Concrete Screed for wet UFH - advice please

[JamieM]

12 minutes ago, DeeJunFan said:

We have 150mm concrete sub-floor, quinn lite blocks at the slab level and then we are currently debating to go either 200mm PIR with Sand/Cement screed or 250mm PIR and 50mm CemFloor screed.

 

I'm the same except trying to decide whether to use 125mm PIR and 100m sand cement or 150mm PIR and 75mm s/c

[Declan52]

150mm insulation. 

[joe90]

I am having 200 mm PIR and 65mm sand and cement with UFH pipes.

[JohnW]

1 hour ago, DeeJunFan said:

 

In NI this is the "Way" it would be very difficult to find a builder locally who would even consider doing anything differently.

 

This is the same build up as i have used.  We have 150mm concrete sub-floor, quinn lite blocks at the slab level and then we are currently debating to go either 200mm PIR with Sand/Cement screed or 250mm PIR and 50mm CemFloor screed.

 

 

What U-values are you hoping to achieve with your 2 options @DeeJunFan?

[JohnW]

1 hour ago, Declan52 said:

I wouldn't want my finished floor put in at the founds stage if I was going to be building blocks. The floor will be ruined!!! 

Plus all that weight on your floor with the ufh piped in it is taking a chance. Depending on how access is you might have to stack blocks up 5 straps high which is 5t sitting on 1m2 which is a lot.

@Declan52 that's a fair point.

[JohnW]

1 hour ago, JamieM said:

 

I'm the same except trying to decide whether to use 125mm PIR and 100m sand cement or 150mm PIR and 75mm s/c

What U-values are you hoping to achieve with your 2 options @JamieM?

[JohnW]

45 minutes ago, joe90 said:

I am having 200 mm PIR and 65mm sand and cement with UFH pipes.

What U-value are you hoping to achieve with this @joe90?

[JamieM]

very roughly the u value for the PIR would be about 0.176(125mm) and 0.1467(150mm)

 

and if you include the screed/sand/cement would be 0.168(125mm) and 0.143(150mm)

 

 

 

[JamieM]

	thickness (mm)	W/m K	m2 K/W	u value
PIR	125	0.022	5.681818	0.176
screed	100	0.41	0.243902
			5.925721	0.168756
PIR	150	0.022	6.818182	0.146667
screed	75	0.41	0.182927
			7.001109	0.142835

Edited March 1, 2017 by JamieM

[joe90]

18 minutes ago, JohnW said:

What U-value are you hoping to achieve with this @joe90?

Well, I don't really know!!! I have not done a PHPP, originally I was going with 300mm of EPS but because of ground conditions, my reluctance to pay over the odds for a structural engineer to design a passive slab and being impressed with the Golcar passive house foundation design I decided to do it this way. I confess to being a bit of a Luddite, I know I am building in a very mild part of the country ( if a bit damp) . There are many on this forum who are whizzes at maths and will know exactly what their U value will be but I firmly believe that micro climate can affect what actual temps you get more than all the maths can evaluate. Jeremy,s build is a good example, because of his very sheltered location it turned out that cooling was a bigger issue than heating. Some will say I am mad but whatever, all I know is my heating will cost far less than A.N.Other house.

[PeterW]

The P/A value also affects the overall floor uValue as I found out. As we have a retrofit and new build element then we end up with a whole raft of different calcs but due to P/A of some we end up with a lower uValue for the same thickness of insulation in a couple of places. 

[Declan52]

You need to work out the perimeter of your build and divide this by the floor area.

Go to this site with whatever your figure you got from working out the sum above and put it into the calculator.

http://www.kingspaninsulation.co.uk/Services/U-Value-Calculator.aspx

[Jeremy Harris]

Bear in mind that for a new build Part L ( https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/540326/BR_PDF_AD__L1A__2013_with_2016_amendments.pdf ) has tightened up on things like thermal bridging and heat losses from UFH, so what may have been acceptable under the last edition may not under the current regs.  The changes are mainly intended to stop the horse trading big developers were doing, by fitting a handful of solar panels on the roof and then cutting back on insulation, as the PV improved the TER enough to get a pass despite the house being pretty poor in terms of insulation.  There are also notional model design guideline values for fabric U values that are suggested as a way to get a low enough TER/TFEE, and the floor U value in that table is now 0.13 W/m².K, not the limiting fabric value of 0.25 W/m²/K.   UFH must also not lose more than 10W/m² into the ground, maximum.  This isn't a tough target to meet if you have a low overall heating requirement, but would be tough to meet if you build to the limiting fabric U values, I think.

 

Edited to add:

This is the compliance guide for new build domestic building services, including UFH: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/453968/domestic_building_services_compliance_guide.pdf

 

Edited March 1, 2017 by JSHarris
Added the compliance guide link

[JohnW]

I suspect this thread could roll on for weeks as there appears to be many ways to skin this particular cat.

 

I think, I now have enough information to make a well informed decision, which will definitely involve increasing the insulation in the floor, reducing cold bridging and attempting to achieve a floor U-value as close to 0.1 as the budget will allow.

 

So, thanks again to everyone who contributed to this thread. I really appreciate the combined expertise and the unselfish time & effort you have all spent.

[DeeJunFan]

1 hour ago, JohnW said:

What U-values are you hoping to achieve with your 2 options @DeeJunFan?

 

For me it will be the difference between 0.086 and 0.107.

 

I was always of the mind to go 200mm PIR and 100mm Sand/Cement but my wife is concerned about the level of the screed and has always wanted a liquid screed.  I had convinced her that it would be a bad idea because of the drying times and adhesion but those issues dont seem to be the same with the CemFloor so she wants us to go that route now.

 

I need to just agree or find a compelling argument to the contrary.

 

D

 

Edited March 1, 2017 by DeeJunFan

[JohnW]

19 minutes ago, DeeJunFan said:

 

For me it will be the difference between 0.086 and 0.107.

 

I was always of the mind to go 200mm PIR and 100mm Sand/Cement but my wife is concerned about the level of the screed and has always wanted a liquid screed.  I had convinced her that it would be a bad idea because of the drying times and adhesion but those issues dont seem to be the same with the CemFloor so she wants us to go that route now.

 

I need to just agree or find a compelling argument to the contrary.

 

D

 

You are wrestling with a similar dilemma to mine, except my wife's not involved!

 

My only hesitation with the Cemfloor product is that it's Thermal Conductivity is 2.9W/m K compared to concrete 0.8W/m K which I interpret as...the floor will release the heat into the room much quicker than concrete, i.e. quicker response times, however the converse is also true in that, when the heating is off the heat will leave the floor quicker i.e. cool down quicker.

 

My thinking is that the quicker response is attractive if you plan to run your heating intermittently, e.g. have it switch on 1 hour before you get home from work, however we have been advised to run an ASHP using cheap tariff electricity overnight and let the thermostats determine when the house needs additional heat during the day. with this approach concrete seems better because it releases the heat more slowly and therefore the ASHP is less likely to switch on during the day, however with the Cemfloor screed losing the heat quicker I assume the ASHP would switch on more frequently. Does this make sense?

[DeeJunFan]

1 minute ago, JohnW said:

You are wrestling with a similar dilemma to mine, except my wife's not involved!

 

My only hesitation with the Cemfloor product is that it's Thermal Conductivity is 2.9W/m K compared to concrete 0.8W/m K which I interpret as...the floor will release the heat into the room much quicker than concrete, i.e. quicker response times, however the converse is also true in that, when the heating is off the heat will leave the floor quicker i.e. cool down quicker.

 

My thinking is that the quicker response is attractive if you plan to run your heating intermittently, e.g. have it switch on 1 hour before you get home from work, however we have been advised to run an ASHP using cheap tariff electricity overnight and let the thermostats determine when the house needs additional heat during the day. with this approach concrete seems better because it releases the heat more slowly and therefore the ASHP is less likely to switch on during the day, however with the Cemfloor screed losing the heat quicker I assume the ASHP would switch on more frequently. Does this make sense?

 

Indeed, I suppose it depends how you intend to run your heating.  If you use the ASHP to heat a large Store up to a particular temp you will be running your ASHP at night and then calling for heat from the store during the day. 

 

There are so many variables that its hard to know what is the best way to go.

[Jeremy Harris]

To be honest, once you get down to low heat energy input levels it doesn't much matter what you use to get that heat energy.  In our case, the additional overall cost of an E7 electricity tariff here makes it pointless, we're better off using the lowest standing charge standard tariff.  We find that the input power to our ASHP rarely exceeds about 800W, and then only for an hour or so every couple of days, and most of the time it's on for that hour or so it's only drawing around 400W.  The cost is so low as to be less than the cost of running things like the sewage treatment plant air pump and the UV water disinfection unit we have.

 

Hot water is far and away the highest energy demand, far greater than heating the house, and it was a challenge to come up with a reasonable compromise.  I've concluded that once you get down to only needing such a low level of heating, it makes more sense to separate out hot water system design from the heating system design, because they have such differing requirements.

Edited March 1, 2017 by JSHarris
typo, "how" when I meant "hot"

[Nickfromwales]

10 minutes ago, JSHarris said:

We find that the input power to our ASHP rarely exceeds about 800W, and then only for an hour or so every couple of days, and most of the time it's on for that hour or so it's only drawing around 400W.

Have you established a CoP ratio from your data so far Jeremy? 3:1 more / less?

Best to reiterate that the 400w with a CoP of 3:1 would net ~1200w of heat energy, give or take losses / efficiency etc.   

[PeterW]

1 hour ago, Nickfromwales said:

 Have you established a CoP ratio from your data so far Jeremy? 3:1 more / less?

Best to reiterate that the 400w with a CoP of 3:1 would net ~1200w of heat energy, give or take losses / efficiency etc.   

 

I've just completed mine again using the JSH sheet - I have a max of 9kw available at A7W35 (COP 3.2) and I need 2.3kw... but at A7W52 I'm down to COP 2.3 but will boost the DHW with immersions. 

 

Also helps to plan out when your heating and water will be needed and draw it out on a block of 24hrs and you can soon see where your ASHP or alternate heat source  will kick in to provide heating or DHW

[Jeremy Harris]

1 hour ago, Nickfromwales said:

Have you established a CoP ratio from your data so far Jeremy? 3:1 more / less?

Best to reiterate that the 400w with a CoP of 3:1 would net ~1200w of heat energy, give or take losses / efficiency etc.   

 

I should be able to take a stab at this with the next lot of data, but I think it's a bit better than 3:1 pretty much all the time, with the relatively low flow temperature and no defrost cycling.

 

It seems that, in winter, I'm probably putting between 0.5 and 1.5 kWh as heat energy into the house per day, so around £0.02 to £0.06 per day in electricity cost.  The air pump in the sewage treatment plant costs about £0.135 per day to run, so a fair bit more than the heating for the house.

Edited March 1, 2017 by JSHarris

[Laith J]

Sorry to hijack an old thread... Some very useful information to be fair.. Im currently helping my father out with is extension and we have 190 mm to build up to finished floor, fow which he has already bought 100mm PIR and is going to run 16mm UFH pipe. He currently is planning on a 90mm s/c screed to take him up to FF but personally I think he should put more PIR and reduce the screed amount.. what would be the best option in this scenario?

[JamesP]

1 hour ago, Laith J said:

Sorry to hijack an old thread... Some very useful information to be fair.. Im currently helping my father out with is extension and we have 190 mm to build up to finished floor, fow which he has already bought 100mm PIR and is going to run 16mm UFH pipe. He currently is planning on a 90mm s/c screed to take him up to FF but personally I think he should put more PIR and reduce the screed amount.. what would be the best option in this scenario?

I would increase the PIR by up to 50mm. Do double check your levels and thresholds.

My floor build up was 150mm celotex on top of the slab, 16mm ufh and 40mm screed. Check with the screeders minimum screed recommendation.

 

 

Edited January 30, 2020 by JamesP
spellin

[Temp]

Google says 20 and 30mm PIR is available so you could add a layer of that to the 100mm PIR he already has. Think I'd put it under the 100mm.

[joth]

19 hours ago, Laith J said:

Sorry to hijack an old thread...

Well it was actually  very opportune timing for this thread to pop up this morning! Just after reading this I attended a site meeting to discuss issues with the ground works for the extension - digging was completed once for a 200mm raft foundation for timber frame, but building inspector had a change of mind and has now decided it must be done as strip founds to match the existing house  (Easy to be fickle when it's someone else's money eh? Best bit is we even get to pay VAT on all the wasted work too. But I digress)

 

Anyway, long shot is we need to raise the level back up by 100mm as it's no longer a structural raft slab, and having paid to excavate it I was reluctant to pay more on MOT to fill it back in. Having just read this posting this morning I thought why not put more insulation instead. Architect and contractor all agree that's a goer.  So now the plan is (top to bottom):

• 75mm cement screed
• extension 260mm Celotex / main house retrofitted with 160mm Celotex
• 100mm slab

From what I can tell Celotex is no more expensive that MOT anyway - maybe it's actually cheaper, lets see.