Energy Frame for New Builds: Calculation & Compliance

The energy frame is one of the few requirements in the Building Regulations that ties an entire project together across disciplines. It sets a ceiling on how much energy a new building may consume in total per square metre per year for heating, ventilation, cooling and domestic hot water — and because it sums the effect of the façade, the windows, the building services and the ventilation plant into a single figure, it is sensitive to choices made very early. An insulation thickness, a window area facing north or a heat-recovery unit left out all move the final calculation.

It is also where mistakes get expensive. The energy frame forms part of the application for a building permit, and it is directly linked to the energy label the building ends up with. If you only discover late in the design phase — or worse, after the façade is locked — that the frame does not hold, you are left with a redesign that hits both the budget and the schedule. The good news is that the energy frame is a calculation model, not a guessing game. Once you understand what goes into it, you can steer it from the sketch phase.

What the energy frame for new construction covers

The energy frame is a combined, calculated demand for supplied energy to run the building, expressed per m² of heated floor area. It covers the building’s demand for space heating, cooling, ventilation and domestic hot water — for dwellings it also includes a fixed allowance for the lighting share, while commercial and institutional buildings count the electricity used for lighting directly.

It is important to keep two things apart:

• The building’s energy demand is what the building would use in isolation, before you account for how efficient the energy supply is.
• The supplied energy is that demand weighted by factors for the individual energy carriers (electricity is weighted differently from district heating). It is this weighted figure that is compared against the energy frame itself.

The consequence is that the choice of supply — district heating, heat pump, solar PV — can be just as decisive for whether the frame holds as the insulation of the building envelope. Two buildings with an identical façade can end up on opposite sides of the limit purely because of their heat supply.

What numerical value the frame lands on depends on the building type (residential vs. other commercial use) and on the size of the building, since an area-dependent term is typically included. The specific requirement levels are set in the Building Regulations and are adjusted over time — always verify the current value in the applicable edition of BR before you start calculating.

What goes into the calculation itself

The energy-frame calculation is a building simulation, which in Denmark is performed as standard in the Be calculation tool (Bsim is used for more detailed simulations of larger commercial buildings). It draws together a long list of inputs that the engineer and architect must supply jointly:

The building envelope

• U-values for exterior walls, roof, ground-bearing floor and foundations
• The windows’ U-value, g-value (solar gain) and total area and orientation
• Linear thermal bridges at junctions (cold bridges), e.g. at the foundation, window reveals and balconies
• The building’s airtightness, verified by a pressure test

Building services and supply

• Heat supply and its efficiency (boiler, district-heating heat exchanger, heat pump)
• Ventilation plant, including heat recovery (recovery efficiency) and electricity for fans
• Any cooling plant
• Local renewable energy, e.g. solar PV, which is deducted in the accounts

Operating assumptions

• Internal heat gain from people and appliances
• Hours of use and room temperature based on standardised assumptions

The point for the design phase is that the heavy parameters — window area and orientation, heat recovery and form of supply — should be conceptually in place as early as the sketch phase. It is cheap to adjust a window area in an early model and expensive to do so once the façade has been detailed.

Rule of thumb: run a rough energy-frame calculation as soon as the façade concept and form of supply are known. An early calculation with reasonable assumptions almost always catches the problems that would otherwise only surface during the authority-stage design.

How the frame is documented to the authority

The energy frame must be substantiated as part of the building case. In practice this means that an energy-frame calculation is attached to the building permit, showing that the calculated supplied energy lies below the applicable requirement for the building type. The calculation must rest on the designed values — not on loose estimates.

The documentation is not a one-off document. It follows the project:

1. At the building-permit application: a calculation based on the project material that substantiates the frame.
2. During construction: the assumptions the calculation is built on — insulation thicknesses, window types, ventilation plant — must actually be executed as assumed. If a component is changed on site, the calculation must be revised.
3. At completion notification: documentation that the building lives up to the assumed values, typically including a completed pressure test of the airtightness and an energy label for the finished build.

The pressure test is worth highlighting, because it is an actual measurement and not a calculated assumption. If you assume good airtightness in the calculation and cannot document it by measurement, the assumption fails — and with it, potentially, compliance with the frame. This is a classic source of problems late in the process.

The energy frame does not stand alone among the BR18 requirements. It interacts with a range of other requirements triggered by new construction — for a complete overview of which requirements apply and when, the review of the key BR18 requirements for new construction is a good starting point.

Where the energy frame collides with other requirements

The energy frame is not optimised in a vacuum. The choices that make the frame easier to hold can make other requirements harder — and vice versa. This is where an experienced consultant earns their fee.

• Daylight vs. heat loss: Large windows provide daylight and solar gain, but also heat loss and a risk of overheating. If you cut the window area to ease the energy frame, you may come into conflict with the daylight requirements. The trade-off is real and should be handled as a whole — see how the daylight and indoor-climate requirements in BR18 set the lower limit that the energy frame must not push through.
• Operating energy vs. climate footprint: The energy frame is about the building’s operating energy. It is in principle separate from the climate requirement (LCA), which looks at the building’s total CO2 footprint over its lifetime, including materials. A thicker building envelope improves the energy frame, but costs materials and therefore embodied CO2 — the two optimisations do not always pull in the same direction. The relationship between the two accounts is explored in the article on climate requirements and LCA in the Building Regulations.
• Overheating: A tight, well-insulated building with large solar gain can have a low heating demand and at the same time an indoor-climate problem in summer. The energy frame does not necessarily penalise this directly, but it must be handled as a separate substantiation.

The practical consequence is that the energy frame should be optimised alongside the other substantiations — not as an isolated after-the-fact calculation once everything else is locked.

When you are in doubt about a specific requirement

The energy frame is a calculation model, but it lives within a set of rules that keeps changing: requirement levels are tightened, weighting factors are adjusted, and what applies to this precise project depends on building type, use and timing. The manual exercise of looking up the current BR edition, finding the right requirement and understanding the interplay with adjacent sections is manageable — but it is time-consuming and easy to get wrong when several disciplines are working in parallel.

That look-up exercise is exactly what Arcili’s chat module Rex is built to shorten. Rex specialises in BR18 and can quickly identify which requirements apply to a given building type, and how the energy frame connects to the other provisions — so the consultant spends time on the trade-offs themselves instead of on the search. It does not replace the engineer’s calculation or the final substantiation, but it removes the heavy look-up work in the early phase.

Want to see how it works on a real project? Read more about Arcili or book a walkthrough, and we’ll show you on your own cases.