

[swimmydeepo]

the plywood (under the tile's adhesive), then I can only use 3mm electic c= ables or mats.

I've read that electric systems offer more uniformity in the way the heat i= s radiated - wet systems might be affected by cooler areas... There are benefits on both systems (wet is more economical on med-big proje= cts) - but for small projects I've figured that an electric UFH is the way = to go.

[Fredxx]

There's nothing efficient about heating using electricity. If you have gas central heating piping nearby I can only recommend a wet UFH.

[Andy Burns]

It's almost 100% efficient, that doesn't make it cheap though, especially if the slab isn't insulated properly.

[Roger Mills]

If the slab isn't insulated, it's not efficient either - if you use a sensible definition of efficiency.

[The Natural Philosopher]

Indeed. The amount of heat into the room as a proportion of the total is the U value upwards into the room divided by the U value downwards into the soil.

Which is why screed and tiles make the best UFH floor as thats a relatively good conductor.

Once above ground floor its less an issue because heat going downwards still heats the house.

Where I have rugs and a sofa over my UFH the floor underneath is way hotter than the rest of the room due to the insulation of the carpet and the sofa!!

If you take a typical UFH of say 50W/sq meters at something like 40 degrees internal screed and a suspended concrete floor such as I have - and assume an icy 0 dec C blast under it, and 60mm of styrene insulation..a U value of 0.45.. then the losses at 40C are around 16W/sq meter downwards. That could easily be one third of the total input into the floor..

This is why you need MASSIVE insulation under a screed for UFH - far more than you need for just a conventionally heated room. Because the floor is far hotter internally than the room would normally be. And the more stuff you put on top of the floor the hotter it gets.

I wish the architect and BCO had been more clever on the UFH effect as I should have stuck 100mm in.

[swimmydeepo]

I've just contacted Warmup UFH systems...they are adamant that their insula= ting 10mm boards is all we need for uninsulated concrete floors - yep, 10mm= insulating boards from them would guarantee an heating up time of 45min to= 1hour...while on uninsulated concrete it would take 5-8hours and insulated= concrete (insulation under the screed) should take 3-4 hours?! So, if I de= cide to bury some 100mm Celotex I'd wait 3 hours to heat up my bathroom whi= le if I opt for the 10mm boards I get a speedy 45 minutes? This 10mm boards are magic!! Surely, I'd like to buy their marketing waffle= - beside...the techs specs aren't that different than any other similar UF= H boards from other manufacturers (only theirs are much more expensive).

Yet, we must distinguish between undertile UFH and under-screed UFH (there = are so many UFH systems out there I'm losing my mind...).

[The Natural Philosopher]

Well of course they are adamant.

They are trying to sell you snake oil.

No that is true. 10mm is better than nothing. Instead of losing 95% of the heat it will only be 75% and that's 5 TIMES BETTER SIR.

And it will reduce warmup times, true. My screed takes several DAYS to cool down. And about 10 hours to really warm up.

So, if I decide to bury some 100mm Celotex I'd wait

Its all very simple once you grasp the basic principles.