Digging Up Concrete Floor To Retrofit Wet UFH
Posted 12 June 2014 - 09:24 PM
My name is Steve. Long term goal is working towards off-grid self-sufficiency starting with a 19C stone farmhouse in France.
Short term worry is fitting wet underfloor heating on the ground floor. Existing floor is tile on freezing concrete c1980. We want UFH to make best use of new wood burner + solar + 2000L heat store that we are currently installing.
I'm expecting to have to dig up the entire existing floor and dig down some way into the old earth floor to make enough space for new stackup. I've got no idea how hard it is going to be, or what I will find, but will know shortly.
My understanding is that once I have got down to an adequate depth , I have to first put a new concrete base down, then DPM , then insulation, then pipes and then about 7cm screed.
Floor area is 54m2 ( about 9 x 6 m ). Does anyone have tips on how thick the new concrete base should be, what kind of mix, should I use reinforcing mesh? Any tips from the experienced and knowledgeable gratefully received. This is first time I've done this and I don't want to fall into a trap if I can help it.
Posted 12 June 2014 - 09:34 PM
I think it's better to put the insulation down first, then the DPM, then lay steel fabric on chairs, with the UFH pipes cable tied to the steels, then 100mm of concrete. It does need a bit more work to get the concrete floor dead level, but is cheaper and easier as a heated floor system.
Use EPS as insulation, as it's cheap and proven to work well underground.
You will need to make sure the house is well insulated and draught proofed, too, as 54m² of heated floor isn't going to give you a massive amount of heating. Also, you will need to mix the high temp water from your wood burner down to UFH temperatures.
Do the heat loss calcs first, as otherwise you may be disappointed as to how little heat you get from such a small floor area (a couple of decent sized radiators running at a higher temperature would give you a lot more heat than the best UFH system, I think, but UFH should work very well if you've got the heat loss of the building right down).
Edited by jsharris, 12 June 2014 - 09:35 PM.
Posted 12 June 2014 - 10:17 PM
Are you saying we can put insulation directly on an earth floor , without first making a concrete base??
Re: Heating power . We're aiming to pack in the pipes to maybe 10cm pitch using 4 loops. Also we've got a little 8kw wood burner + wood cooker for backup.
I've come to believe UFH is the only way to use the heat in the store once it gets below rad temp ( apart from DHW heating - I'm putting in a separate 200L DHW tank with heat exchangers and will heat this from the store indirectly ). We're at 1000m so get wild swing between day and night temps . Lots of sunshine and often hot in day , but can be severe here in winter (e.g. -15C for weeks with metres of snow ) . Solar will be great to help even out this night-day delta .
Insulation is not great it is true. But I don't think it is the worst because the walls are very thick and we are dug into the hill on the north side to the first floor beam level and have reasonable double glazed windows in most places. There is definitely some insulation in the roof from the last renovation in the 1980s - but I haven't looked to see exactly what. There is definitely work to do to improve the insulation in the roof.
We are planning to use hemp plaster insulation on the inside , one we've take off the old concrete and false back wall ( damp covering wall ). We did a lot of digging work out the back last year, digging out a 2m trench behind the house and filling it with 20 tonnes of gravel in a geotextile bag and putting proper drains in to keep everything dry behind the house, so we're optimistic we can take the false wall off and most of the 1980s concrete render on the inside and replace it with hemp plaster - and get some improvement this way.
Posted 13 June 2014 - 06:39 AM
The best you'll realistically get from UFH is around 50 to 60 W/m², if you try for any more that that the floor will be a bit hot and the losses increase. So if you can heat the whole of the 54m² (remembering you can't really put UFH pipes under places where things may need to be screwed to the slab) then you could get a bit over 3kW of so of heating from the floor. Realistically I think it would be better to run the slab a bit cooler, reduce the losses and get maybe 2 to 2.5 kW max from it as a design target. You will need a fair bit of insulation underneath it, as UFH increases the floor heat loss to ground a lot, as you're heating the floor to a higher temperature than the room, and the ground underneath will be fairly cool, around 8 to 10 deg C. As a rule of thumb, UFH pretty much doubles the floor heat loss for any given insulation thickness over other forms of heating, because of the higher floor temperature. We have 300mm of EPS foam under our slab, but we will only ever run it at around 20W/m² absolute maximum, most of the time it will run at between 5 and 10W/m² I think.
EPS has the big advantage of being pretty much the cheapest rigid foam insulation you can get, the downside is you need a thicker layer, as it isn't as good an insulator as PIR foam, for example.
The better you can get the airtightness the lower your heat losses. For many houses ventilation heat loss is as great as fabric heat loss through lack of insulation. Once you have good insulation then ventilation losses are often the greatest source of heat loss (they certainly are for our new build, without MVHR around 70 to 80% of our heat loss would be from ventilation loss alone).
Posted 13 June 2014 - 09:42 AM
If your house isn't well insulated the floor temperature might have to be high in order to deliver the required power. Might need as much as 55-60C flow temperatures particularly if you were putting carpet down.
If the ground is a bit soft you can put compacted hardcore (eg crushed limestone), sand blind, DPM, insulation, plastic, Mesh/UFH/Conc. The insulation should probably rated for ground contact to stop water absorption.
Another option would be a suspended floor (ventilated below) using either timber or beam and block on stub walls but that's probably more work as the resulting floor is thicker and needs a 200mm+ void below it.
Posted 13 June 2014 - 11:03 PM
Since you tipped me off about the idea of ditching the 'concrete subfloor' I found this photo blog which seems to show pretty much the process you are describing
This guy was having some problems avoiding treading on the pipes while he finished the job. Those 'castellated pipe positioning' plates (sat on insulation) look like luxury by comparison.... although stupidly expensive. But your suggestion seems to have the big advantage of cheapness and less work.
I wonder if a 10cm reinforced slab as the main floor surface might be less likely to crack than the 7.5cm screed alternative ( ontop of those expensive castellated panels). But my main worry is damaging the pipes - and being forced to dig it all back up in my lifetime!
Thanks also for focussing my mind on the heat loss realities. I will have to keep this in the forefront when those devils in my head start telling me I have dug down far enough! Dig and keep digging is what I'm taking from this, and allow some big money for EPS or some other THICK insulation.
Thanks both for letting me know how little power I will actually get from a 54m slab! My estimates were completely wrong. But the existing freezing concrete floor makes life on the downstairs in winter miserable even if a fire is blazing. If I can insulate it with 15cm+ of EPS and get 3kW to radiate off it I think/hope it will be a big step forward. I'm not going to run the slab hotter than standard because of fear of shortening the life of the pipes or cracking the floor... I'll just accept the 2.5-3kW that it seems I will get. I'll be happy if I can get it working before the winter! Thanks again
Posted 14 June 2014 - 07:12 AM
With only 150mm of EPS and with the floor surface (not the slab, which will be a degree or two warmer) running at 26 deg C (to give you a heat output of about 3.4 kW with a room temperature of 20 deg C, assuming all of the 54m² is able to radiate heat into the room), then you will lose around 7% of your heat input to the slab into the ground under the house, through the insulation. 150mm will also only just meet building regs, which assume a non-heated slab (and the heat losses for a non-heated slab with this thickness of EPS would be around half those for a slab with UFH). If you can get a thicker layer of EPS in there then I would do it. If you can't, then look at using more expensive PIR foam instead if you can, as 150mm of this will be around the same performance as 230mm of EPS. If using PIR, then be aware that it needs to be the right grade and type for below ground use, and may be better being above the DPM, rather than below it, in my view, which may make it a bit more awkward to lay, and runs the risk of the insulation floating up when the slab is poured (not a problem if you keep the EPS underneath the DPM).
Edited by jsharris, 14 June 2014 - 07:12 AM.
Posted 14 June 2014 - 02:59 PM
I was assuming I should keep the slab from touching the walls with some edge insulation , like you would for pipe in screed. Do you think a 100mm reinforced slab can sit on the EPS without being bound into the walls ok?
7% heat loss doesn't sound too bad - but I'll see how far down I can get.
Posted 14 June 2014 - 03:27 PM
Bear in mind this 7% heat loss is after the efficiency of the boiler or wood stove or whatever, so in reality it represents a fuel increase that is probably between 10% and 15% more than for radiators.
Posted 01 August 2014 - 12:53 PM
Now I am having trouble deciding what EPS to buy. Difficult to find something that thick - so I'm beginning to think of two 10cm layers. Does this sound sensible?
I can't find anything less than 10euro per m2 ( 100mm EPS ) that is clearly rated to lay on the ground. But plenty at 6 euro per m2 clearly rated to support concrete floors. Perhaps I should use the more expensive board on the ground layer only. Do you think this is worth worrying about?
Thanks for the tips!
Posted 01 August 2014 - 10:21 PM