u-values How to Calculate U-Values for Ground and First floors?

[Chendy]

Hello,

I am completely new to the building control process, stumbling my way through, any help very appreciated...

I have a maisonette flat situated on the ground and first floors of a 4 storey residential block.

The block is a reinforced concrete frame construction.

 

All floors are reinforced concrete slab, with uninsulated screed over the slab.

I want to remove the existing screed.

Replacing with floor insulation, then a finished concrete floor containing underflooring pipes.

 

I understand from LB1 the new floor should ideally meet a U-value of 0.25 W/m²K, "feasibility" aside.

 

Q1.A How to calculate the U-value of the ground floor? Is this a correct approach?

• https://warmafloor.co.uk/support-centre/u-values/ is a guide to calculation of the U-Value of the uninsulated floor.
  • based on The IP 3/90 formula is U = 0.05 + 1.65(P/A) – 0.6(P/A)²
    Where:
    U = U-Value of the uninsulated floor (W/m²K).
    P = Length of the exposed perimeter (m).
    A = Area of the floor (m²)
•  https://insulation-uk.com/member/u-value-calc is an example of a insulation manufacturer provided calculator
• I have cross compared and they line-up close enough.

 

Q2.A How to calculate the U-value of the first floors within the same dwelling?

• I haven't been able to find any guides how to do this
• Whilst it seems a U-value of 0.25 W/m²K still applies, I would assume that first floors within the same property would require less insulation??
  • the heat only really escapes from thermal bridging of the floor with the wall, heat wasted downstairs is in the same property??
  • and/or the room below can be assumed to also be heated or normal room temperature, so there won't be such a large temperature difference, so less insulation is needed??
  • http://files.nu-heat.co.uk/core/media/media.nl?id=237176&c=472052&h=f809f347f001eb168007&_xt=.pdf this datasheet from a underfloor heating company states
    • "In ground floors the insulation beneath the screed should be 70mm ‘Celotex’ or equivalent, or conform to Part L of the Building Regulations; whichever is greater. In upper floors insulation should be to a minimum of 30mm ‘Celotex’ or equivalent to prevent downward heat transmission"
• Q2.B can anybody link to a formula or calculator to use?
• Q2.C or is the calculation just the combined U values of the concrete slab + floor insulation?

 

 

any help very appreciated

 

: )

 

 

 

 

Edited September 23, 2021 by Chendy

[Chendy]

... for background...
I'm massively constrained with ceiling height and lining up with existing door thresholds - its an ex council flat.
So want to calculate everything and trade off against these two other considerations.

[Ferdinand]

Building Control are usually quite  approachable for a phone chat, and if it is a flat they may have dealt with it before.

 

If you are nervous, you could chat without telling the address in general terms, or even (I did not say this) mention the address of another identical one (eg one for sale) so it does not go on your record.

 

In my area, the general stance is "better something than nothing, even if it does not quite meet regs". Though I would not get that in writing.

 

Remember that air tightness with controlled ventilation is very important.

 

One trick is to trim slab doors, or look or secondhand ones that can trim more,  or look for doors with a bigger trimming margin. There are doors out there with trimming margins of up to about 65mm, which would alone give you 50mm of celotex at a squeeze. You may not want to replace them, but it can be a good option.

 

F

[Mr Punter]

The upper floor insulation is not a matter for building control as it does not form part of the external envelope.

 

Just do the calcs for the ground floor.  Others on the forum are not so keen on nu-heat and may give you pointers to better systems.

 

You don't need to have concrete or screed, you could have a dry system that will accept any number of floor finishes but reduce the height build-up.

 

I am assuming you have done the maths on ufh v radiators?  You will lose less heat with rads, they are far simpler to install and you will save some ceiling height.