The terms "passive house", "A++" and "nearly zero energy" have been used so heavily that they barely say anything any more. Yet the physics behind them is simple and comes down to four items. If you understand those four, you can judge any quotation.
1. The insulating envelope — the largest single item
To a first approximation, a house loses heat according to how large the surface is and how easily heat escapes through it. A CLT structure is in itself a better insulator than most masonry materials, but the decisive factors are the thickness and the continuity of the insulation laid over it.
Continuity matters at least as much as thickness here: interrupted, gappy insulation behaves like a coat left slightly open.
2. Airtightness — the most underestimated item
In a poorly built house a significant part of the heated air simply escapes through the gaps: at window and door junctions, at penetrations, where the floor meets the wall. This loss is independent of how thick the insulation is.
Airtightness can be measured: the blower door test puts the building under pressure and shows how many times per hour the internal air exchanges itself. That single number is the one that speaks honestly about the quality of the workmanship.
Prefabricated structures have a structural advantage here: the junctions are made to factory tolerances, so there are fewer improvised, on-site joints.
3. Thermal bridges — the invisible holes
A thermal bridge is a point in the structure where heat gets out "short-circuited": a bracket passing through, a floor slab running out uninterrupted, a badly installed window frame. There are two problems with it: it carries away heating energy, and it creates a cold surface on the inside where moisture condenses — the most common cause of mould.
Thermal bridges should not be eliminated afterwards; they should be avoided at the design stage. Again, this is a point where early decisions are worth more than later repairs.
4. Summer heat protection — the other half of the year
A well-insulated house needs little heating in winter, but in summer the same envelope also keeps heat in once it has got inside. Summer comfort is therefore decided not by cooling but by shading: external shading of the south- and west-facing glazing is worth an order of magnitude more than any internal blind.
Add to that the ability to purge-ventilate at night, and correctly specified glazing. These four items together produce what you see at the bottom of the bill.
What is worth investing in first?
For most family houses the order is this:
- Airtightness and thermal-bridge-free detailing. Cheap if designed well, expensive to fix afterwards.
- Sufficient, continuous insulation. Those extra few centimetres usually pay for themselves quickly.
- External shading. Few people plan it in at first, and everyone misses it later.
- Building services last. A good envelope needs only a smaller — and cheaper — heat pump.
That last point is the most common misunderstanding: it is not the building services that make a house energy efficient, it is the structure. The services only cover the remainder that the envelope has left over.
