Rear single storey ground floor extension- Roof U-value

[ab12]

What is the required U-value according to the last (2022) building regs for pitched roof for single storey extension?

 

Is it 0.15 or 0.13?

 

If it was 0.15 can this be achieved by fitting Knauf mineral wool 44 loft insulation 150mm thickness between the joist and 150mm above the joists?

 

Thanks

[Redbeard]

0.15

 

https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf

Table 3.2 Limiting U-values for new fabric elements in existing dwellings.

 

And in answer to the 2nd Q: It depends. You'd probably get sign-off from many BCOs on the basis that 0.3/0.044=6.818(R value) and 1/6.818 (U value) = 0.1466(W/m2K). However your description (

39 minutes ago, ab12 said:

150mm thickness between the joist and 150mm above the joists?

conjures up for me a picture of lots of timber to contain the 'above' bit. You can do an area-weighted U value calc (Part L always used to have a guide to doing so - I suspect it still does) by notionally 'lumping together' the timbers into 300 thick 'portions'. If, say, you end up with 2m2 of wood at 300 thick, then your U value calc is for (say, on a 20m2 roof) 18m2 of insulation, and 2m2 of wood. I suspect you could end up slightly over (worse than) the required U value, but possibly no-one will quibble. I have often done area-weighted U value calcs (for my own 'happiness') and I do not think I have ever been asked by a BCO to show them.

[ab12]

7 hours ago, Redbeard said:

0.15

 

https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf

Table 3.2 Limiting U-values for new fabric elements in existing dwellings.

 

And in answer to the 2nd Q: It depends. You'd probably get sign-off from many BCOs on the basis that 0.3/0.044=6.818(R value) and 1/6.818 (U value) = 0.1466(W/m2K). However your description (

conjures up for me a picture of lots of timber to contain the 'above' bit. You can do an area-weighted U value calc (Part L always used to have a guide to doing so - I suspect it still does) by notionally 'lumping together' the timbers into 300 thick 'portions'. If, say, you end up with 2m2 of wood at 300 thick, then your U value calc is for (say, on a 20m2 roof) 18m2 of insulation, and 2m2 of wood. I suspect you could end up slightly over (worse than) the required U value, but possibly no-one will quibble. I have often done area-weighted U value calcs (for my own 'happiness') and I do not think I have ever been asked by a BCO to show them.

Now to the fitting part, So current joist size is 7 by 2 or depth of 175 mm in metric units.

 

Now These loft rolls come in depth of 150mm or 200mm. If I select 150mm depth to go between the joist  and fit on the furthermost end away from ceiling level there'll be a 25mm gap between the ceiling plasterboard and start of insulation.

 

Is there potential for condensation in this place?

 

I have the impression that the insulation should be fully fitted to the depth of the joist.