UFH on top of chipboard flooring

[David R]

Hi Folks,

 

I have a question regarding placing underfloor heating pipes on top of 22mm chipboard flooring.

 

I am building an extension, the original design was to place heat dispersion plates between the 195mm x 50mm joists at 400mm on top of 165mm insulation.

 

We have decided to place pipes in a 50mm pumped screed on top of the joists / chipboard flooring in order to make use of the thermal mass. the extension is around 70 Sqm. 

 

Our engineer says the weight is not an issue and we have allowed for the height difference of the screed, by building the block work lower.

 

Does anyone know of any issues with placing the UFH pipes on top of chipboard?

 

Second question, the UFH will be heated with an oil boiler and ASHP. The room will be a living room, kitchen, dinning room open plan.  Would it be beneficial to have 2 heating loops as opposed to one?

 

Thanks in advance

 

 

[Russell griffiths]

So when your floor flexes what happens to the screed on top.??

 

its not a method I would want, has your architect done this before?

[Mr Punter]

You often get this in flats.  I guess they use a decoupling mat on top of the chipboard.

[David R]

The joists are on 3400mm span. would there be any flex or do you always get flex in a wooden floor no matter how rigid?

 

My architect has not done this before as far as I can tell. 

 

I will look into decoupling mats, I have never heard of them.

[Scoobyrex]

have a look at the timber solution from screedflo,  It is a different screed mix and they have guidance on the wooden floor makeup.

[Temp]

Sorry if this is obvious but you still need the insulation below the floor.

 

I've not used them but there are systems like this..

 

https://ambienteufh.co.uk/tiling-over-underfloor-heating/

 

 

[Temp]

We have a mixture of wet ufh systems in our house. Some rooms are stone over ufh in screed. Others are Engineered wood on battens with UFH in a foil covered insulation system between. I quite like the faster response time of the latter. 

[David R]

Thanks for your responses.

 

Scoobyrex, Thanks, I have spoken to a closer located screed company and they have a similar product to Speedflo and say that screeding on top of boards is not an issue, they said the membrane / tanking system used (plastic sheet ) acts as a slip layer plus the screed would flex.   They are sending a rep round next week to have a look at the job. 

 

Temp, Thanks, there will be 165mm of insulation between the joists, ( my badly worded initial question ) now you have me worried about thermal bridging and should not fit the boards, but fit a layer of 20mm insulation on top of the joists. I like the system you used, it would save lifting boards in a retro fit.

[AliG]

If the floor is already well insulated underneath then  the 20mm on top of the boards would probably suffice. But if you didn't see it fitted I would be wary of how well insulated it is underneath. Usually builders just hang a little bit of rockwool and don't do it very carefully. You should probably be looking at a build up with a least 0.15 U-value if putting UFH on top of it.

 

On 03/08/2021 at 09:30, David R said:

Second question, the UFH will be heated with an oil boiler and ASHP. The room will be a living room, kitchen, dinning room open plan.  Would it be beneficial to have 2 heating loops as opposed to one?

 

You will need more than one loop due to the size of the room as the max loop size is usually 100metres of pipe.

 

You should do a calculation of how much heat output you need for the room and how close to space the pipework for your desired floor temp.

 

Why are you connecting it to both the ASHP and oil boiler?

 

 

 

[David R]

AliG,  you comments are really appreciated. they are helping me get my head round the UFH.

 

The floor has 165mm Celotex well fitted between the joists, I am doing it right now, what a pig of a job to do!

 

It would have to be 22mm boards or 20mm insulation as I do not have the build up space for both. I prefer the boards as it would give a more solid floor.

 

The U-Value is around 0.15 or better, with a PA of 1 and according to the architect plans it is 0.15. There is about 50% glass in the extension.

 

The reason for wanting to keep the oil boiler and fit an  ASHP is that the older part of the house, which we are living in, could be upgraded with larger radiators etc at a future date, plus the older part is not so well insulated. Also, I do not want to fit an ASHP to find out that my electricity are unmanageable or that it cant cope.  I am also fitting a wood burning stove in a lounge area of the old part of the house and perhaps another one in the extension once it has all been signed off. The is in an exposed sight, the winters can be a bit harsh.

 

Any input is appreciated.

 

 

[Nickfromwales]

I would get a Hybrid ( oil uplift ) ASHP and condense the solution. That would allow ASHP only where minimal heating is required, but then burn oil to fortify when, and only when, necessary.

The 70m2 Will likely need 4 loops, as you’ll want to be well under 100m for each individual loop length to maximise even heat output across the whole area.

Pipe size and centres ( distance from pipe to pipe ) will be dictated by the system you choose, but given this is retrofit which is incredibly tricky to get right I would 100% be using an overly system. If height is an issue, recess battens to accept 22mm P5 and finish them flush with the joist tops. Then I would skin the whole floor with 9mm ply glued and screwed for integrity. Then fit your overlay system on top of that with as much insulation in the build up as possible.
Forget “thermal mass” you just want the heat getting to the room as directly as possible. 
Minimum 100L buffer tank, because of oil / ASHP, but if both they will need to become a multiple coil thermal store to hydraulically separate the different systems to suit. 

On 03/08/2021 at 09:30, David R said:

50mm pumped screed on top of the joists / chipboard flooring in order to make use of the thermal mas

This will only give you a slightly better emitter, but at a cost. I would buy insulation not screed. 

Finally, draught-proofing the subfloor will be your biggest return, more so than insulation. Use foam a plenty, run the 9mm ply 8mm short of the external walls and then fully foam the perimeter. This will pay huge dividends. 

[AliG]

The Celotex will give you enough insulation. Make sure it is well sealed as @Nickfromwales points out.

 

It sounds like you can use either the kind of build up shown above - 22mm board, overlay panels, tiles or board plus screed on top of it.

 

I might disagree with Nick here. If you have room for the 80-90mm build up of 22mm chipboard plus screed plus tiles I would do that.

 

The reason is that you are going to need a high flow temp / floor temp to heat that area. I think this will work better in a heavier floor.

 

TBH I would be worried that you are going to struggle to heat the space. What kind of glazing will it have? 50% glazing is close to a conservatory. Is it 50% including the internal walls to the rest of the house? If it is not triple glazed you will struggle to put out enough heat from the floor. You might well need the oil boiler for colder times when the required flow temp will be too high for the ASHP.

 

In my last house we retrofitted the kitchen with UFH in this manner and I had to run the flow temp at 62C to keep the room warm in winter. It really struggled. I think we only had 100mm Celotex under the floor however and I didn't do it myself so don't know how well it was done.

 

[Nickfromwales]

40 minutes ago, AliG said:

I might disagree with Nick here. If you have room for the 80-90mm build up of 22mm chipboard plus screed plus tiles I would do that.

6 and two 3’s. More insulation, less losses, lower flow temp > better CoP from HP. There’s only so high a flow temp you can run at the end of the day. 
Look at how well electric UTH works in a bathroom   

[David R]

23 hours ago, Nickfromwales said:

If height is an issue, recess battens to accept 22mm P5 and finish them flush with the joist tops. Then I would skin the whole floor with 9mm ply glued and screwed for integrity. Then fit your overlay system on top of that with as much insulation in the build up as possible.

 

Thanks Nick your thoughts are appreciated, most of the insulation is in already. Taking it back out and cutting boards to fit between the joists, is going to be too much work. Our intention is to use Vinyl flooring. I worry that an overlay system would be able to achieve a flat enough surface, plus the time / cost of fitting it. I take on board what you say about insulation it makes sense.

 

I have been taking care and foaming gaps in the insulation between the joists and could fit rockwool insulation underneath to help with draught-proofing 

 

Reading a few educating posts on here  on "thermal mass" and don't think I will use that term ever again!  If I could start again, I would have of used block and beam construction, with insulation and concrete slab on my sloping site, plus educated myself far more on correct floor build up. I like the idea of a hybrid system. My Ex ships engineer neighbour  has a similar stone built property, he has a back boiler in his wood burner, ASHP, PV, Oil and solar thermal, I need to keep up a bit.

 

AliG,   Thanks,   I have been a bit of a numpty in understanding the PA ratio,  it is not "1" it is .35 as I included the internal wall as you alluded to.  It is double glazing with a triple glazed roof window.

 

I do not have 80 / 90 mm build up for tiles, this would have been a good solution.

 

[Nickfromwales]

This kind of thing

[Mr Punter]

16 minutes ago, Nickfromwales said:

This kind of thing

£9,384.00!

At least there is free delivery.

 

Is there much benefit in having the hybrid compared to a modern ASHP and oil boiler separately?

[Nickfromwales]

2 hours ago, Mr Punter said:

£9,384.00!

At least there is free delivery.

 

Is there much benefit in having the hybrid compared to a modern ASHP and oil boiler separately?

If you already have an oil tank and have retrospectively upgraded your dwelling, then it seems sensible, eg if the insulation and heat emitters aren’t quite ‘there’ and an ASHP alone will struggle wintertime. Otherwise I’d not really go forward one. The cost is high, but it’s a cheap be box external unit solution that is quite neat and effective imho. 
That means you can use the HP for the brunt of the heating and just have oil kick in only when absolutely necessary. 

[David R]

I had looked at the Grant oil / ASHPs and think it is good that the industry recognises that there is a need for them. We have a Grant oil boiler and it has performed really well, if a bit noisy. The idea of fitting an oil boiler outside is appealing.  The next step will be getting my head around controlling both the ASHP and oil boiler to get seamless uplift.

 

Could you recommend a thermal store / controls ?