U-Value Required for Building Regulations for Retrofit/Renovation Underfloor Heating Heating?

[Chendy]

Hello,

Does anybody know the U-Value required when retrofitting / renovating underfloor heating?

There seems to be plenty of resources talking about new builds but finding it hard to find any definitive information for retrofit.

 

For my specific property, I have a ground and first floor. Both having a concrete slab floor with screed over it.

I will remove the screed, then add insulation, then UFH pipes, then concrete poured over.

Trying to work out how much insulation I need. I'm assuming less required for the first floor.

 

Once I determine the maximum U-value permissible for building regulations, I can use various services such as this U-Value calculator to work out how much of what insulation I need.

 

Any help appreciated!
 

Edited September 14, 2021 by Chendy

[Chendy]

I have got off the phone from Kingspan who thought but couldn't confirm that the U-Value is 0.25 W/m²K for floors, irrespective of whether there is underfloor heating or not.
Is this true?
 

[SteamyTea]

The Building Regulations are here:

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/697629/L1B_secure-1.pdf

 

The L1B is for existing buildings.

Seems to be 0.22W.m^-2.K^-1 unless it is an upgrade and then it is 0.25

 

While you may not have to improve the losses though the floor, you may want to.

Part of thermal calculations is temperature difference, and as the ground does not change temperature much, and UFH heats the floor more than an unheated floor, it makes sense to insulate it more.

Edited September 14, 2021 by SteamyTea

[Chendy]

Thanks @SteamyTea

 

I checked L1B, and that's what I found too.

1. Is this the correct value irrespective of whether underfloor heating is to be installed?

 

2. Is it still 0.25W.m-2.K-1 for upgrading intermedia floors of existing buildings?
If so, seems strange, as like you point out, there is less heat loss, and much of the 'loss' will be into the room below.

Thanks
 

[SteamyTea]

2 minutes ago, Chendy said:

1. Is this the correct value irrespective of whether underfloor heating is to be installed?

Seems to be, probably because when the regs were written, UFH was not that common.

But that should be of little concern to you, add as much insulation (smallest U-Value) as you can below the UFH pipework.

 

It is not compulsory to work to the minimum standard.

[saveasteading]

25 minutes ago, SteamyTea said:

It is not compulsory to work to the minimum standard.

Agreed. as the floor is constantly warmed with UFH, it is important not to heat the earth, hence take this once only chance to insulate well, and it will repay over time.

 

If the floor is large then the earth in the centre will provide decent insulation in itself (to the extent that a very large building does not need flor insulation at all).

but for a small building, most of the floor is near the outdoors and there is heat loss.

 

Because of this it is probably best to use the most expensive insulation (PIR) to whatever thickness you can. 

 

I see the first floor insulation as being control rather than economy. you will be keeping the heat in the relevant room, especially keeping the ground floor heat where intended.

[SteamyTea]

1 minute ago, saveasteading said:

 

I see the first floor insulation as being control rather than economy. you will be keeping the heat in the relevant room, especially keeping the ground floor heat where intended.

Yes, not many people live in walls, ceilings and floors, most live in the space between.

We also breath the air in that space, so warm enough air helps reduce the bodies thermal losses (which may not be beneficial if trying to loose body mass, but that is another issue).

[CotswoldDoItUpper]

21 minutes ago, SteamyTea said:

Yes, not many people live in walls, ceilings and floors, most live in the space between.

We also breath the air in that space, so warm enough air helps reduce the bodies thermal losses (which may not be beneficial if trying to loose body mass, but that is another issue).

You burn more calories to cool the body then heat it…. Therefore a warmer environment will assist those trying to reduce mass of body fat. #OffToppic #sorry 

[SteamyTea]

36 minutes ago, CotswoldDoItUpper said:

You burn more calories to cool the body then heat it

Have you got some reference source for that?

I thought that perspiration, which uses the the latent heat of evaporation, helped cool the body, and that relies on a external heat source and humidity level differences.

[CotswoldDoItUpper]

1 hour ago, SteamyTea said:

Have you got some reference source for that?

I thought that perspiration, which uses the the latent heat of evaporation, helped cool the body, and that relies on a external heat source and humidity level differences.

Your cardiovascular system works harder to pump more blood to extremities when warm so that the process off evaporation from perspiration can take place. 
 

the exception is when you’re very cold and continually shivering. This can increase calorific burn by about 200/hr over non-shivering cold temps, but that is strictly not a reflection of an increase of your Basel metabolic rate.

 

have a quick google for refs.

[RobTristram]

The latest (2023) U-value requirement for ground floors, whether in new houses or extensions, is 0.18.

 

Apparently there is some complex way of calculating a reduction in the requirement based on the area and perimeter. You're allowed to use the whole house if you're designing an extension. The larger the floor area is compared with the perimeter, the less heat will escape, so the U-value of the floor can be higher than 0.18. The trouble is they don't tell you how to do the calculation, they just refer you to other documents. If anyone knows how to do it I would be grateful for their advice.

[SteamyTea]

17 minutes ago, RobTristram said:

If anyone knows how to do it I would be grateful for their advice.

I took a stab at it and posted it up somewhere on here.

 

Really just a case of taking the slab thickness i.e. 0.15m, add it to each edge, to a 0.15m thick slab that is 5m by 10m become 5.3m by 10.3m.

Then multiply that by the U-Value below the UFH system, and the temperature difference.

The temperature difference is the ground temperature in mid winter i.e. 4°C and the mean UFH system temperature i.e. 35°C. So a delta T of 31°C.

 

 

Edited July 3, 2023 by SteamyTea

[Gus Potter]

1 hour ago, RobTristram said:

Apparently there is some complex way of calculating a reduction in the requirement based on the area and perimeter.

Yes it's a bit of a mine field.

 

Starting with the design codes a lot of the ground floor heat loss calculations you get using the online calculators are based on on BS EN ISO 13370 (2017) and BRE 443.

 

I'll leave suspended floors out for now as a different approach but take ground bearing slabs. The following is a rough explanation.. but this is kind of how it works.

 

You have UF in some kind of screed or could be in the structural slab, that sits on insulation. We know the thermal conductivity of the insulation. But to make things efficient we need to look at what is under the insulation and around the edges.. the ground will heat up a bit and once it does where does this heat go?

 

Heat leaks out in two dimensions, down and sideways. Now often the online calculators often attribute a thermal conductivity (the Lambda value based on clay soil) of the ground below of 1.5λ (W/m.K) . Imagine we had a huge warehouse.. that ground has an insulating effect. Now as we don't have a warehouse we modify the calculation for the size of the slab.. and that is where you get the perimeter / area value from as this takes account of the heat that leaks out the sides. A small slab tends to have a P/A closer to one, a large slab say P/A of 0.6... big area vs perimeter.

 

Now when you work out the perimeter area you need to make a judgement on whether is is just the edges of the slab next to the external walls or do you need to include the edges that face into the building.. matter of judgement on how you are detailing it all but you can make savings here if you are struggling. Often if need be you can offset by adding a bit more insulation in the easier parts of the build.

 

Turning to the maths. We start with the thermal conductivity of each material, its thickness .. add up and reciprocate to get the U value.

 

@SteamyTea and @Carrerahill  et al. The ground thermal conductivity works I think on the principles of an elastic half space.. an infinite depth, same kind of principle pressure in soil modelling but coupled with flow of electricity through a bus bar. The maths I think are based on the Laplace equations for three dimensional flow... which can be water / electricity or heat.

 

Now @RobTristram if you start to struggle you can put extra insulation down and around the perimeter. Say take 50 -75mm of insulation board down the inside of the underbuilding to the top of the founds. This could get you out a hole and will actually work. @saveasteading knows more about this and the practicalities.

 

For me I would would take the online U value calculators assumption of perimeter thickness of insulation (often 25mm). Then if I at least double the depth and increase the thickness buy 3 times I would have a stab at demonstrating that I can reduce the P/A value by say 25- 33%... not below say 0.6.

 

Now that thickness seems a lot but if doing a TF / maybe dot and dab insulated plasterboard on masonry it can work a dream as it marries up closely with the thickness of insulated plasterboard... you get some continuity of insulation.

 

Life does have a bonus from time to time.

 

Once you have got the thermal conductivity established you can then look at your delta T and play about with that.

 

 

 

 

 

 

 

 

 

 

Edited July 3, 2023 by Gus Potter

[SteamyTea]

9 minutes ago, Gus Potter said:

The maths I think are based on the Laplace equations for three dimensional flow... which can be water / electricity or heat.

With a bit of Fourier thrown in I seem to remember.

 

I once, over at the other place, model the heat flow in two dimensions though a solid, seem to remember that it is just a cone made with exponential curved sides.  That is the losses for a cylinder, but should not be hard to chop it into a rectangle and take away the 'off cuts'.

[Gus Potter]

3 minutes ago, SteamyTea said:

With a bit of Fourier thrown in I seem to remember.

Yes Fourier could be applied but does it work with an infinite depth of material when you are dealing with a soil?

 

 

 

[SteamyTea]

7 hours ago, Gus Potter said:

Yes Fourier could be applied but does it work with an infinite depth of material when you are dealing with a soil?

I don't see why not.

Not really infinite depth, semi infinite.  So long since I had to do this sort of thing that I would have to do a lot of research.

My simple method of expanding the slab size, by the slab thickness will be close enough.

[saveasteading]

There used to be a simple graph in an insulation catalogue, summarising the more complex calculation.

But it was not repeated in subsequent issues. 

 

Cynical? Yes I think so, because i don't think it was well understood.

 

I noticed this and labelled the old  book,

" never throw this out".

.I may have it in a box.  I know I found it online once too, so it may still be there.

 

 

The logic is clear. A heated warehouse might be 100m x 100m. So the heat in the slab at the middle has at least 50m of ground to spread through before if reaches the cold outside. 

Contrast that with a complex house shape with a high proportion of outside wall, that may only be maximum 3m away.

 

There is another way of dealing with small buildings, by placing vertical insulation around the perimeter. The building regs had a section in it, but I always thought it had an error, the benefit was so low.

 

It's all as if the insulation manufacturers cared more about selling more stuff than about best practice and energy efficiency.

 

I'm not in a position to look for the old document, but will af some stage.

 

From memory, you don't need any slab insulation beyond 25m x 25m. Anything wider than about 10m allowed some reduction.

[Dave Jones]

On 14/09/2021 at 14:10, Chendy said:

Hello,

Does anybody know the U-Value required when retrofitting / renovating underfloor heating?

There seems to be plenty of resources talking about new builds but finding it hard to find any definitive information for retrofit.

 

For my specific property, I have a ground and first floor. Both having a concrete slab floor with screed over it.

I will remove the screed, then add insulation, then UFH pipes, then concrete poured over.

Trying to work out how much insulation I need. I'm assuming less required for the first floor.

 

Once I determine the maximum U-value permissible for building regulations, I can use various services such as this U-Value calculator to work out how much of what insulation I need.

 

Any help appreciated!
 

 

its not clear cut for a refurb. If you look at Part L regs the target is 0.18 for a floor BUT the threshold limit is 0.70 if achieving the target U value is not technically feasible or would not achieve a payback of 15 years or less.

[saveasteading]

10 minutes ago, Dave Jones said:

Its not clear cut for a refurb

I think this is sensible. There are many  tales of old buildings falling down due to attempts to dig the floor out and underminng the walls.

 

We aimed for new build standards because it is in our long term intetest,but had to adapt and adjust.

[Dave Jones]

55 minutes ago, saveasteading said:

I think this is sensible. There are many  tales of old buildings falling down due to attempts to dig the floor out and underminng the walls.

 

We aimed for new build standards because it is in our long term intetest,but had to adapt and adjust.

 

totally agree. old buildings can be destroyed by adopting modern insulation and building methods. Im same going above the standards where its financially feasable. I think the most payback will be in targetting passiv levels of air tightness. I'm considering buying the 'lite' version of the blower door tester which gives the ACH so can do it at first fix stage where its still possible to sort them out.

[Sparrowhawk]

1 minute ago, Dave Jones said:

I'm considering buying the 'lite' version of the blower door tester which gives the ACH so can do it at first fix stage where its still possible to sort them out.

How much is that 'lite' version?

[Dave Jones]

https://www.buildtestsolutions.com/air-leakage-testing/leak-checker-diagnostics-fan

 

[saveasteading]

What a great idea. Better yet if it could be hired.

@ProDave s diy version was £1200 cheaper but didn't have an app giving figures.