Minimum U-Value for effective UFH on ground floor?

[JohnW]

I know the lower U-value I achieve for my ground floor the better but is there a maximum U-value beyond which under floor heating is not advisable?

[Jeremy Harris]

Depends on several variables.  First off, you need to know the heating requirement, which is derived from the house heat loss, as that determines the temperature that the floor surface needs to be in order to adequately heat the house to your particular target temperature (some like houses warmer than others, so that's another variable).  The floor insulation contributes to heat loss more with UFH than with other forms of heating, so that also needs to be taken into account.  Once you have a feel for the heating requirement, and hence the floor surface temperature, you can start to work out the heat loss through the floor, and this is best done by not always focussing on the percentage heat loss, as that tends to highlight the inefficiency of UFH in a low energy house in a way that can tend to cover up the fact that the actual heat loss power is pretty small, IMHO.

 

I put together this rather simple spreadsheet to calculate floor heat loss etc that may help.  Save the file and rename it, changing the .txt suffix to .xls and it should open in most spreadsheet applications OK:

 

Floor heat loss and UFH calculator.txt

[JohnW]

Thanks @JSHarris - I've been playing with your fabulous spreadsheet (never thought I'd ever use those 2 words together) for some time now and it appears that changing the floor U-Value doesn't have a very big impact on the overall results. I tried Floor U-Values of 0.14, 0.18 and a Regs matching 0.25 for comparison and the results are shown below;

 

What surprised me most, apart from the low values, was that the adjusted July & August figures appear to show the house over-heating more with a U-value of 0.25 than at 0.14 or 0.18. Can you confirm that this is what you would expect?

 

 

[Jeremy Harris]

That's not really an over-heating indication, as the spreadsheet doesn't account for incidental heat gains at all, so just ignore the negative numbers.

[frankquietly]

@Jeremy Harris Thank you for the floor heat loss calculator. Can I please check something, it seems to not consider the exposed perimeter / area ratio. My understanding is that ground floors in contact with the ground can have quite a low starting U value due to this ratio. For example my mid terraced house has an exposed perimeter of 16.1 metres, area of 36m2, hence a ratio of 0.45 m/m2. Using an online table gives a starting U value of 0.65 without any insulation (Gov scot floor u paper).

 

Am I missing something here?