We’re planning an extension which will add new space and reconfigure some internal space too, one of the outcomes being we have a nice big open plan kitchen diner, most of which will be on the new footprint.

Current heating is oil fired boiler, gravity fed system. Part of the plan might involve moving (and/or replacing) the boiler and locating it outside the house as it currently sits in the kitchen making loads of noise and also requires a vent in the kitchen that make the kitchen cold!

Ideally we’d want wet UFH in the new space and possibly some of the existing footprint (e.g in the utility, downstairs loo & part of a sunroom).

I know some UFH systems can be fitted on top of existing concrete slabs with a thin layer of insulation underneath. Does anyone have any experience of these? Are they as effective/efficient as putting the pipes under a new screed…obviously ripping up and relaying a load of existing slab will be messy and expensive.

Does anyone have any experience of a hybrid approach with part being laid over existing slab and part under a new floor screed? Is this possible? Any pitfalls to avoid?

Are there any other considerations we should think about?

If the extension is the majority of the floor you could get away with just having underfloor heating that area. With the pipes spaced for the load of the whole room. if your clever with the planning of the kitchen layout you can make sure the cold bits coincide with the areas you want to get to in a hurry first thing in the morning (kettle, fridge, toaster etc). (My kids seem to be drawn to the bits of the kitchen I want to get to in a hurry making them the bits which they will get cold feet would seem to be an advantage).

You could do a hybrid system the thinnest build up for a water based system over an existing slab I’ve seem is about 40-50mm including the floor finish but you could use a timed electric system on the existing slab area is small just to take the bite out of the cold first thing in the morning. They can be fitted within the tile adhesive layer so effectively there would be little if no build up.

Rather than digging up 45sqm of floor and relaying it with built in wet UFH (which would cost £1000’s) we’re just going with electric.
That said we have solar PV to help a little plus 10sqm of the floor will be occupied by an island so we don’t need to heat under that.

We have used Omni floor boards which sit on suspended wooden floor. You have to insulate between the joists. Not sure it that’s an option but it works well if you have suspended floors in older sections of the house

Have laid timber flooring over boards like those ^ Can work quite well for retro-fitting UFH into a house with a suspended floor. Another option could be to have your extended area as a new suspended floor with joists level with old. Take up old floorboards, fit UFH ready chipboard as above then your new flooring on top.

What I would say is that floor board solution works better with a tile rather than timber floor. We have a combination and because there is no screed on the timber areas then there isn’t much to hold the heat. Whereas the areas of the house with stone floor stay warm for AGES after the UFH has gone off. You have to plan your heating totally differently too because of the thermal performance of the different flooring materials #firstworldproblems 😉

Thanks for the comments so far. A couple of clarifications:

– I’m keen to avoid electric UFH as have previous experience of the running costs.

– The existing areas we’re considering retro fitting wet UFH too are concrete floor, not timber.

– We’d be looking to have porcelain/stone tiles as the final finish where the UFH is

I’m keen to avoid electric UFH as have previous experience of the running costs.

Please do not be under the illusion that wet UFH is cheap to run – my sister has oil fired wet UFH and it churns through the oil even with a modern efficient boiler.
It’s a bit ‘cheaper’ but you certainly need to look at the costs of digging up your existing floors, relaying them [to the right level] and then installing the UFH. That cost would pay for a lot of ‘leccy.

I would go for an overlay system as the existing screed probably isn’t thick enough to be used for underfloor (traditional screed needs to be 65mm ideally) and you would want to be adding insulation too so would be faced with breaking out the oversite too.

Some of the overlay systems are very minimal makeup, but you might be able to build up your existing a little more if you are remodelling most of the ground floor and add some more insulation.