Underfloor heating - old house

[Waterdropsdown]

I’m doing a kitchen extension/renovation and the final room is going to be a large kitchen/diner/living room. Only about 25% is an extension the rest is part of the old footprint redone with walls knocked down etc.
I am struggling with understanding the underfloor heating. My builder is telling me if I want the floorboards ripped out and concrete slab input for then concrete to be laid down and underfloor heating applied to the whole area it will cost £22k! I had previously been told it would be about £6k but he tells me this is with the concrete slab only in the new part and then insulting between the existing joists etc laying underfloor heating on top (although he is saying this will now be about £8k not £6k). Anyway has anyone had underfloor heating in similar room which is part old part new and done it with leaving the joists in place? I can’t afford the £22k cost but really want the underfloor heating.

we have underfloor heating throughout our whole victorian house. In the kitchen which is extended this is a solid concrete slab floor, but the rest of the house is wooden joists. Its great - the difficulty with having a mixed system is that the temperature you run UFH at is lower than radiators, so having it everywhere avoids these issues. Our house is nice and warm and we don't have walls taken up with radiators.

@northerngoldilocks thanks for the response
so is your kitchen part old house and part new build? Therefore the floor in that room is part concrete slab base and part joists base? Both bits of floor are insulated and underfloor heating laid on top? Can you tell which bit is joist and which bit is concrete slab?

No- the whole kitchen is concrete slab as we could replace as no cellar under that bit. The rest of the ground floor is on a different level and is wooden joists

Thanks @northerngoldilocks

does anyone else have any experience of a kitchen with wet underfloor heating partly laid on a insulated suspended joist floor and partly laid on a insulated concrete slab flooring? Does it work ok temperature wise?

I would say that whilst our concrete vs joist heating isn't in the same room, the same principle will apply. Overall, the heat from the concrete slab area is probably better - and so i'd imagine that you might notice it retains heat better than the joists section, and therefore this could mean that parts of your kitchen are more heated than others. I doubt it would be so noticable that there would be cold sections or difficulties with room thermostats though. In the rooms that are laid over joists we don't have to supplement the heating at all.

We have this. UFH in the new extension with concrete and then UFH with joist floor in the old which they then put screed in. We chose not to continue with UFH and joist (although we were given that option from our builder). This was because we were told the insulation wouldn’t be as good and due to new building regs. It did cost us £30k which we couldn’t believe but we don’t regret it as it’s now amazing.

@Fretfulmum sorry I don’t quite follow. were you given the option of partial underfloor heating on top of the existing joist floor and partial concrete sub floor but you chose to replace the joist flooring with concrete sub floor? Then the whole room cost £30k?

or you actually have underfloor heating partly on joist partly on concrete?
thanks for your feedback

I meant the former. Insulation in the joist flooring was an issue

I should mention the bulk of the cost was because we had to drill out concrete first (some of our old floor was concrete too) which didn’t have UFH.

Ok so my £22k quote doesn’t seem ridiculous compared to your £30k!

anyone else have experience of Ufh on top of joists versus concrete sub floor? Thanks

There are several ways of tackling this. I suggest you speak to a company that supplies retrofit UFH who will have more experience than your builder on ways to get the right fit for you. They are generally really helpful. UFH in a concrete slab takes a lot longer to heat up than overlay/retrofit UFH as it has a lot more concrete to heat up before you feel it but also takes longer to cool down so is more efficient but not as responsive as overlay UFH. Retrofit overlay UFH can either be laid over floorboards (and the new slab) or under the floorboards (which will need insulating) and over the new slab but the level of the slab will be needed to be adjusted so on the former so that there isn't a difference in levels. We had an extension on the back of our 70s house and had an insulated slab laid on the new part but chose to use wet overlay/retrofit UFH throughout the ground floor to ensure equal responsiveness. Wasn't too expensive but we installed it ourselves, total of about 120 square metres, eight zones over five rooms.

thanks @Cherubimbum that's great to hear someone has done it! Sounds like it might involve a step from the hallway then. I’m having a meeting with the builder tomorrow about it and it’s useful to have as much info as possible on the scenarios.

to clarify you only got the new concrete slab on the new part of the ground floor not the existing? And the existing part you had underfloor heating added on top of the existing joists?

@Waterdropsdown

We got concrete on the new and did an overlay, the existing room was concrete slab too but from the 70's so uninsulated. I have a friend who did overlay on new slab to marry up with existing floorboards. They insulated under the floorboards, lined boards on top with ply/OSB for stability (can't remember which) and then put the insulated tiles and overlay UFH throughout the room. This is why you need to know what method and materials you want to use as the height of the new slab will need to take into account the additional of the ply/OSB.

This isn't where we got ours from but look at the profix combined scenario at the bottom of the page
https://www.underfloorheatingsystems.co.uk/underfloor-heating-design/floor-construction/

Thanks @Cherubimbum
will take a read of this

Hi @Waterdropsdown - I’m currently facing your conundrum and wondered what you ended up doing back in 2023, and the result? Thanks

https://www.wundagroup.com/water-underfloor-heating/overfloor/
after getting all the usual quotes of 30 grand to dig up a kitchen, the size of a postage stamp and relay this and relay that we ended up going with the retrofit option which cost me just under three grand including tiling the Floor.

We’ve only had a little bit of a winter with it, but the dog is extremely happy with this new option