My proposed floor is concrete (ground-bearing) slab above 180mm XPS insulation in order to have some thermal mass in room with lots of SE glazing. I was going to put some UFH in even though I don't expect to use it more than once in blue moon. The normal way of doing this seems to be to finish concrete ~70mm below FFL then later put down screed and put UFH pipe in that. 2 questions:

1) how do you attach the pipe to the existing concrete in order to pour the screed? There are various plastic formers you can get to wrap pipe round but they assume you have insulation underneath. Using them above concrete would just separate most of your useful thermal mass. Similarly there are various fixings to attach pipe to insulation - but I haven't seen any to attach pipe to concrete.

2) why separate the concrete and the screed? Why not pour it all in one? Is it just some practical reason to do with what's easy in building (concrete can be all wonky - screed will sort it out?) We'll be putting tiles on top so it only has to be 'quite flat'. And I've seen people polishing their concrete in order to use that directly as the floor surface. Is that expensive (this is a relatively small 28m2 area)? We can dig out less deep and save a skip or three of soil-shifting if we just skipped the screed and did it all in one, making efforts to ensure it was flat enough. Is there a good reason not to? In this case the UFH is normally tied to the rebar grid that goes in the floor slab, which seems simple enough

We used the slab - insulation - mesh - pipes - screed method, using ties to fix the pipe to mesh.

Concrete slab and screed are different beasts, screed for UFH usually containing fine fibre to provide reinforcement to prevent micro-cracks. Not sure if they also have slightly different thermal properties.

Posted By: davidfreeboroughThe other issue as CWatters has mentioned is that you may end up with a very long response time. In some situations it may be unacceptable, but in a Passivhaus with an Economy 7 heat pump this can be a plus point.

First, I very much favour the idea of insulation under the slab, especially if joined up to external wall insulation, to minimise thermal bridging.

However, I think that integrating the UFH pipes into the full slab will result in too much thermal mass and a very slow response time. You would basically need to have your heating on 24/7, at a very low level. I suspect that getting the temperature stable in a low energy dwelling, especially one with large passive solar gain, and therefore subject the weather variations (even on a short timescale) would be tricky.

I think that separating the structural slab from the screed using a thin layer of insulation is probably the best compromise. This uses the thermal mass of the screed, but is not slowed too much by the thermal mass of the structural slab.

Posted By: wookeyAnd I've seen people polishing their concrete in order to use that directly as the floor surface. Is that expensive (this is a relatively small 28m2 area)?

No, but I've also read that it's not as cheap as you might think as well!

Nick Grant described his on the AECB forum - http://www.aecb.net/forum/index.php?topic=35.0

I'm sure I have some more links somewhere but I can't get at them at the moment. Anyway the photos at the top of this page - http://www.concretenetwork.com/staining-concrete/ - and in the linked photo page give an idea of what you can do.

When considering how much thermal mass is attached to the UFH bear in mind that we're essentially planning not to use it except very rarely. The room will be a largely unheated area for plants, bike fettling, welly boots etc. I'm much more worried about it being too hot in the summer (when it may get used quite a lot) than too cold in the winter (when it will mostly serve as a thoroughly overspecced porch). It would only get heated if one needed to do some bike spannering in the evening, and then just enough to take the chill off.

I was considering 10mm of acoustic foam between screed and slab but had mostly decided that in summer it would just isolate the slab unhelpfully and I was probably better off without it. It would be nice to be able to model this properly and not just guess.

I quite agree that in normally-heated areas we'd want fast-acting heating not really-slow heating.

Oh and whilst we are on the subject is 'less un-green' (i.e high PFA) concrete fairly standard now or am I going to have to search it out and pay extra?

FYI for anyone who finds this page. We (well my builders) did one of these deep UFH and it works well.

From COLD, it takes 3 days of log burner (16kw max) to get the 20 tons of deep concrete, UFH up to a 35 deg return temperature.

Then simply measure the slate temperature with an IR gun (thermometer), turn on the pumps to measure the deep heat in the slab and top up as needed.

In worst weather that's an 8 hour burn in the snug every afternoon / evening.
Our logs, coal in desperate times.

Posted By: ShevekI'm also looking for a way to connect UFH pipes directly to a slab. None of the UFH systems a designed for this. They all assume either insulation or suspension between joists.

Eh, I'm no expert on this subject but fastening UFH to the reinforcement in the slab is surely a very common situation?

Or are you saying the slab is already poured? In which case I suppose embedding the pipes in a screed would be the way to go.

how deeps your screed . lay a light mesh on top of slab and cable tie to this
A98 or A142

Can't you use the cliprail and drill/screw it to the concrete slab through the holes that would normally take the staples into the insulation?

I can't imagine you'd need that many screws as the clips don't look like they hold to the insulation very tightly normally.

I've seen the plastic formers cut up and fixed down and used to form the bends at each end of a room and the pipe left loose across the room. If tight, the pipes lay across the concrete , if they kink up too much add the odd plastic pipe clip where required.

Insulation is important when you are heating up the slab with a fixed high temperature...used with on off controls....quite simply you need to avoid introducing too much heat into a too big slab...

If you are going to use water approaching ambient temp you can get away with less insulation.