CarbonLite, clearly: how the AECB's standards help us design low-energy buildings that work

CarbonLite is the AECB's framework for buildings that are genuinely low-energy, comfortable and healthy. It takes Passivhaus thinking, modelling in PHPP with a focus on fabric, ventilation and airtightness, and turns it into standards that UK designers and builders can actually deliver at sensible cost. The point isn't to chase a label. It's to predict how a building will perform, build it to match, and then test that it does.

Ribbon House exterior view showing rear extension in Queen’s Park with precast concrete canopy, large glazing and landscaped garden as part of an AECB CarbonLite retrofit.

The three routes

CarbonLite runs along three routes, which lets the level of ambition match the project rather than forcing the project to match the standard.

The New Build route is for new homes and non-domestic buildings using widely available technology and standard construction methods. The AECB reckons it cuts operational CO₂ by around 70% against typical UK stock, and it does that without exotic materials or unusual methods.

The full Retrofit route is for deep retrofits that meaningfully cut energy demand and winter bills while improving comfort and air quality. It runs on PHPP and is quality-assured by independent AECB approved certifiers, with the finished project uploaded to the Low Energy Buildings Database. Airtightness is held far tighter than Building Regulations ask for.

The Step-by-Step route is the pragmatic one for when budget, access or sequencing rule out doing everything in one go. The key is that the first step is planned so it doesn't block later measures, and every phase is modelled in PHPP to keep it on track technically and financially. It's retrofit without painting yourself into a corner.

The targets, in plain English

The thread running through all three routes is that performance is modelled, not hoped for.

Everything is calculated in PHPP, the same engine behind Passivhaus, so we predict energy use and comfort before building and verify them afterwards. Airtightness is held to real, achievable levels: an entry-level step might target around 5 m³/hr·m² at 50Pa, while a deep retrofit aims tighter, in the region of 2 air changes per hour at 50Pa. Fabric comes first, which means insulation, thermal-bridge control and high-performance windows and doors are coordinated as one system rather than upgraded in isolation, because that's how you get comfort without pushing a problem like condensation or summer overheating somewhere else. And ventilation is planned from day one, usually MVHR, or well-designed mechanical extract where MVHR isn't yet viable, so the air indoors stays clean and filtered. That matters most in an airtight retrofit, where you can't rely on draughts to move air around.

The principle is simple: design with evidence, build with care, prove it works.

Ribbon House: the indoor-outdoor connection, with sliding timber doors onto the garden, designed to AECB CarbonLite standards. RISE Design Studio.

Why we use it

CarbonLite is rigorous without being rigid. It sets clear, testable outcomes but leaves us the freedom to design for context, character and craft. It scales, from a single London terrace to a community or cultural building, because we can dial the ambition up or down through the route we choose. And it's certifiable: an independent AECB certifier reviews the PHPP model, the details and the evidence, and successful projects go onto the public Low Energy Buildings Database, which keeps the whole thing comparable and accountable.

How we run a CarbonLite project

We start by mapping your goals (budget, phasing, comfort problems, bills, carbon priorities) against the most suitable route, and sketch early options. Then we model the building's energy balance in PHPP before the design is fixed, testing fabric upgrades, window specs, shading, airtightness and ventilation to find the balance between performance and cost.

From there, the detailing has to make airtightness and moisture-safe insulation buildable rather than just desirable, so we coordinate the junctions and services and plan testing and QA with the contractor from the outset. We tender to contractors who understand low-energy delivery and agree the testing and commissioning plan up front. On site, blower-door tests and MVHR commissioning are managed as the work goes, so handover isn't a last-minute scramble. At the end we close the loop with the certifier and, where it's useful, add light-touch post-occupancy checks to see how the building performs in use.

Questions clients ask

Is CarbonLite just Passivhaus under another name? No. It's built on Passivhaus methodology and PHPP, but it's set up as a flexible framework for the UK market, with separate routes for new build and the different realities of retrofit.

What if I can't afford a full retrofit now? Start with Step-by-Step. We plan the eventual destination and deliver it in logical phases that don't trap you later.

Will it end up looking boxy? Only if it's designed badly. CarbonLite sets the performance outcomes. How the building looks and feels is a separate design question, and material, daylight and proportion are still ours to get right.

How tight is airtight? It depends on the route and the building. As a guide, an entry-level step might sit around 5 m³/hr·m² at 50Pa and a deeper retrofit nearer 2 ACH at 50Pa. We set the right target for the project, make it buildable, then test it.

What you actually get

Lower bills and lower carbon, with measured results rather than a green badge. Even, year-round temperatures and filtered air. And a building set up for where things are heading: ready for a heat pump and a decarbonising grid, with a staged path to get there if a single large project isn't realistic right now.

Whether it's a new build or a much-loved Victorian terrace, the work is the same in spirit: pick the right route, then design the details so the building performs in the real world rather than only on paper. If you'd like us to suggest the right CarbonLite route for your project, send us the address, your goals and a rough timeline and we'd be glad to talk it through.

→ Email us at architects@risedesignstudio.co.uk
→ Or call the studio on 020 3947 5886

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