HE 0 and HE 1: energy consumption and demand limits

Section HE 0 limits the building's energy consumption through two indicators calculated under standardised use conditions: non-renewable primary energy consumption and total primary energy consumption, both expressed in kilowatt hours per square metre per year. The distinction matters: the first penalises fossil energy and rewards renewables, the second prevents renewable abundance from excusing a wasteful building.

The limit values depend on the site's winter climate zone and on use, with differentiated tables for private residential use and for all other uses, where the limit is also modulated by the building's internal load. The climate zone is determined from the province and altitude according to the document's corresponding annex, and it is the first box to be filled in any early check: the same building has appreciably different limits in Seville, in Burgos or in a mountain municipality of the same province.

Section HE 1 works one step before consumption: it limits demand, the energy the building would need to maintain its comfort conditions regardless of the performance of its systems. Its philosophy fits in one sentence: fabric first, machine second. An excellent heat pump on a mediocre envelope achieves little.

Control is articulated with several complementary indicators. The envelope's global heat transfer coefficient, known by its symbol K, limits the assembly's conduction losses and depends on the building's compactness. The solar control parameter limits July gains through the openings, protecting against summer overheating. The envelope's airtightness bounds uncontrolled infiltration, with requirements on joinery and, in certain cases, on the building as a whole. And the transmittances of each element, walls, roofs, floors, openings, also have individual limit values acting as a safety net, preventing a very poor element from hiding behind an acceptable average.

The section is completed by condensation limitation requirements and the treatment of thermal bridges, the junctions where the envelope loses continuity: slab edges, columns in façades, opening reveals, roller shutter boxes. In well-insulated envelopes, thermal bridges concentrate a disproportionate share of losses and are the typical origin of surface condensation pathologies, which explains the growing attention design and verification devote to them.

Compliance with both sections is verified by calculation with recognised tools, HULC, the unified tool, being the official reference. Verification produces the building's indicators, consumption, demand, K, solar control, which are compared with the tables' limits, and its result is incorporated into the justification report and later feeds the energy performance certificate.

For the design workflow, the operational recommendation is to treat verification as a design tool rather than a final formality: an early energy model, even a simplified one, reveals whether the design sails comfortably or tightly, and allows correction with the cheap levers. Verification arriving with the design closed can only correct with the expensive ones.

The economic usefulness of understanding HE 0 and HE 1 separately lies in the map of levers, worth ordering by increasing cost. The cheap levers run out in preliminary design and the expensive ones remain for the end.

Lever	Acts on	Typical cost
Compactness and orientation	Demand	Zero at preliminary design, impossible afterwards
Size and protection of openings	Demand	Low, a design decision
Insulation thickness	Demand	Moderate, with decreasing marginal effectiveness
Joinery and glazing quality	Demand	High per unit of improvement
Thermal bridge treatment	Demand	Moderate, high return in avoided pathology
Generation efficiency	Consumption	Moderate to high, with aerothermal heat pumps as the standard
On-site renewables	Consumption	Variable, covered under HE 4 and HE 5

The optimal sequence is almost never the one followed by a rushed design, which oversizes insulation and joinery to compensate an unfavourable shape. With the model on the table, the minimum-cost combination for compliance is found by iterating between levers, and the differences between compliance-equivalent combinations can be appreciable in the envelope package, which as the article on the cost of CTE compliance recalls is the budget's largest regulatory block.

It is also worth designing with an eye on the next step: the CTE reform going through approval, with the zero-emission building as its destination concept, points to a further tightening of these limits, and long-horizon projects will do well not to comply to the millimetre. The current frontier of the requirement, the nearly zero-energy building, is developed in its own article.

The first is verifying late, the section's capital sin, already described. Verification belongs to the stage where the form can still change.

The second is setting the climate zone by ear: altitude changes the zone within the same province, and with it every limit. The zone is checked from the site's altitude rather than from the province.

The third is optimising average transmittances while ignoring the individual limits per element and the thermal bridges, with verifications failing on the fine print. Each element is also checked against its own limit.

The fourth is specifying joinery by U-value alone, forgetting the solar factor and airtightness, which are part of the same compliance and the same price. The three parameters are stated together in the item.

The fifth is divorcing ventilation from the energy calculation: the DB-HS 3 flow rates are a major addend of demand, and heat recovery decisions belong to both documents at once. Both documents are verified against the same flow rate.