Ufh install in bungalow

[vik2001]

I'm doing a extension where it will join into the existing floor in bungalow which is just a slab of concrete. I want ufh throughout.

NEW extension is block n beam concrete floor which will have 100mm pir insulation.  The old bungalow is concrete with no insulation. 

Builder is saying he will add 20mm insulation to existing concrete so both floors match up, and will add overlay ufh board floating over the pir insulation in the extension and old floor. Then lay a floating engineered oak floor on top.

 

Can the ufh overlay board be left floating over insulation board ?  

[Conor]

Don't do it. 20mm insualtion is nothing, you'll be chucking away loads of heat and it'll cost you a fortune to run.

Either rads or dig up your existing floor and lay an insulated slab.

[JohnMo]

2 hours ago, Conor said:

Don't do it. 20mm insualtion is nothing, you'll be chucking away loads of heat and it'll cost you a fortune to run.

Either rads or dig up your existing floor and lay an insulated slab.

Plus even with 100mm insulation you will still be loosing plenty of heat downwards.

[Redbeard]

+1. Even the 100 PIR on the B & B does not sound that exciting, but 20mm with a delta T much higher than typical air-outside transference is - how shall I put it - a little (*) sub-optimal. (*)By which I mean a lot, lot, lot.

[vik2001]

See attached the block and beam floor, and you can see the existing house past the acroos.  

I dont have much height to play around with. Once I put the pir boards on the block and beam what can I put in top that doesn't increase height. 

Ideally I want to overlay with ufh boards everything 

[Dave Jones]

your block and beam needed to be 225mm lower than finished floor. 150mm insulation 75mm screed.

 

elementary mistake if a builder was involved.

[vik2001]

I did think as well the block and beam floor needed to be lower, and raised this with builder. But there's a sewer pipe that runs through the garage that cant be touched without smashing garage floor. This is directly below the block and beam floor, so if the beam floor was lower it be at level of drain pipe

Edited March 1 by vik2001

[Mr Punter]

15 hours ago, vik2001 said:

I want ufh throughout

 

Without 150mm insulation you should have a rethink.  You could put UFH in the extension and rads elsewhere, but on the concrete floor it will leak loads of heat.

[Iceverge]

Just put in large radiators. 

 

In an efficient setup then UFH runs below skin temp anyway so still feels cool to touch.

 

If you have ever experienced "hot" UFH then the system is lightly to be very very expensive to run as the losses to need the high flow temps will be correspondingly large. 

 

If you do need a bit of extra comfort in the bathrooms electric UFH just under he tiles is an option. As the area is small and the run times intermittent then it won't be too dear to run. 

 

 

 

 

 

[Chris D]

Hi @vik2001, I’m in a very similar situation in that I have a bungalow with solid floors and no existing radiators either and very open plan so not many walls so wanting to install ufh. 

I ran the calcs with 25mm PIR and my P/A and it dropped the u value by about half to ~0.3 which is not ideal but not catastrophic. 
I also was reading around and the site below seems to agree. I’ve obviously tried to look at flow temps etc so for example using @Jeremy Harris heat loss calculator with a room/floor temp of 30c I think I loose ~2.3kw into the floor rather than 900w if I meet building regs of 0.13 but as a function of the total 14kw didn’t seem too bad to me. 
 

https://www.homebuilding.co.uk/advice/retrofit-underfloor-heating

The only possible issues overlaying on PIR is compressive strength so thinking maybe XPS might be needed and a bit thicker. 

I realise it’s going to loose some heat downwards but it s it really that bad or am I missing something? 

[JohnMo]

13 minutes ago, Chris D said:

Hi @vik2001, I’m in a very similar situation in that I have a bungalow with solid floors and no existing radiators either and very open plan so not many walls so wanting to install ufh. 

I ran the calcs with 25mm PIR and my P/A and it dropped the u value by about half to ~0.3 which is not ideal but not catastrophic. 
I also was reading around and the site below seems to agree. I’ve obviously tried to look at flow temps etc so for example using @Jeremy Harris heat loss calculator with a room/floor temp of 30c I think I loose ~2.3kw into the floor rather than 900w if I meet building regs of 0.13 but as a function of the total 14kw didn’t seem too bad to me. 
 

https://www.homebuilding.co.uk/advice/retrofit-underfloor-heating

The only possible issues overlaying on PIR is compressive strength so thinking maybe XPS might be needed and a bit thicker. 

I realise it’s going to loose some heat downwards but it s it really that bad or am I missing something? 

Your missing something

Be careful with Jeremy spreadsheet as it's only really works well with just about airtight and very well insulated.

 

Your downwards heat loss is ground temp and means UFH flow temperature - not room temperature.  Rubbish insulation means high mean flow temperature, your mean temp could be around 30 to 40 degrees, so 4 to 5kW downwards or 100kWh a day.  Rethink - Everyone mentions 150mm min for good reason.

[Chris D]

@JohnMo thanks for the quick response. I'm trying to be very careful with the spreadsheet and have done the following but may be making mistakes. 

 

Increased air changes per hour to 1 to reflect lack of air tightness. 

Used 'room temp' of 30 and 40 degrees to simulate the UFH with the increased delta T and then reverted to the standard room temp of 20 to calculate other fabric losses. 

The scenario above was for 30C because I figured for a flow temp of 40 it wouldn't result in a floor temp of that as some insulation in the overlay boards though for a room temp of 40 that would be 3.5kW. 

 

If I approach from a couple of other perspectives.... If building regs are 0.13 then at 0.3 I will have 2.3 times the downward loss of a relatively good setup. 

 

I used the link below for u-value calculation which again looking at the thermal resistance it's the diminishing returns. i.e. the first 25mm halves the u value. then 100mm halves again so whilst 150mm would be great but I don't have the headroom. 

 

https://www.gov.scot/binaries/content/documents/govscot/publications/advice-and-guidance/2020/02/tables-of-u-values-and-thermal-conductivity/documents/6-c---u-values-of-ground-floors-and-basements/6-c---u-values-of-ground-floors-and-basements/govscot%3Adocument/6.C%2B-%2BU-values%2Bof%2Bground%2Bfloors%2Band%2Bbasements%2B%2B.pdf

 

From another perspective, if room/floor temp was 30 then double the downward heat loss I have now. By adding the 25mm PIR I half this so roughly the same downward heat loss I have currently. 

 

I don't disagree at all it would be better to have more insulation but I don't have the space and also don't have any plumbing for radiators so that isn't an easy solution either so am trying to make an informed decision. If for example I lost an extra 1-2kW downwards it's not ideal but equally compared to the >£25k to dig the floors up seems like a more sensible option.