Since May 24, Ireland's building energy rating (BER) scale has had a new summit.

The A0 category, reserved for zero-emission buildings, now crowns the simplified eight-point scale, and by 2030 every new home in the state must reach it.

It is tempting to read A0 as simply 'a bit better than A1 used to be', one more rung on a familiar ladder. That reading is wrong, and the sooner the design professions internalise why, the fewer expensive disappointments there will be. An A0 is not a grade a building drifts into by being generally excellent.

It is a conjunctive test: six separate conditions, assessed within DEAP, every one of which must be satisfied simultaneously. Fail any single one, by a single kilowatt-hour, a single kilogram of carbon, or a single gas hob – and the building is excluded from the top of the scale, however magnificent it is in every other respect.

The six conditions

Under the zero emission building (ZEB) definition implemented in DEAP, a dwelling attains A0 only if it meets all of the following: 

• First, a primary energy value of no more than 42 kWh/m² per year;
• Second, a carbon emissions value of no more than 5 kg CO²eq/m² per year;
• Third, no on-site use of fossil fuels of any kind;
• Fourth, the capability to respond to external signals: the building must be able to adjust its energy use in reaction to the grid;
• Fifth, its energy demand must be met by carbon-free sources or otherwise comply with national requirements;
• Sixth, full compliance with nearly zero energy building (nZEB) standards, which is itself not one check but three, since DEAP demonstrates nZEB compliance through the Energy Performance Coefficient, the Carbon Performance Coefficient and the Renewable Energy Ratio together.

Six conditions on paper; eight distinct pass-or-fail gates in practice. Each deserves a moment's attention, because each fails buildings differently.

1) The energy ceiling: 42 is lower than it looks

The primary energy threshold of 42 kWh/m²/year sits in deceptive territory. Under the old scale, an A1 required 25 kWh/m²/year or better, while an A2 stretched to 50. The A0 ceiling therefore falls between the two, which means a large share of the homes Ireland has spent the last decade celebrating as A-rated will not clear it.

A compliant, heat-pumped, well-insulated new apartment certified A2 at 45 kWh/m²/year is, by the arithmetic of the new scale, three kilowatt-hours from the summit and might as well be 30.

Primary energy, remember, is not the energy a household uses but delivered energy multiplied by fuel-specific conversion factors, so the figure is sensitive to heating system efficiency, hot water strategy, ventilation choices and the DEAP treatment of on-site generation. Getting under 42 without photovoltaics is possible for compact, well-oriented forms; for most dwelling types, the solar array is doing real work in the calculation.

2) The carbon ceiling: Five kilograms and falling grid intensity

The second condition, 5 kg CO₂eq/m²/year, runs in parallel and is not automatically satisfied by passing the first. Carbon and energy are correlated but not identical: the carbon figure depends on which fuels serve which loads and on the emission factors DEAP assigns to them.

For an all-electric home, the two conditions tend to rise and fall together as the electricity grid decarbonises, but any residual combustion – a solid fuel stove retained for character, a gas connection kept 'just in case' – drags the carbon number disproportionately. In practice this condition polices the same territory as the third, but with a numerical tripwire rather than a categorical one.

3) The absolute bar: No fossil fuels, no exceptions

Condition three is the simplest to state and the most unforgiving in practice: no on-site use of fossil fuels. Not efficient use, not occasional use, not backup use – none. A single gas hob in an otherwise exemplary passive-standard house is disqualifying.

So is an oil-fired backup boiler that never runs, and so, for retrofit projects, is the gas boiler the client wanted to keep 'until the heat pump proves itself'. This is a binary condition, and binary conditions are where good buildings with one sentimental compromise go to lose their rating. For architects, the implication lands early in the design process: the fuel decision is now a rating decision, made at RIBA Stage 2, not a services detail resolved at Stage 4.

4) The novel condition: A building that listens

Condition four is the quiet revolution in the list. The requirement that a dwelling be capable of responding to external signals imports the logic of the smart grid into the building certificate: an A0 home must be able to modulate its demand – shifting heat pump operation, water heating or vehicle charging in response to signals from outside. This reflects the recast Energy Performance of Buildings Directive's vision of buildings as active grid participants rather than passive consumers, and it is the condition least familiar to Irish design teams.

It cannot be retrofitted with a certificate in hand; it requires controls, metering and communication capability specified from the outset. The profession has spent 20 years learning to design for low demand. Condition four asks it to design for flexible demand, which is a different discipline.

5) The moving target: Carbon-free supply

Condition five, that carbon-free sources meet energy demand or comply with national requirements, is, at present, the easiest gate to pass and the least understood. Because DEAP evaluates supply at the level of the national grid, an all-electric dwelling currently satisfies this condition by default: the carbon-free share of Irish grid electricity assumed under 2026 conditions comfortably exceeds the compliance threshold. But 'currently' is the operative word.

The condition is defined against a moving national energy system, and its generosity today is a function of the grid's progress, not the building's virtue. Designers should treat it as passed but not banked.

6) The compound condition: nZEB, three tests deep

Then comes condition six, and this is where the conjunctive character of A0 becomes fully visible. Compliance with nZEB standards is demonstrated in DEAP through three simultaneous checks: the Energy Performance Coefficient and Carbon Performance Coefficient, which compare the design against a notional reference dwelling, and the Renewable Energy Ratio, which requires a minimum share of the building's energy use to come from renewable sources.

A dwelling can hold its primary energy under 42 and its carbon under 5 and still stumble here most plausibly on the renewables ratio, where an efficient but renewables-light design falls short, or on the coefficient checks in an unusual built form that diverges awkwardly from its notional reference. Condition six means A0 is not merely a performance destination; it is a performance destination reached by a prescribed route.

One slip from zero

Add it up, and the character of the new summit becomes clear. A0 is not a spectrum position; it is a logical AND across eight gates, and DEAP does not average. There is no partial credit, no compensating excellence, no 'A0 minus'.

The house at 43 kWh/m²/year and the house at 143 receive the same answer to the A0 question. This severity is deliberate: the category exists to give unambiguous meaning to 'zero-emission' ahead of the 2030 mandate, and an unambiguous category cannot be a fuzzy one. But it changes the nature of design risk. Under the old scale, a late-stage compromise cost a sub-grade, a B1 slipping to B2.

Under the new one, a single substitution during construction, an undocumented insulation product that forces a DEAP default value, or a value-engineered PV array can drop a project from the best rating in Ireland to the second best, with everything that implies for marketing, green mortgage eligibility and, from 2030, regulatory compliance for new-builds.

The professional response is not despair but discipline. A0 must be designed for explicitly, verified continuously and protected contractually: every one of the eight gates identified at concept stage, tracked through procurement, and evidenced with certified documentation the assessor can accept because in DEAP, what cannot be proven does not exist, and default values are where marginal A0 go to die.

The rating rewards exactly the habits the profession claims as its own: whole-building thinking, early systems integration and rigour sustained to handover. Ireland has set its new gold standard behind six locks. The architects who learn the shape of every key will be the ones handing clients the door.

References

1. SEAI. (2023). Domestic BER Assessors - Dwellings Technical Bulletin #5. https://www.seai.ie/publications/Technical-Bulletin-05-Issue-1-February-2023.pdf
2. Department of Housing, Local Government and Heritage. (2022). Building Regulations Part L Amendment - Buildings other than Dwellings. https://www.gov.ie/en/publication/d82ea-building-regulations-part-l-amendment-buildings-other-than-dwellings/
3. Department of the Environment, Climate and Communications. (2021). Climate Action Plan 2021. https://www.gov.ie/en/publication/6223e-climate-action-plan-2021/
4. SEAI. (2022). DEAP 4.3 Software Updates. https://www.seai.ie/blog/deap-4-3-software-update/
5. SEAI. (2021). NEAP Technical Bulletin #1: nZEB Compliance. https://www.seai.ie/publications/NEAP-Technical-Bulletin-01.pdf
6. Eirgrid. (2022). Annual Renewable Energy Constraint and Curtailment Report 2022. https://www.eirgridgroup.com/site-files/library/EirGrid/Annual-Renewable-Constraint-and-Curtailment-Report-2022.pdf

Author: Lala Rukh (LinkedIn) is a doctoral researcher at the University of Galway. She is an electrical engineer and has master's degrees in energy systems and marine plastics abatement.