Hi All,

I'm getting a small lean-to at the back of the house rebuilt next year, which will make an open plan kitchen / dining area when it's done. The existing floors are all suspended wood and the drawings I've had done add a new suspended floor in the new part (the new floor area is only about 2.5 m x 6 m).

I would quite like to get liquid underfloor heating throughout the dining / kitchen area (total area maybe 6m x 6 m).

I think underfloor heating is generally considered to be much better with a solid floor, but my budget is already a bit stretched so I'm fairly reluctant to get the whole area filled in and concreted.

There are some options for underfloor heating with suspended floor and I have seen examples of people doing it by insulating between the joists then embedding the pipes in a screed that sits flush with the top of the joists. My worry with this is - won't it crack with time? Also it is putting a lot of weight on the joists. The spans between the sleeper walls are small - roughly 4 feet, so I suppose it can take a bit of weight but I've no idea how much.

The other option I've seen is to replace the floorboards with a thick chipboard that has pre-cut grooves for the pipes.

My question is are either of these good options, or am I better off just installing radiators and spending my budget elsewhere?

You could fill the underfloor void with EPS beads while you're at it.

Just a couple of thoughts
How much space beneath the 1) existing joists and 2) will be beneath the new joists? Is there a possibility of full fill insulation for the existing and for the new floor void?

If you want UFH, in the new build why not put insulation (15cm EPS?) down then 10cm - 15 cm concrete slab as the new floor with the UFH on that (10 cm slab with 150 x 150 x 6 rebar or 15 cm without rebar.(I prefer the 10 cm option)) Budget considerations are that you have nothing to rip out on the new build and insulating between / under joists is always a pain - do some costings.

Any works done with the extension and insulation have to consider the ventilation of any other ground floor suspended floors.

Will the installation of UFH in the existing area cause any issues with doors and thresholds.

If the existing central heating is rads. then a separate zone with its own heat exchanger will be needed for the UFH.

I find radiators in kitchens a pain as they take up valuable wall space.

Dominic - thank you, that's really useful to know. Can I ask, are you happy with the system and how it performs? Do do you have UFH downstairs on a solid floor? If so, how does it compare?

Peter, there is about 50 cm space under the existing floor, so I guess it is too much to fill. In the front room and hall I have got 220 mm wood fibre between and around the joists (100 mm deep), so I was thinking of doing the same. During the work, I'm planning to level all the joists and adjust their height based on what I decide to do, so hopefully levels will line up with doors etc.

For the new part, what I currently have drawn is 200 mm joists supported on joist hangers (ca. 2.5 m spans). These are packed with 200 mm wood fibre insulation in between then there is a 200 mm air gap, then 100 mm concrete on top of 150 mm hardcore. It would be easier to change this to a solid floor but then I would have two types of floor to install UFH over so I'm not sure whether that complicates things?

Posted By: Peter_in_HungaryIf you go for a solid floor you can put the insulation above the concrete slab and lay the UFH pipes in preformed EPS panels. Doing it this way would mean that the UFH reacts the same way as that on the existing floor as both will be on insulation rather than one on insulation and one having a large thermal mass.

For the suspended floor option you don't mention a VCL which should be on the warm side of any insulation

BTW 200mm joists seem a bit over-kill for a 2.5m span

OK, thanks for the advice. I'll chat to the architect and see what he thinks. Maybe he designed the joists that thick to make it easier to fit insulation in between.

Thanks to both of you. Dominic's setup sounds really nice and you have convinced me that it is worth getting some UFH in the room (both old and new parts) even with a suspended floor.

Apparently the 200 mm joists are so that it's easier to fit 200 mm insulation between.