Fabric efficiency against the carbon target in an energy statement

A planning energy statement has to satisfy more than one metric, and the fabric energy efficiency figure is the one a heat pump cannot rescue. Under Approved Document L Volume 1 a new dwelling has to meet a target primary energy rate, a target emission rate and a target fabric energy efficiency rate, and the last of those measures space heating and cooling demand alone. Change the heat source and two numbers move. The fabric figure does not.

That is why schemes arrive at planning with a compliant emissions result and a failing fabric result, usually after the glazing has been fixed and the elevations signed off.

The separate metrics a statement has to satisfy

Three targets apply to dwellings under the current Approved Document L Volume 1, and all three have to be met.

Planning policy then layers its own requirement on top. In London, the energy hierarchy of be lean, be clean, be green and be seen applies, with major development expected to achieve a substantial improvement on the Building Regulations baseline and a stated minimum proportion of that improvement from energy efficiency measures alone at the be lean stage. Outside London, adopted local plans set percentage improvements, carbon reduction targets or fabric standards that vary by authority, so read the policy before choosing the specification.

Where a local plan sets a fabric standard directly, an Energy Statement that reports only carbon savings answers the wrong question.

Why a scheme passes one target and fails another

Fabric energy efficiency is a demand metric. It is calculated from the envelope, the airtightness, the thermal bridging and the form of the building, with no credit for how the remaining demand is met.

So an air source heat pump cuts the emission rate sharply, improves primary energy once the generation factors are applied, and leaves fabric energy efficiency untouched. Photovoltaic panels do the same. A communal heat network with a low carbon source does the same. All of these are legitimate and none of them helps the fabric result.

The reverse also happens. A compact block with modest glazing and good detailing can pass fabric comfortably and still fail primary energy if it is heated by direct electric panels, because the fuel factors penalise that choice.

What follows is a sequencing rule. Fix the fabric at concept stage, because it depends on decisions that are expensive to revisit, then choose the systems, because those can change late without redrawing the elevations.

Glazing ratio, form factor and orientation

Three geometric properties set the fabric result before any insulation thickness is chosen.

Form factor is the ratio of heat loss envelope area to treated floor area. A mid terrace house or a flat in a block has a low form factor because it shares walls. A detached house with a complex plan, projecting bays and a wrap around single storey element has a high one, and every additional square metre of envelope is additional heat loss. Two schemes with identical constructions can differ by a wide margin on fabric energy efficiency for this reason alone.

Glazing ratio works in both directions. Glass loses more heat than wall at any realistic specification, so a high glazing ratio raises space heating demand. It also admits solar gain, which reduces heating demand and raises cooling demand, and the cooling side is where Approved Document O and an Overheating Risk Assessment come in. A south facing wall of glazing with no external shading will often fail overheating assessment and fabric efficiency at once.

Orientation decides whether solar gain helps or hurts. South glazing with a modest overhang gains in winter and is shaded in summer. West glazing is the hardest to shade and the biggest overheating risk, because late afternoon sun arrives at a low angle when the building is already warm.

Here is the honest uncertainty in all of this. Modelled demand and measured demand diverge, often substantially, and the performance gap is well documented in the industry without being resolved. The metrics are what the Regulations and the planning policy test, so design to them, but do not treat a modelled figure as a prediction of bills.

Where renewables cannot rescue a weak fabric result

Once the fabric target is missed, the options narrow to fabric measures. Adding generation does not change a demand metric.

That leaves a familiar list, in rough order of cost per unit of improvement: better thermal bridging detailing, a tighter airtightness target with the detailing to deliver it, reduced glazing area or improved glazing specification, thicker insulation within the agreed wall thickness, and finally a change to the envelope geometry.

Thermal bridging is the one most often left on the table. Junction psi values from accredited or calculated details can produce a worthwhile improvement in fabric energy efficiency at low capital cost, and using default values in the calculation is a self inflicted penalty. If the modeller has used defaults, ask why.

Airtightness is the next. A tighter target costs little in materials and a lot in site supervision, so it is only credible where the construction method supports it and the contractor is briefed.

Reducing glazing is the measure clients resist and officers rarely mind, provided daylight standards still work. Our note on energy statements for warehouse and logistics schemes covers the equivalent trade off on large roof and wall areas in non-domestic buildings.

What the officer reads first

Case officers are not building physicists and they do not read the calculation output. They read the summary table and look for the policy percentages, so the statement should present them in the policy's own terms.

Give the baseline, the result after energy efficiency measures alone, the result after efficient supply, the result after renewables, and the residual, with percentage improvements at each stage. Then give the three Approved Document L metrics as targets and results with a pass or fail. Two tables, one page, no narrative required to interpret them.

Officers also look for consistency with the rest of the submission. If the energy statement assumes 40 per cent glazing and the elevations show 55 per cent, the consultee will find it. If the overheating assessment assumes openable windows on two facades and the layout has single aspect flats, the same applies. Where a Sustainability Statement accompanies the energy work, the assumptions in both have to match.

Fixing the problem at concept stage

Six decisions settle most of the fabric result, and all of them cost less before planning than after.

1. Compactness. Reduce envelope area per square metre of floor area by simplifying the form and avoiding small projections.
2. Glazing budget. Set a percentage by elevation early and hold it, with the daylight assessment run in parallel rather than afterwards.
3. Orientation and shading. Place living space glazing where solar gain is useful and shadeable, and treat west glazing as a liability.
4. Junction detailing. Commit to calculated psi values and buildable details rather than defaults.
5. Airtightness strategy. Choose the air barrier line, draw it as a continuous line on section, and set a target the contractor can hit.
6. Services only after that. Choose the heat source, ventilation and generation once the demand is settled.

On a listed building or in a conservation area the order changes, because fabric change is constrained by consent rather than by cost, and our note on energy statements for a listed building with limited fabric change deals with that case.

Reports drafted on PlanScope are AI-assisted and should be reviewed before submission, and we do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. Enter your postcode on PlanScope to see which energy and sustainability documents your scheme is likely to need.