Can minimum u-values be averaged across all flat roofs?

[miike]

For a pre-2022 Part L building reg on new builds, the min u-value is 0.13 for flat roofs. If the building has multiple flat roofs and terraces, does the SAP assessor/bc work out the worst performing part of the roof and pass/fail based on whether this is within the min values? 

 

I'm wondering if the u-value can be averaged out across all areas - for example equal parts are 0.11 and 0.15. 

 

I've heard that the SAP assessor can also consider the building performance as a whole, and a below standard roof can be offset by improvements in other areas. However, I haven't been able to find confirmation of this. 

 

Is anyone familiar with these rules who can advise? 

[ADLIan]

If there are roofs (or walls or floors) with different U-values they are normally entered into the sap software as different elements along with associated area and U-value. The software will check the area weighted U-value and ensure that worst case U-values are not exceeded. The sap assessment allows a degree of ‘trade off’ between elements.

[Dave Jones]

16 hours ago, miike said:

For a pre-2022 Part L building reg on new builds, the min u-value is 0.13 for flat roofs. If the building has multiple flat roofs and terraces, does the SAP assessor/bc work out the worst performing part of the roof and pass/fail based on whether this is within the min values? 

 

I'm wondering if the u-value can be averaged out across all areas - for example equal parts are 0.11 and 0.15. 

 

I've heard that the SAP assessor can also consider the building performance as a whole, and a below standard roof can be offset by improvements in other areas. However, I haven't been able to find confirmation of this. 

 

Is anyone familiar with these rules who can advise? 

 

yes you can do this. Put an opening in the roof for example and it drops the efficiency big time so you have to account for it. You can also beef up other areas if you can only reach the bare minimum on a single element.

[ADLIan]

Door and window openings are a separate input input SAP - area and U-value of each opening. A roof window does not degrade the average U-value of the roof, same applies to walls.

[kandgmitchell]

The pre-2022 Approved Document for new build L1A had the limiting fabric U value for roofs as 0.20W/m2K with the caveat that in creating the target CO2 emission rate and the target fabric energy efficiency rate (the SAP calcs) a U value higher than this would be necessary - this was a "bottom line" so to speak, to avoid a really high U value compensating for a really poor one.

 

The latest post June 2022 Approved Document has that bottom limit as 0.16W/m2K. Not sure where 0.13W/m2K you mention comes from. However, should you be able to achieve 0.13 and 0.15 on different roofs then the SAP calcs done under the pre-2022 regime will take account of what were for then, very good U values and ought to show a compliant set of SAP results.

[miike]

1 hour ago, kandgmitchell said:

The pre-2022 Approved Document for new build L1A had the limiting fabric U value for roofs as 0.20W/m2K with the caveat that in creating the target CO2 emission rate and the target fabric energy efficiency rate (the SAP calcs) a U value higher than this would be necessary - this was a "bottom line" so to speak, to avoid a really high U value compensating for a really poor one.

 

The latest post June 2022 Approved Document has that bottom limit as 0.16W/m2K. Not sure where 0.13W/m2K you mention comes from. However, should you be able to achieve 0.13 and 0.15 on different roofs then the SAP calcs done under the pre-2022 regime will take account of what were for then, very good U values and ought to show a compliant set of SAP results.

 

Ah I've been looking at the 'notional dwelling specification' and reading it as the standard which must be achieved. The limiting fabric u-value of 0.20 makes things much easier. The 0.13 came from the target value for the pre-2022 regs.