Ofgem U-Values Consultation Response Published

In February this year, Ofgem published a consultation on overwriting U-values on cavity walls for ECO. The response to this consultation has now been published and there are new policies for overwriting U-values that must be implemented for any measures installed from 1st June 2016. 

There are three key outcomes from this consultation: 

  • A 'cavity wall U-value checklist' 
  • Threshold U-values for cavity walls for each age band
  • An audit of CWI measures already installed under ECO2


Cavity Wall U-value Checklist

The cavity wall U-value checklist must be completed for any cavity wall insulation (CWI) measure where the wall U-value has been overwritten. This checklist has two sections; one completed by the person collecting the inputs for the U-value calculation, including evidence from an intrusive survey if required and a second section that must be completed by a suitably qualified person as defined by RdSAP convention 3.08. The checklist requires each element of the wall (brick, block, wall ties etc) to be defined and supporting evidence provided.

Threshold U-values

If a cavity wall has an overwritten U-value that is higher than the threshold value, additional evidence will be required. The threshold U-values for age bands A-E are the RdSAP default values, for the later age bands (F-K) the RdSAP default value has a multiplier applied, so these are higher than the RdSAP default.

The additional evidence required if the threshold is breached is:

  • Relevant building plans
  • Core sample tested in a laboratory


Or by one of the routes already specified in RdSAP convention 3.08:

  • Verified by BBA/TIMSA member
  • Evidenced by relevent building control approval
  • Evidenced by any other schemes formally agreed between Accreditation Schemes/Approved Organisations and Government. This evidence will need to be agreed with Ofgem prior to notification.


Ofgem will be carrying out an audit of CWI measures with overwritten U-values under ECO2. More information about this audit will be published on the Ofgem website soon. 

What does this mean for NHER Assessors?


 EPCs audited under EPDB will still need to adhere to RdSAP convention 3.08. However, you should be aware that if the EPC you are producing is for a CWI measure under ECO and you are overwriting the U-value of the wall, additional evidence is required by Ofgem. We suggest you make your client aware of this.


If you are providing U-value calculations for ECO purposes, you should make your client aware that for CWI measures additional evidence will be required. 

Read the full consultation response here

See the CWI checklist here


Dyfrig Hughes, Technical and Product Development Director at NES, says: 

“I am pleased that Ofgem have gone some way to address the issue of unrealistic U-values being used in RdSAP for ECO scoring. This will improve the accuracy of scoring and also the quality of EPCs lodged on the register. It’s good to see that the new policy builds upon the current RdSAP conventions and does not require disruptive industry updates to ECO software. I’ll be interested to see if the evidence requirements can be met in practice or whether we see the end of overwritten U-values for cavity wall insulation measures.”