Underfloor Heating without Insulation Directly Underneath

Brianb8802

Hi all,
I have a question about how practical electric under floor heating mats are.

We're going through a new build currently (new estate) that has gas heating with rads throughout the house. My wife and I would like to have underfloor heating to the tiled areas purely as a comfort thing, not for heating the house. Approx 90sqm downstairs and 15sqm upstairs.

The builder is on board with this and has given a reasonable quote for the works but wants to sit the UFH mat on top of the ground floor screed without insulation directly under the mat. They say the insulation under the ground floor slab will be enough and there is no need for a layer directly below the mat. They also recommend no insulation upstairs on timber substrate. They probably can't fit the insulation either as the screed is already in to height. From what I've read this is not what most manufacturers recommend, granted we're not actually heating the house, just taking the chill out of the tiles in winter. Planning on having the house a no shoes house for us and the kids.

I have a few questions:
We haven't lived in an A rated home before so does a tiled floor actually get that cold in winter in new houses, compared to your typical 30 year old houses?

Is not having a layer of insulation directly below the heating mat a deal breaker? At the moment I feel it is.

I've read once you turn on the mats without insulation, it heats the slab below first then the tiles above, could take 30min to feel any difference in the tiles. Has anybody else experienced this?

Given that the mats are about 100W/sqm, that's approx €2.25/hr to run downstairs using 25c/kwhr. That would soon add up in the winter! It doesn't really sound very practical?

If anybody has some experience I'd love to hear your thoughts, it will help us decide on if we actually install the mats or not.

Thanks

Yellow_Fern

Brianb8802 wrote: »

Hi all,
I have a question about how practical electric under floor heating mats are.

We're going through a new build currently (new estate) that has gas heating with rads throughout the house. My wife and I would like to have underfloor heating to the tiled areas purely as a comfort thing, not for heating the house. Approx 90sqm downstairs and 15sqm upstairs.

The builder is on board with this and has given a reasonable quote for the works but wants to sit the UFH mat on top of the ground floor screed without insulation directly under the mat. They say the insulation under the ground floor slab will be enough and there is no need for a layer directly below the mat. They also recommend no insulation upstairs on timber substrate. They probably can't fit the insulation either as the screed is already in to height. From what I've read this is not what most manufacturers recommend, granted we're not actually heating the house, just taking the chill out of the tiles in winter. Planning on having the house a no shoes house for us and the kids.

I have a few questions:
We haven't lived in an A rated home before so does a tiled floor actually get that cold in winter in new houses, compared to your typical 30 year old houses?

Is not having a layer of insulation directly below the heating mat a deal breaker? At the moment I feel it is.

I've read once you turn on the mats without insulation, it heats the slab below first then the tiles above, could take 30min to feel any difference in the tiles. Has anybody else experienced this?

Given that the mats are about 100W/sqm, that's approx €2.25/hr to run downstairs using 25c/kwhr. That would soon add up in the winter! It doesn't really sound very practical?

If anybody has some experience I'd love to hear your thoughts, it will help us decide on if we actually install the mats or not.

Thanks

Interesting case which I think may relate to many people, but like yourself I don't have too many answers. From what I can see electric UFH seem like an inferior option for all but the wealthiest because of the high running costs. No point going to install a compromise that you are reluctant to switch on.

If the floor is already in, options are more limited but there is hope. I think default floor insulation is pretty good these days but if not, there are products that allow a lot of extra insulation without enormous build like Spacetherm but they are pricey. You need to fact it in as early on as possible. There are various other systems water based UFH systems that sit on existing screed some with their own insulation and others without. So if the floor is already done you don't have to give up the idea of wet UFH which is indeed a lot superior. You might lose 1.5 cm to 5cm downstairs depending on system and 0-2 cm upstairs depending on system. However to meet regulations you'd have to think about how to sure that this installation doesn't make your house inaccessible to wheel chair users. I have never used these products but I can PM you names. It is a hassle but not rocket science. I don't have UFH but everyone says its objectively better when done well.

Perhaps other people who have experience of A-rated homes in winter could explain their thoughts on the coldness of tile in such a home, however IMO it is inevitable that tile will feel cold since it is conductive and always going to be cooler than a human foot unless the tile is specifically heated. It would be less cold than a cold house. This all assumes your A rated house is actually built well and is warm which isn't always the case

CelticRambler

Brianb8802 wrote: »

Given that the mats are about 100W/sqm, that's approx €2.25/hr to run downstairs using 25c/kwhr. That would soon add up in the winter! It doesn't really sound very practical?

Your maths is a bit off: you won't be continually heating every m² with 100W because you'll be heating a m³ - but once it's heated, it'll stay warm/hot if the insulation below the floor slab is decent, and that heat will keep the tiles warm for a good while without pumping in any further Watts.

You will, in effect, be creating a giant storage heater, which is not necessarily a bad thing. In practical terms, you'll lose the reactivity of a shallow screed, but underfloor heating is not really meant to be reactive. The only immediate problem that I can see (based on my own [positive] experience of putting in a mat a little over a year ago) is that the contribution to the room heat will be such that you'll have the floor temperature set quite low, to the extent that the floor although not cold isn't warm under bare feet either.

Calahonda52

What energy rating will you get for this setup?.
UFH from a heat pump COP: 4 at night rate, UFH from mats, COP of 1

Brianb8802

Yellow_Fern wrote: »

From what I can see electric UFH seem like an inferior option for all but the wealthiest because of the high running costs. No point going to install a compromise that you are reluctant to switch on.

You might lose 1.5 cm to 5cm downstairs depending on system and 0-2 cm upstairs depending on system.

Perhaps other people who have experience of A-rated homes in winter could explain their thoughts on the coldness of tile in such a home

Yeah that's the worry, we're really keen on putting it in but the worry is it doesn't work as intended and uses way to much energy to make it practical.

Ideally would prefer a level floor finish throughout, we're planning on running floor finishes level through the internal doors thresholds, with no door saddles. Screed is already in so unfortunately it's too late to add the insulation layer.

Hopefully others that have it installed can chime in.

CelticRambler wrote: »

The only immediate problem that I can see (based on my own [positive] experience of putting in a mat a little over a year ago) is that the contribution to the room heat will be such that you'll have the floor temperature set quite low, to the extent that the floor although not cold isn't warm under bare feet either.

Ah that's not ideal. I was hoping I could turn it on for an hour at 5am before everybody is up and it'd last until lunch time before you'd have to give it another blast for an hour or so. But that doesn't seem doable.
Do you have it installed over a large area or just one room? Have you ended up using yours much since it's been installed? Notice a large increases in energy cost?

Calahonda52 wrote: »

What energy rating will you get for this setup?.
UFH from a heat pump COP: 4 at night rate, UFH from mats, COP of 1

I'm not sure to be honest, I don't know much about calculating the rating. I do know the standard base building is A2, before we talked about adding anything. Can this UFH be excluded from the calculation as it is not intended to heat the house, and is an optional extra to add comfort underfoot. The builder hasn't mentioned that there would be a problem achieving A2. Should I question it?

CelticRambler

Brianb8802 wrote: »

Ah that's not ideal. I was hoping I could turn it on for an hour at 5am before everybody is up and it'd last until lunch time before you'd have to give it another blast for an hour or so. But that doesn't seem doable.
Do you have it installed over a large area or just one room? Have you ended up using yours much since it's been installed? Notice a large increases in energy cost?

No increase - it knocked 500€ off my annual bill the year after I put it in, and I've just received a refund of another 160€ for 2019 (overpaid by DD based on 2018 usage). When I put it on the winter, I don't turn it off till the end of the spring. My thermostat allows me to set two high and low temperatures (everyone-in-bed, up-for-breakfast, out-at-work, home-for-the-evening) aswell as constant heating and frost protect.

I'm away from home at the moment and forgot to drop it down to frost protect, but (thanks to my smart meter - don't know how anyone can live without one! :pac: ) can see that it's not going to bankrupt me before I get back. 18-20kWh per day Thu to Sat, up to 25kWh yesterday which is probably because the outside temp was -1°C. So [(20x3)+25] x0.25ct/kWh = about 22ct for four days using your prices. (actually less than that, because those kWhs include the broadband router, two fridges ... and the Christmas lights with incandescent bulbs that I forgot to unplug too! :eek: )

I have one mat heating three rooms (bathroom, utility room with staircase, walk-through pantry/larder/second kitchen: a total of about 30m² floorspace, of which 22m² was intended to be heated (cold zones left for the fridge/freezer, potato-holding zone, under-worktop "recycling centre" etc) using an 18m² mat.

MicktheMan

CelticRambler wrote: »

No increase - it knocked 500€ off my annual bill the year after I put it in, and I've just received a refund of another 160€ for 2019 (overpaid by DD based on 2018 usage). When I put it on the winter, I don't turn it off till the end of the spring. My thermostat allows me to set two high and low temperatures (everyone-in-bed, up-for-breakfast, out-at-work, home-for-the-evening) aswell as constant heating and frost protect.

I'm away from home at the moment and forgot to drop it down to frost protect, but (thanks to my smart meter - don't know how anyone can live without one! :pac: ) can see that it's not going to bankrupt me before I get back. 18-20kWh per day Thu to Sat, up to 25kWh yesterday which is probably because the outside temp was -1°C. So [(20x3)+25] x0.25ct/kWh = about 22ctEuro for four days using your prices. (actually less than that, because those kWhs include the broadband router, two fridges ... and the Christmas lights with incandescent bulbs that I forgot to unplug too! :eek: )

I have one mat heating three rooms (bathroom, utility room with staircase, walk-through pantry/larder/second kitchen: a total of about 30m² floorspace, of which 22m² was intended to be heated (cold zones left for the fridge/freezer, potato-holding zone, under-worktop "recycling centre" etc) using an 18m² mat.

FYP;)

CelticRambler

MicktheMan wrote: »

FYP;)

Well spotted ... Knew there was something odd about the calculation but couldn't see the misplaced decimal point. (I don't pay as much as that for my nuclear juice).