Why this EPC got it wrong

Findings

1. 01

   The certificate does not measure your walls, it assumes them

   In a solid-wall Victorian terrace, RdSAP records the wall construction from a small handful of visible cues, most importantly the wall thickness at a reveal and the visible brick coursing. The assessor then applies a national default U-value for that construction. If any insulation is present, the assessor records it only when there is documentary evidence of a specific installation, which most Victorian homes do not have on file. The certificate ends up recording what the average solid brick wall in the national stock is understood to do, rather than what the wall on this particular house is doing in an east London winter.

   Wall U-value on the certificateAssumed, not measured
2. 02

   A C rating usually means the assessor recorded something else the walls are not

   For a solid-wall Victorian terrace to reach a C, the certificate typically records replaced windows to modern double glazing alongside a modern condensing boiler with weather compensation, on top of full loft insulation at current thickness. Any one of these can push the rating up by around a full band, because RdSAP weights them heavily relative to the walls. This is why two neighbours on the same street can have EPCs one or two bands apart while sharing the same wall construction, the same orientation and the same heat-loss reality. What the certificate describes is the kit; what it under-weights is the fabric.

3. 03

   The room the certificate cannot see is the room the household actually notices

   The RdSAP survey records the home as a single zone, weighted by floor area. It cannot record that the back bedroom over an unheated kitchen extension runs three degrees colder than the front bedroom, that the bay window forms a cold pool the sofa sits in, or that the party wall to a poorly-heated neighbour is doing more heat-loss work than the assessor's model can capture. These are the observations that decide whether a home feels warm or cold; they are outside the certificate's field of view by design.

   Rooms recorded individuallyZero
4. 04

   The improvements list is a national table, not a plan for your house

   The improvements shown on an EPC are drawn from a national table of typical costs and typical savings for each measure applied to each construction type. The assessor does not choose them and cannot vary them based on what they saw. That is why the same certificate can recommend internal wall insulation, low-energy lighting and a wind turbine within three lines of each other. Some of the recommendations are excellent for the archetype; some are default entries that the methodology cannot suppress. The list is a starting point for the household's own thinking, not a plan to be executed top to bottom.

5. 05

   The reported energy demand is halfway to the truth for this archetype

   Our worked example reports a space-heating demand of around one hundred and forty kilowatt-hours per square metre per year. Monitored data on comparable Victorian terraces, most notably from the Energy Saving Trust and Historic England's own studies, tends to land between one hundred and eighty and two hundred and twenty. The gap is not the assessor's failure; it is the methodology accepting a portable assumption in return for being able to survey ten million homes in a reasonable time. For a household deciding what to do next, the practical response is to read the certificate as a starting hypothesis about the home, and to spend an afternoon with a thermal camera and a room thermometer before treating any of its numbers as facts about the house.

   Gap to monitored benchmarkRoughly one third under-stated

Headline figures

• Certificate ratingCreported
• Reported space-heating demand, RdSAP138 kWh/m²/yrmodelled
• Independent monitored benchmark for similar homes180 to 220 kWh/m²/yrmeasured
• Survey durationTypically 30 to 45 minutesreported

Measured is monitored data from a real project. Reported is a publisher's stated figure. Modelled is our own estimate, worked from public references.

What this investigation cannot tell you

• We are describing one archetype; the same critique applies less strongly to modern homes where the assessor can verify the fabric directly.
• This piece does not argue that EPCs are useless; a rating trend on the same home over time is a useful signal, and the certificate remains the gateway to several grants.
• The reported figures in the metrics table are typical, not universal; specific certificates on specific homes will fall inside or outside the ranges we show.
• Assessors follow RdSAP as required by law; nothing here should be read as a criticism of individual assessors working within the methodology.

Where the figures come from

• RdSAP methodology and conventionsBRE Group
• Energy Performance of Buildings Register (public search and guidance)gov.uk
• Traditional buildings and energy performance researchHistoric England
• Solid-wall insulation field trialEnergy Saving Trust

How we built this

We assembled a representative EPC for a solid-wall Victorian terrace using the RdSAP conventions published by BRE and the standard defaults that assessors apply during a survey. Reported ranges for space-heating demand come from Energy Saving Trust and Historic England field data on similar homes. No specific certificate, address or household is referenced; the certificate we describe is a worked composite, and any resemblance to an individual home's rating is the point rather than the exception.