Topic: underfloor heating with concrete slab

Re: underfloor heating with concrete slab 

thanks Pete - I've followed your decision making re your build with a lot of interest! :) Have had a look at the link you suggested, which demonstrates we aren't the only ones who've been going round in circles on this issue!
Dean Buckeridge (architect) has also got back to me and reassured me that the solution we are using is the most practical & cost efficient for our situation and the heat lost will not be excessive.
Thanks for the advice.

Re: underfloor heating with concrete slab 

Nicky

I have thought about this also being an architectural draughtsman, and what I was thinking is to cut some sheet polystyrene that will be under your slab and insert it into the bottom the the poly blocks before reinforcing is placed in. Then you have a complete slab and foundation insulated with no concrete touching the ground.
Will be doing this for the next house I build without a doubt.

Mike

Re: underfloor heating with concrete slab 

Hi Mike, apparently this may cause issues with spreading the load of the house to the ground?
not sure of the technicalities myself, all I know is I want a warm & efficient house for my family and we don't have a huge budget to spend achieving this. So have been working hard to try and get the best efficiency 'bang for buck'. Has been an interesting process!

Re: underfloor heating with concrete slab 

Hi Pete
we are regularly in Wairarapa (Rathkeale) and would love to see the build and your experience with the UFH system- we are in Waikanae and just starting the journey with slab design currently underwAy with engineer. Is there a chance of meeting up?
Brent

Re: underfloor heating with concrete slab 

Mike,
You cannot use standard S grade polystyrene as you would use under a floor slab under foundations or even H or VH, it simply doesn't have anywhere near the compressive strength requirements to cope with roof, wall and floor loads.

It is possible I understand to get a extremely high strength xps board that could be used but I understand that the costs are prohibitive.

Re: underfloor heating with concrete slab 

Hi Brent,
sure - as we're building with polystyrene panels the construction tends to attract visitors like cattle attract flies.
I'm thinking of charging a gold coin donation to pay for my beer money :))
Give me a call on 021 530480 next time your over in Masterton (early evening or weekend is probably best)

cheers Pete

Re: underfloor heating with concrete slab 

I'm not an architect but I would echo what Dean has said. From the advice I've been given from several sources, poly under any load bearing area is a no-no
Pete

Re: underfloor heating with concrete slab 

I would suggest that you keep your underfloor piping about 500mm away from the outside wall.
Concrete is a not a bad insulator. Some loss will occur but it will be very small. This also has the beneifit of reduceing losses through window and wall, (convection cuurent) is reduced in these areas. Very few of us actually use this space (people) normally got furniture and nick nacs at these spaces. You can compensate for the reduced heating surface area, by reducing the space between your pipework
TC

Re: underfloor heating with concrete slab 

Staying 500 mm away from the exterior walls with the piping is complete nonsense.
UFH should always be evenly spread to avoid tensions in the concrete
You may create a colder zone along the wall DANGER OF CONDENSATION SPOTS
There is a good reason why radiators are placed on exterior walls mainly under the windows as the cold needs to be eliminated where it comes from.
Unless you use triple glazing (U-value .7 or R 1.42)in uPVC or timber frames you may create a very cold zone around the windows
DANGER OF CONDENSATION AND MOULD
plus discomfort.
The best method is still the 50mm overlay concrete on PU or Polystyrene insulation.

Re: underfloor heating with concrete slab 

its only 30C water, not 60C+ like the old boiler systems. There wont be any tention in the slab.

500mm from the outside wall is old school. It was used before edging insulation of the slab was common and it certainly is not nonsence. At the time it was the best in a bad situation.

VB

Re: underfloor heating with concrete slab 

Must be very old school 1960,s early 1970's or so.
It is category nonsense for a long time now

Re: underfloor heating with concrete slab 

Max "bang for buck"
How much extra are going to save by insulating the sides, compared to 500mm from the wall. Not that I am not recommending insulation. But if money is limited then this is a better option.
I covered the cold spot as you indicated, by saying practically that these areas are not used for people.
As far as condensation, bulls**t, a some heat will conduct through the concrete, other wise why would you need insulation, and for this reason 500mm is picked. As far as room temp goes it will be even, as you have more surface area than a raditior. Hot spots occur.
You talk about window frames, you discounted wood and pvc (a must with the 500mm system in your opinion) so therefore you must be suggesting aluminium for a insulated system. haha.
Do you think having a low level heat grade is going to stop a cold area around windows, I think not.
Yes rads to help in this area. But this is not a rad system.
Each system (any type) has pro and cons, you can not expect a medium priced system to cover all heating wants.
It far to easy to go on the net read a few articles, and believe you know it all, as these articles only tend to show benefits.
TC

Re: underfloor heating with concrete slab 

I did not suggest aluminium windows but I said the effect is worse with aluminium windows.
New Zealand houses have big condensation problems because the thermal envelope is neglected.
Even a few degrees less along the outer walls can reduce the dew point and cause condensation on the concrete, walls and of course windows.
Heating on the perimeter is paramount and more important than in the middle of the room.
You need to tackle the cold where it comes from.
Perimeter insulation on concrete slabs has been done in Europe for decades and it costs very little and I don,t know what all the fuss is about.....
If you have to compromise on the thermal envelope like this you must have an unrealistic budget.

Re: underfloor heating with concrete slab 

If air temp and RH remain the same the dew point does not change.
Condensation is caused by poor ventilation, the effect is increased by poor thermal enverlopes.
The insulation would seem to be a problem here, for what ever reason (but as we are not in Europe we have to deal with the here and now)
Heating the perimiter is not paramount, heating is paramount.
You talked about rads, do they heat the perimeter, no they heat a very small amount of the perimeter.
TC

Re: underfloor heating with concrete slab 

You guys all talk about u/f heating like you have been doing it all your lives - and if you tell me you have I will consider it a lie unless your 20 years old or from another country.

NZ building standards have only started really improving this last decade. underfloor heating in this country has only really gone anywhere in the last 20 years and most of the improvements in NZ where in the last 10 years. So by "old school", I mean over 10 years ago. Hell, there are still people selling electric boiler systems for u/f heating.

I bet in the 60's and 70's I could count the amount of underfloor installs in NZ on one hand.

VB

Re: underfloor heating with concrete slab 

This is a very interesting debate, and one played out in our own design office over the last couple of years. we DO insulate all our slabs (even the ones that we are not destined for hydronic underfloor heating.)

The reason being that we have several German designers on our staff and it became such a major point for discussion (code word for argument) that we had to sit down and develop a reliable, cost effective and code compliant way of dealing with the perimeter beam of our floor system.

We tend to use raft (waffle floor) type systems which leave a rather larger edge exposed to the elements than a conventional slab.

see http://www.facebook.com/pages/ebode/98244388735?ref=nf
for pictures of our raft and edge insulation set into the boxing before the pour.

My understanding is that most heat loss from a slab is through the edge and then (in the case of an unheated slab) through the top portion directly under the bottom plate.
Our German slab engineer feels that in many ways people would get higher gains by simply changing to 140mm framing than fafing around with elaborate slab edge details. We do both lol

People also seem to be fixated in making small gains in the slab but are perfectly happy to have enormous (and relatively thermally inefficient) glazing units.. French door / ranch sliders and the like around a good portion of the perimeter.

Again our kiwi obsession with indoor outdoor flow works hard to undo the small gains we make in insulation.

Niel de Jong ECO ebode

(Ps not afraid to state my name)

Re: underfloor heating with concrete slab 

Condensation caused by poor ventilation ?? b......t
Excessive relative humidity is caused by poor ventilation.
Condensation as such is caused by cold surfaces.
If the house is heated properly with a fully functional thermal envelope the humidity is ventilated out before condensation occurs.
Yes we are here and now with lots of crap building and silly theories about heating and insulation.
But we still don't want to listen to those who are 20 years ahead of us in this matter.
Why are the rads under the windows? Because it is the coldest surface?

Re: underfloor heating with concrete slab 

What do you think an increase in RH does, increase dew point temp.
You have answered your own question "humidity is ventilated out"
Yes I agree NZ building standards are crap, but this is the excepted standard, and overtime it will improve, trying to change to quickly only puts peoples back up.
Is better to do a better job over the avergae house building regiem or just do a great job on the limited who can afford the best, and F*** the rest.
Rads are under windows for a very simple reason, nothing to do with perfromance, efficiency or comfort. A window wall is a dead wall, a rad makes a wall a dead wall, so both are combined to increase usable wall space. in europe most common practice is to have curtains and pelmets (style permitting), these in them selves cause a thermal barrier.
I agree with abode, especially about windows.
TC (no name not looking for work)

Re: underfloor heating with concrete slab 

Yes it does but if the room temperature is maintained evenly at 18-22 C and no cold surfaces on the thermal envelope no condensation will occur as the air is capable to hold the moisture until it is ventilated out.
But if there are "holes" in the thermal envelope the room will be colder and the amount of moisture the air can hold is decreasing.
Rads are under windows because the window is the coldest surface. That is a fact at least in Europe.
Of course it might be different here as there is a kind of obsession around here to do the opposite ;-)
Is it because we are upside down???
If we push to close the twenty year gap now it will be for the benefit of everyone apart from the lazy and change resistant establishment in the building industry.

Re: underfloor heating with concrete slab 

This is a good old debate, here's my 10 cents worth.
Based on personal experience.
Back in the UK, radiators are almost 100% fitted beneath windows. This is to counteract the cold spot and general cold air "down draft" that windows of all material types will produce to some extent (some more than others). If you look at most UK building, plumbing or home energy information this is the reason invariably given. I so also agree with TC's point, the wall beneath a window is usually dead space so a radiator might as well go there. Every house I had (or my mum, sisters, brothers, friends etc) in the UK had radiators under the windows.

Again in the UK, UFH pipes are recommended to run closer together in the floor area beneath windows and outside walls to counteract the cooling effect of these elements. This is what I did back in 2002 when I fitted UFH, the 65mm screed slab (i.e the slab containing the UFH & laid above the structural slab) was fully insulated with 50mm PIR foam board beneath and 25mm PIR round the inner edges of the walls.

I have given some details above of how I have done my UFH recently. Just to recap, 100mm polystyrene sub-slab (apart from load bearing thickenings), 25mm edge insulation to outside of slab block work walls (in hind sight it would have been better to use a polyblock rather than concrete). The walls are 250mm thick polystyrene, any UFH pipes are set back 100mm minimum from the external walls, ie 350mm from the slab edge (getting close to TC's 500). Unfortunately the mesh extends to 100mm to the edge and I have R10 rods at 600mm centres tying the slab to the foundation. These two elements, necessary for NZ codes for a 2 storey house will suck away some of my precious heat unfortunately.
Being POMs we have gone for a more European look so our windows (upvc double glazed - low E) are relatively smaller (I think) compared to a usual kiwi design, but still 50% of the wall. Wall area 67m2, window area 35m2 (including frames). Larger windows on the north side than on the south.

We'll see how the UFH performs once the build is complete and the system is up and running. I'll let you all know.

Pete

Re: underfloor heating with concrete slab 

Hello Pete,

I am about to build soon and strongly considering the UFH inside the screed slab, rather than buried in the structural slab. Also just heard of someone in the neighborhood had a slab crack and broke the pipes for UFH... funny enough, I haven't seen anyone expressing any concern about that part.
Anyway, I hope you are enjoying yur new home and wondering if how did the UFH work out o the end?

Thanks,
Dejan.

Re: underfloor heating with concrete slab 

Thanks Dejan,

not quite there yet - first fix wiring at the moment. Hope to be in by Christmas so UFH may not get a good try out until next winter

Pete

Re: underfloor heating with concrete slab 

Hi, I'm new here and building our first new house soon. We are looking at a woodburner with 6kW wetback and 8kw room heat (and maybe a heatpump).

We want to use some of this wetback water to feed thru our hallway on the sth side of the house which is thru the wall from the fireplace, same place as HW cylinder.

Can someone recommend the kind of pipework used? Is PEX the right type? It will be the std low pressure wetback but could potentially be upto 95 deg C. We will be using a std 100mm pad with 50mm polystyrene underneath, 25mm timber surround footings barrier (still under discussion:). pipes will be tied to the mesh midway thru the pad.

We just want a large heatsink for the wetback that will give a gentle warmth to the south end of the house/hallway, that we can switch on once (or while) the HWC is heated up.

Any help here would be great.

Thanks in advance,
Lincoln from Hamilton.

Re: underfloor heating with concrete slab 

If anyone has had the slab crack so badly that the UFH pipes have failed the house is probably a write off.
The correct specification pipe can take normal slab cracks.

In New Zealand it is best to fit pipes into a conventional 100mm slab - the screed method isn't so suitable if you use an air sourced hot water heat pump (lack of storage capacity).

And at the present time an air sourced HWHP is the most cost energy source.

JK