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Energy·applied

Energy-Efficient Envelope

Why it matters

Asked what heats a house, most people name a machine: a boiler, a heat pump, a furnace. The real answer is the walls. Every kilowatt-hour a building holds onto is one that never has to be generated, stored, or paid for. And unlike any machine, insulation has no moving parts, no COP that collapses in a cold snap, and no maintenance schedule. Its service life is measured in generations. The cheapest, most fossil-free, most wartime-resilient energy in existence is the heat that stayed inside. That is why the envelope — insulation, airtightness, windows, the elimination of thermal bridges — comes first in every serious energy design. And every downstream component shrinks when it's done well.

Role in 001's holistic picture

The envelope is where the whole energy chain begins: it sets the demand that Heat Pumps, Thermal Storage, and the solar array are then sized to meet. It decides whether gentle low-temperature surfaces can heat the building at all. Envelope quality settles whether the Passive House Standard remains an aspiration or becomes arithmetic. It is also where Systemic Affordability bites hardest, because envelope quality is bought once and pays forever. Its fabric is bio-based insulation. And passive solar design orients the envelope to gather free heat before any machine runs.

In the world

When an existing building leaks heat, there are cures short of tearing it down: Energiesprong, a movement born in the Netherlands, wraps existing houses in prefabricated insulated facades and roofs fitted in days, with performance warranted for decades. Per Energiesprong, that comes to more than 10,000 net-zero retrofits across seven countries, around 6,000 of them in the Netherlands. The envelope-first doctrine turned into a mass retrofit industry for existing, often social, housing. And the same doctrine scales upward as far as anyone has dared take it. Bolueta in Bilbao is an 88-metre residential tower, certified in 2018 as the world's tallest Passive House. It holds 108 apartments, with 63 more rented as social housing in the adjacent block. That is envelope discipline carried to the twenty-eighth floor, for people who need low bills most.

How 001 engages with it

This page owns House 001's most honest number. The honesty starts at its edge: what the number says, and what it leaves open. Across a full heating season the living lab consumed 42.77 kWh of electricity per square metre for heating — in air-to-air mode, air leaks included. That figure is measured and firm. Our early write-up multiplied it by an assumed COP of 3 into a heating demand near 128 kWh/m² a year — roughly eight times the Passive House benchmark. That second number we no longer stand behind: the heat pump's real COP was never metered, and the season it describes was heated in a way the house was never designed for. The design put most of the heat into thermal mass — sand in the walls and the floor. But the wall loop died to a pipe bent somewhere inside the frame, beyond even a professional plumber's reach. The floor underdelivered through its badly layered radiant sandwich. And the house fell back on heating air — in a building with known leak paths that punish exactly that mode. The envelope itself was a deliberate experiment. House 001 was built as a first full-scale MVP of this whole catalog, and its walls, floor and ceiling were drawn at a 20 cm baseline for trying natural insulation. Living space alone bounded that thickness: sawdust is almost free, so doubling the layer would have cost only room indoors. And the loose sawdust settled — a real but fixable contributor, its cellulose fix identified and not yet scheduled. What survives our matured reading is narrower and more useful than a single multiple. It is a firm electricity figure, a list of named and separable causes, and a rule: an envelope verdict has to rest on an intact heat-delivery chain. What the same house would draw in its designed mode — air to water to thermal mass — is a number that does not exist yet. Maybe someday we will find out.

Unity Hub is the other end of our range: designed as a low-energy building. Its peak heating load at the −20 °C design temperature is about 10 kW for the entire 460 m² building — roughly 22 W/m², twice the Passive House threshold. That envelope quality is what makes the whole heating debate we published (air-source vs geothermal vs wood) almost pleasantly low-stakes. When demand is that small, even the winter-degraded COP of an air-source heat pump stays workable. A good envelope saves energy. More than that, it makes every other decision forgiving.

Maturity: applied. Measured through full seasons at House 001 — a firm 42.77 kWh/m² of heating electricity, with the honest analysis of why no clean envelope multiple can be read from it. Designed to about 22 W/m² of peak load at Unity Hub.

Further reading