Using wet UFH on existing Solid Floor Slab

[GoodDIYjob]

We have an extension soon starting.

The new built on single storey extension area to the rear of our property is about 20m2 internally.

I was keen putting UFH in there, there new Floor Slab will be insulated with 100mm Celotex done under building regs etc.

The plumber / heat engineer who does most of the work for the Builder wanted to extend the area of wet UFH to include an existing part of the kitchen floor that becomes 1) a small shower / wc room and 2) a small part of the new open kitchen diner.

The floor area of this old solid floor is probably about 9m2 (2.5m x 4.5m roughly). The long side is an external wall. This is likely original 1930s solid floor. No insulation.

I baulked at the idea because of the trouble taking the old solid floor up. Then the builder went back to him and he clarified he was thinking only the screed and you can put some foil reflector or insulation board in without really affecting floor levels much.

I agree with the heat engineer that it would be better to cover the whole area, especially the old floor beneath a new shower room as space is tight in there.....

BUT is it going to produce any heat and be a ridiculous heat drain costing enourmous amounts for virtually nothing?

I can't see it would be viable to dig out the whole solid floor. It will blow our Budgets severely. Not to mention more impact on trying to live here.

Does anyone have any experience of a retrofit of UFH in a small area of existing solid floor and could they comment on the success of low profile insulation sheet or reflective heat system?

I am dubious, as you may gather.

Many thanks for feedback.

 

[Notch7]

The feedback I have had from customers that have had electric underfloor heating installed in a room on top of an existing slab with only the 10mm or so is insulation board is that it is both expensive to run and loses heat quickly. That may of course be combined with a room with poorly insulated walls and roof which all impacts.

A wet system wont be as costly to run as electric, but Im not sure how successful a primary heat source underfloor system will be without the screed insulated beneath with 70mm or so of insulation.

I would be tempted to get a price for digging up the concrete oversite, doing reduced dig then re-concreting lower to allow the floor to be insualted.

If you dig up the screed, does that mean you will be required to insulate as part of consequential improvement anyway?

 

[GoodDIYjob]

Thanks for reply. Yes, that is kinda what I thought about the performance prospects.

On the consequential improvements, I had not thought of that and neither the builder or plumber mentioned..Good point though.

It seems like unless someone comes on and says yes, I've done it on a similar scale, and pretty good I will have to look at either Wet UFH in new floor area only. Or go back to rads.

Trouble is because of open plan and existing kitchen having no heating at all, the new area UFH has to heat whole area. Also large glass areas. Furrher more kitchen units reduce the net m2 of the new area further for wet UFH. So effectively with wet UFH in new solid floor only, I'd be heating a 25m2 open area of kitchen diner with probably less than 16m2 area of UFH. Maximum I understand is 100w/m2. So that is 1600w absolute max

Rough peak heat loss with the amount of glass in the area is probably north of 2kw at minus 2C design.

I wanted UFH but not sure it is stacking up now, I may have to supplement with a wet plinth or vertical designer rad anyway....

 

[endecotp]

There have been threads here about UFH and existing solid floors and insulation. You can try searching but it's not easy to guess the right keywords.
I can't say I recall any great success stories!

 

[GoodDIYjob]

Had a bit of a search around.

Seems there are retrofit systems that seem to be either:

EPX / XPS sheets with a routered profile for pipework; or
Gypsum fibreboard Sheet.

And often low profile pipe.

Specific brands products I have found are Nu-Heats Lo Pro, Polypipe Overlay and Wundo Overfloor - [several options (EPS seems most applicable).]

Does anyone have any experience of these?

I don't want to be paying £££ to heat up the subfloor and ground below! I'd rather supplement the new UFH with a rad but siting is a bit messy.... Then I start to think about forgetting UFH all together.

I guess all these retrofit systems would need the screed removed unless a floor level change was acceptable (which it is not).

We were thinking of vinyl tiles too like Karndean so depends on how they can be attached, suspect there would need to be some screed depth above sheet systems?

I'm not sure what the heating engineer who works with the builder has in mind, as details are very sketchy in his quote. He did say, even if we didn't run UFH in the old floor sections, it would have to run through it (the screed) to get to the new extension area anyway, but there is a difference between a few metres of pipe in an old uninsulated solid floor versus running dedicated pipe spacing throughout the old area, to fully heat up the old floor.

I could also try in the plumbing forum if no first hand experience in here.